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Diversity of Microbes and Cryptogams
Pteridophyta
Prof. S.P.Khullar (Retd.)
Department of Botany,
Panjab University
Chandigarh - 160014
Significant Keywords: Pteridophytes, fossils, stellar system, classification, Psilopsida,
Rhynia, Psilotopsida, Psilotum, Sphnopsida, Equisetum, Lycopsida, Lycopodium,
Selaginella, Pteropsida, Pteris,Marsilea, Apogamy, Apospory, Reproductive Biology,
Polyploidy, Gene-block hypothesis.
Legends to FIGURES
Fig. 1. The Geological periods of the Earth.
Fig. 2. The stele in Pteridophytes, A. Protostele, B.Actinostele, C. Plectostele, D. Siphonostele,
E. Solenostele, F. Polycyclic Solenostele, G. Polycyclic stele, H. Dictyostele.
Fig. 3. Reconstruction of Rhynia gwynne-vaughani ( A. after Kidston & Lang; B. after Edwards; C. after
Banks). D. T.S.Stem.
Fig 4. Reconstruction of Agalophyton (Rhynia) major. A after Kidston & Lang.; B after Edwards); C.
Longitudinal section of sporangium; D, E. Lynophyton rhyniensis, the supposed gametophyte of Agalophyton
major (after Remy & Remy, 1980).
Fig 5. Psilotun nudum
Fig. 6. Psilotum nudum. A-C Transverse sections of : A. Rhizome; B. Stem; C. Distal portion of stem. D.
Gametophyte.
Plate 7. Psilotum nudum. A-F. Development of Archegonia. G. Antheridium. H-J. Embryogensis.
Fig. 8. A. Equisetum diffusum; B. E. arvense; C. E. ramosissimum; D. A field of
E. arvense.
Fig. 9. Equisetum. A. Young cone; B. Mature cone; C. Spore; D. Sketch of a spore to show the elaters; E.
Sporangiophore; F. Gametophyte.
Fig. 10. Equisetum. A. T.S.of aerial internode of Equisetum ramosissimum showing an outer and an inner
endodermis; B. internode, C-D. Endodermis in E. debile. E. Path of the vascular supply in the stem
Fig. 11. The stele in various species of Lycopodium. A. L. selago Type; B. L. clavatum Type; C. L. cernum
Type. D. L. squarrosum Type.
Fig. 12. A. Lycopodium setaceum; B. L. hamiltonii, C. L. cernuum D. L clavatum.
Fig. 13. Lycopodium: Sporophylls in various species. A. L. selago; B. L. inundatum;
C. L. complanatum; D. L. clavatum
Fig 14. Gametophytes in various species of Lycopodium. A,B L. selago; C.L. cernuum; D. L. clavatum
Fig. 15. Lycopodium. A-D. Embryogensis; E. Protocorm.
Fig. 16. Selaginella chrysocaulos.
Fig. 17. The various types of stele in Selaginella. A. S. spinulosa Type (rhizome); B. Stem;
C. S. chrysocaulos; D. S. uncinata Type; E. S. willdenovii Type; H-J. S. lyalli Type.
Fig. 18. Selaginella. A. Microspore; B-I. Development of the male gametophyte; J. Megaspore (photograph
kind courtesy Prof R. Mukhopadhya, Burdwan, India); K-Q. Development of the female gametophyte; R-W.
Embryogenesis.
Fig. 19. Pteris vittata
Fig. 20. Pteris vittata A, B. Close up to show the marginal sori; C. Mature Prothallus D.Gametophyte loaded
with only antheridia
Fig. 21. Pteris vittata: development of the Antheridium.
Fig. 22. Marsilea minuta; A . Plant growing on land; B.Plant with Sorocarp.
Fig. 23 Marsilea A. Sporocarp; B Path of Vascular supply; C Receptacle with two lateral microsporangia and
a terminal megasporangium; D-H Development of male
gametophyte; I.A developing megaspore; J-M Stages in the development of the sterile tissue and
archegonium of a megaspore.
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Pteridophytes
(Greek pteris – pteron = a feather; phyton = a plant; i. e. plants with a feather-like
appearance).
It is now generally believed and accepted that life has existed on earth for over 3000 million
years. For most of this time it was restricted to water where it flourished and evolved. But
nothing is permanent and nature always desires a change. Thus from the very favorable and
luxuriant environs of water, for reasons unknown, life decided to invade land. It was only
400 million years ago that the first multicellular plants left water to invade the much more
hostile environments on land. Here these first land invaders had to face severe and harsh
conditions like extremes of temperature, strong winds and dust storms and erosion of the
land surface. There was a general shortage and scarcity of water, resulting in less or its
complete non availability. Further periods of dry drought alternated with wet monsoons
ones, and wet-lands with arid regions. In spite of these unfavorable conditions, life not only
managed to survive out of water, but evolved and rapidly spread over the barren land in the
absence of competition. To overcome the hardships on land, plant life developed some
necessary adaptations to survive here. Some of these basic features were:
i)
A system of anchorage or fixation (to the substratum) to prevent being blown
away, but more importantly, to absorb water and other nutrients from the soil.
ii)
A transportation system- the vascular system composed of specialized cells (to
transport food, water and nutrients from where they were available to where they
were not available) and also to enable the plants to withstand the strong winds and
be able to stand erect.
iii)
A covering of desiccation resistant cuticle, to prevent evaporation and loss of water
from the surface but perforated with small holes (stomata) to allow for the
exchange of gases (oxygen and carbon dioxide) between plants and air.
iv)
The change from a haploid gametophytic dominant phase to a diploid sporophytic
dominant phase. The haploid gametophytes are generally delicate tiny little plants
which apparently have little possibility of survival under the conditions prevailing
on land. Therefore, for successful colonization on land, during the course of
evolution, a diploid sporophytic constitution must have been necessitated under
Natural Selection. This situation had further advantage and survival value in
throwing up various recombination of characters for Natural Selection to operate.
These features enabled plant life not only to survive out of water but to spread on land.
Plants that had these essential features were tiny plants, at first only a few centimeters tall,
without leaves and roots but only a stem system that was forked or simple and terminating
in structures (sporangia) that produced spores not seeds (a spore differs from a seed in not
having an embryo) for reproduction and survival (to overcome the unfavorable weather
conditions like drought or very low or very high temperatures). Such simple land plants
were essentially Pteridophytes. They spread rapidly along the shores and river banks.
Without any visual competition for new habitats and with their simple genetic make up, a
very rapid evolution was stimulated and witnessed amongst these invaders of land. So fast
was the rate of evolution that within 100 million years or a little more, these simple plants
had evolved to such an extent and at such a rate that some of the major groups of vascular
plants like ferns, equisetums, lycopods, amongst the pteridophytes and cycads, maiden-hair
trees and conifers amongst gymnosperms had evolved. The maximum growth of such plants
was during the Carboniferous and Permian periods, when the forests of the world were
composed mainly of Pteridophytes- Lycopods, giant Equisetum and Gymnosperms (mainly
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Cycads).The fossil record suggest that early land plants (bryophytes and vascular plants)
established in terrestrial environments many times. How and when the necessary steps
involved in the transition for an algal life in water to that of land took place will continue to
intrigue and be an enigma clothed in mystery.
The main features of the Pteridiphytes (always follow this sequence) are:
i)
These plants have an independent gametophyte and an independent sporophyte.
(This is in sharp contrast to the bryophytes where the sporophyte is a parasite on the
gametophyte; and the Gymnosperms and Angiosperms where the gametophyte is a
parasite on the sporophyte).
ii)
The dominant phase in the life-cycle is the sporophyte.
iii)
This was the first group of vascular plants to invade the land.
iv)
This was the first group to have a vascular system (containing xylem and phloem).
v)
The xylem is mainly composed of tracheids. True fibers and vessels absent.
vi)
Secondary growth is absent in the living pteridophytes but was present in the extinct
forms.
vii)
They do not produce seeds but instead have spores.
Pteridophytes have had a long history on the earth. They probably had their maximum
development during the carboniferous and started dwindling in numbers and luxuriance
thereafter, till the present times when other than the ferns, only seven living genera are now
available. These are: Psilotum, Tmesipteris, Equisetum, Lycopodium (in the conservative
sense), Phylloglossum, Selaginella and Isoetes). The rest are extinct and represented by
fossils.
Fossils are the preserved remains of organisms that were once living on the earth but are
now extinct.
The study of plant fossils is known as Paleo botany. Fossils are present in different strata of
the earth and their age is calculated by either counting the number of strata or through other
means.
Formation of fossils: Organic substances rot and decay due to the action of microorganisms. If this activity can be prevented the organism can be preserved. Fossils may be
formed by any of the following methods:
i)
Impressions: When an organic substance comes in between soil layers, it decays
there but leaves behind its impression. Nothing of the original tissue of the organism
is left behind.
ii)
Casts or molds: Any hollow portion may get filled up by organic substances and
later soil gets in there. The organic outer portion decays and perishes but an
impression or cast of it is left behind.
iii)
Compressions or Mummifications: Organic substances get carbonized, become
dull and slowly turn to coal (under anaerobic conditions), or at times they may get
carbonized, become dull and get mummified (distortions in size etc. may occur and
correct results may not be obtained through such fossils).
iv)
Incrustations: (In all the above cases nothing of the original tissue of the organism
is left behind). Minerals which happen to be present in a super saturation condition,
in lakes, ponds, springs etc., get deposited layer by layer on any organic substance
which happens to fall in such water, thus preserving it as an incrustation.
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v)
Petrifactions: If the minerals are not present in a super saturated condition ( in the
water of lakes, springs etc.) then the minerals are absorbed and all molecules of the
mineral replace the organic matter molecule by molecule and ultimately all the
organic substance is replaced by minerals resulting in its preservation becoming a
solid rock-like structure.
Such fossils are the most important as they preserve all the original tissue since
decay was prevented right from the beginning.
An alternate method of petrifaction has also been suggested. Accordingly, when any
organic substance gets enclosed by any chemical, then the organic matter starts
decaying. During this process of decay, it gives off certain gasses or other
substances which precipitate on the tissue of the organic matter and thus preserve it.
When such fossils are treated with any weak mineral acid (4% hydrochloric acid),
only the inorganic deposition absorbed by the tissue dissolves away, while the
organic tissue remains as it had been preserved, by the formation of the precipitate,
which thus appears to protect the organic matter.
vi)
Amber formation or Preservation: Seeds, pollen grains, spores etc. get stuck or
engulfed in nature, in the resin exuding out of trees and thus get preserved there. The
resin or amber solidifies, becomes hard and transparent with the organisms
imbedded therein.
Study of Fossils: Petrifactions:
i)
Thin section method: By cutting thin sections
ii)
Peel Method: The surface is cut and then polished to make it smooth. The fossil is
then dipped in 4% hydrochloric acid or any appropriate acid. Minerals dissolve
leaving the organic tissue. Cellulose acetate or any such suitable chemical is poured
on the tissue and peels prepared.
iii)
Maceration: (Mainly for pollen grains, spores, microfossils etc.) Before study,
carbonized parts are oxidized by treating with potassium chlorate and nitric acid or
any other suitable chemicals.
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ERA
PERIOD
EPOCH
QUATERNARY
Recent
Pleistocene
TERTIARY
Pliocene
Miocene
Oligocene
Eocene
Paleocene
Upper (Late)
Lowe Early
CENOZOIC
CRETACEOUS
MESOZOIC
ESTIMATED AGE IN
MILLIONS OF YEARS
1
10
40
60
63
125
JURASSIC
Upper (Early)
Middle
Lower (Late)
Upper (Early)
TRIASSIC
150
Middle
Lower (Late)
PALEOZOIC
PERMIAN
PERMIAN
Upper
Lower
CARBONIFEROUS
CARBONIFEROUS
Upper
Lower
180
255
DEVONIAN
Upper
Middle
Lower
315
SILURIAN
Upper
Middle
Lower
350
ORDOVICIAN
Upper
Middle
Lower
430
CAMBRIAN
Upper
Middle
Lower
510
PRE-CAMBRIAN
3000/4500
Fig.1. The Geological Periods of the Earth.
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THE STELE OR THE VASCULAR SYSTEM OF THE PTERIDOPHYTES
The vascular tissue of the axes is known as the stele. It mainly comprises the xylem and
phloem. The various types of steles found in Pteridophytes are:
Protostele: This is the simplest and the most primitive type of stele. There is a solid core of
xylem surrounded by phloem followed by the pericycle and the endodermis. Pith is absent.
If the xylem is circular it is a Haplostele. In Actinostele xylem is star-shaped with radiating
protuberances or ridges), in Plectostele- the phloem is interspersed in masses between plates
of xylem.
Siphonostele: Here in the centre there is a parenchymatous tissue-the pith, which is
surrounded by the xylem, phloem, pericycle and endodermis.
Solenostele (Amphiphloic siphonostele): It differs from the siphonostele in the xylem
being surrounded on both sides by phloem, pericycle and endodermis.
Polycyclic solenostele: A stele within a stele.
Dictyostele: Overlapping of the leaf gaps results in the stele (siphonostele or solenostele)
breaking up into smaller steles known as the meristeles.
Polycyclic dictyostele: The outer ring is a dictyostele and within there may be either one or
more rings of another type of stele.
(The steles can be compared to the various types of poles used by engineers. The protostele
is comparable to the solid poles used in ancient times; siphonostele is comparable to the
hollow poles; while the dictyostele is similar to the perforated poles).
CLASSIFICATION:
In ancient times when medical virtues were the main basis for plant classification,
Pteridophytes received little or no attention. It was only as late as 16th century that an
approach to the type of classification as we know understand, was made. This later
developed into taxonomic botany. In 1583 Andrea Cesalpino proposed the first plant
classification based on the vegetative and reproductive features. Plants with flowers and
seeds were segregated from those without them which later became the cryptogams. In this
group Ceasalpino included ferns, horsetails, lycopods, mosses, algae and fungi.
The present scientific system of classification of plants began with the publication in 1753
of “Species Plantarum” and in 1767 of “Systema Naturae” by Linnaeus (the author of the
binomial system of nomenclature, wherein it was stated that every organism has a generic
name and a species name). Linnaeus based his classification on the knowledge acquired and
assembled by botanists for over around a hundred years preceding him. Linnaeus
recognized 24 major categories of plants. Twenty three of these, often called Phanerogams
(Greek phaneros = apparent, gammos= marriage; with visible flowers and sexual
reproduction) included vascular plants, and the last Cryptogamia (Greek kryptos=hidden;
gammos =marriage) was considered by Linnaeus as plants having “hidden flowers” not
visible to the naked eye. The Cryptogamia of Linnaeus included ferns and fern-like plants,
mosses, liverworts, algae and fungi. This group has now been divided into a number of
separate categories and their sexual reproduction is no longer considered hidden, nor do
they have hidden gametangia as earlier believed. Now a days plants that do not seeds seeds
are known as Cryptogams, while the seed bearing vascular plants are Phanerogams.
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There are few groups of plants, perhaps none, which have undergone so many changes in
taxonomy and nomenclature as the Pteridophytes. Numerous schemes have been proposed
in the past. With more and more information coming in from study of living and non-living
members (fossils), changes in systems of classification have occurred. The classification on
which this chapter is based is basically the system proposed by Reimers (1954) in Engler’s
“Syllabus der Pflanzen Familien” and Sporne (1982), with modifications.
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Five major groups of Pteridophytes are recognized:
•
Group I: RHYNIOPSIDA- Sporangia terminal or lateral. All fossils
•
Group II: PSILOTOPSIDA- Sporangia laterally fused to form synangia. Psilotum
and Tmesipteris. are the two living genera
•
Group III: SPHENOPSIDA: Sporangia usually borne in a reflexed manner on
sporangiophores.. All fossils except a single living genus Equisetum.
•
Group IV: LYCOPSIDA: Sporangia associated with leaves. Contain four living
genera Lycopodium, Phylloglossum, Isoetes and Selaginella; rest fossilized.
•
Group V: FILICOPSIDA: Leaves megaphyllous (macrophyllous) i.e. leaves with
branching veins The leaf trace leaving a corresponding leaf-gap in the stem stele in
contrast to microphyllous leaves (in the other four groups above) which have a
single unbranched vein; the leaf trace also does not leave behind a leaf-gap in the
stem stele. Microphyllous leaves are usually of small size except in Isoetes and
certain fossils. All ferns are included here.
RHYNIOPSIDA
This group contains the most primitive vascular plants having a horizontal rhizome with
unicellular rhizoids. From the rhizome arose upright axes with simple dichotomizing or
more complex branching patterns, having a centrach xylem strand and terminating in
sporangia. These plants were without roots and leaves but had all the necessary adaptations
for survival on land. Includes three orders:
(i)
Rhyniales (Rhynia, Cooksonia).
(ii)
Trimerophytales (Trimerophyton, Psilophyton).
(iii)
Zosterophyllales (Zoosterophyllum).
(In an earlier classifications, all the early vascular plants were put under a single OrderPsilophytales which had five families: Rhyniaceae ( with 2 genera Rhynia &
Horneophyton), Pseudosporchonaceae (Pseudosporchnus- now shifted to the Pteropsida),
Drepanophycaceae
(Drephanophycus)
and
Asteroxylaceae
(Asteroxylon),
Baragwanathiaceae (Baragwanathia)- all these three families are now considered in the
Lycopsida
Rhyniales: Two families Rhyniaceae and Cooksoniaceae had been recognized earlier but
here only a single family Rhyniaceae with 11 genera is recognized. Some of the genera
included are: Rhynia, Uskiella, Cooksonia (the oldest vascular land plant known) and
Yarravia.
Research is like fashion. It catches on fast and spreads to all parts of the world. Immediately
after the publication in 1857 of Darwin’s, “The Origin of a Species” studies on fossils took
off all around the world in a big way. In 1859, Dawson described Psilophyton princeps, a
fossil from the early Devonian of Canada. It was rootless and leafless. Aerial stems about
2ft. (60 cm) tall, profusely dichotomously branched, spine-like structures (enations) in the
lower unbranched regions, terminal ends circinate; sporangia pear-shaped borne in pairs on
the curved ends of branches.
No note of this publication was taken and it was dismissed as “a figment of imagination”.
So simple a thing could never have existed. Almost half a century later Kidston & Lang
published a number of papers (1917-1921) on their studies on the fossils (belonging to the
lower Devonian) from Rhynie Chert of Scotland. They discovered a number of similar
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fossils like Rhynia major, R.. gwynne-vaughanii, Horneophyton ligieri and Asteroxylon
mackiei.
Rhynia: Kidston & Lang (1917) confused two anatomically similar plants but later
segregated them as two different species. These were R..major and R..gwynne-vaughanii
Kidston & Lang 1920,1921).Using a multifaceted approach, new material and modern
techniques Dianne Edwards and collaborators, re-examined the early vascular plants and
concluded that R..major and R.gwynne-vaughanii are very different both morphologically
and anatomically, it is therefore preferable to recognize them differently and in different
genera. While R. gwynne-vaughanii was retained as the only species of the genus Rhynia, a
new genus Aglaophyton was described to accommodate R.major as A.major. It was further
emphasized that its conducting cells are not tracheids and do not have the thickenings
similar to those of other vascular plants.
R. gwynne-vaughanii: A small plant; rhizome prostrate with rhizoids; aerial axes arose
from the prostrate axes, about 20cm tall, about 2mm in diameter, tapering distally,
dichotomously forked (angle of dichotomy small, 20-30degree), profusely branched, both
prostrate and upright axes with small hemispherical lumps or bulges scattered over the
surface, lateral (adventitious) branches present.. Adventitous branches probably helped in
vegetative propagation. The stele of the adventitious branches was not in continuity with the
stem stele. Sporangia terminal, fusiform, homosporous, usually overtopped by adventitious
branches, 3-6mm long and 2.4mm wide, wall thick, without stomata, sporangia abscise after
spore release. Spores about 40um in diameter.
Anatomy: A transverse section of stem shows a large cortex divisible into an outer cortex
and an inner cortex with large proportion of air spaces and with photosynthetic function in
its outermost cells (since the plant was with out leaves). Fungal hyphae present in the
cortex, possibly entered after the death of the plant resulting in its disintegration. A
hypodermis and an epidermis surround the cortex towards the outside. A few stomata and
cuticle present. Protostele consisting of a slender xylem strand with broad annular or rarely
spirally thickened tracheids and surrounded by phloem.
It may be mentioned that Kidston & Lang had described R. gwynne-vaughanii with a few
dichotomous branches and made no mention of the adventitious branches overtopping the
sporangia as now described and amended by (Edwards (1980). Banks (1992) makes no
mention of the prostrate rhizomatous portion.
Aglaophyton major: Edwards (1986) reasoned that R. major was non-vascular in nature.
Kidston & Lang had initially described it as: Plants rootless and leafless, with an extensive
horizontal rhizome, rhizoids in patches, (not all over), for reasons unknown some branches
of the dichotomously branched rhizome grew upwards into aerial axes, about 60cm tall,
dia.6mm, naked, sparingly dichotomously branched or even unbranched. Sporangia
terminal, homosporous, slightly more in diameter than the branch on which borne, up to
12mm long, 4mm broad; sporangial wall thick, differentiated in to three layers. Spores
tetrahedral trilete about 65um dia. The internal structure of the stem was similar to that of R.
gwynne-vaughanii except in the structure of the conducting cells.
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R. gwynne-vaughanii differed from R. major as follows:
R. gwynne-vaughanii was a much smaller plant with hemispherical humps scattered all over
the axes. It had adventitious branches in addition to the normal dichotomies; sporangia and
spores were smaller. Edwards in 1986 described a new genus Aglaophyton to accommodate
R. major as Aglaophyton major and also emended its description as: Plants sporophytes,
with a semi-creeping decumbent habit, much shorter than originally described, formed
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extensive stands of decumbent axes. Rhizome probably surface living, unlikely to have been
long creeping as originally described. Maximum height of the plant 18cm or so,
dichotomously branched upright axes arise from rhizoid-bearing prostrate axes, upright axes
1.5-6.0mm in diameter, naked, dichotomizing at a wide angle (45-90 degrees) with all axes
terminating in sporangia, single sporangia rare. The reconstruction represents two fertile
units which resulted from axes growing vertically after they had fallen over, and which then
developed swollen areas and rhizoids on their lower surfaces, and eventually produced
sporangia. It seems likely that in many cases vertically growing axes fell over and produced
rhizoids rather than terminated in sporangia.
The most significant finding of Edwards concerns the structure of the central conducting
strand. It consists of three zones, a central region of thin-walled cells, a middle region of
thick walled cells, and an outer region of thin-walled cells. There is no indication of
differential wall thickenings in the cells of any of these regions and the central strand is
interpreted as a conducting strand similar to those found in many bryophytes, the inner two
regions being comparable to hydroids and the outer region comparable to the leptoids found
in many mosses.
The current status of Rhynia major:
Edwards laid much stress on the nature of the conducting strands and expressed that this
plant exhibits characters intermediate between vascular plants and some mosses and cannot
be considered a member of either group. The structures originally interpreted as tracheids in
the central vascular strand upon re-examination were found to be conducting tubes without
any thickenings associated with tracheids of vascular plants. Since unornamented
conducting tubes would be plesiomorphic to the moss lineage directly but does suggest a
close relationship. Also the main plant body of Aglaophyton is clearly a free living
sporophyte (sporangia are attached). Banks (1992) in a reclassification of the early vascular
plants placed Aglaophyton major (along with nine other genera) in the category of
“Aberrant Plants” as they do not fit into the strict definition of Rhyniales which includes
mostly dichotomously branching plants with single, terminal sporangia and centrach
vascular strands the tracheids of which were with thickenings. Sharma & Tripathi (2000)
after a study of the structure of the sporangium and spores of A.(R.) major are of the opinion
that these structures are more like those of the other Pteridophytes than in any Bryophyte.
Moreover would it be fair and justified to remove this plant only on the basis of a single
character (non-vascular tracheids) from the Pteridophytes and ignore all the other characters
that it shares with this group?
The Gametophyte Generation: Generally there is absence of evidence of fossil records of
the gametophytes of the early vascular plants. The gametophyte, it was believed was too
delicate to have been preserved during the fossilization process. In 1968 Lemoigne
suggested that the hemispherical bulges on the surface of R. gwynne-vaughanii were
archegonia. The rhizomatous portion represents the gametophyte, while the aerial erect
portions represent the sporophyte. A similar situation exists in the Bryophytes. However in
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Bryophytes there is a “foot” in the transitional zone between the gametophyte and
sporophyte. Such a structure is absent in R. gwynne-vaughanii. Earlier Pant (1962) had
suggested that the small-sized R. gwynne-vaughanii (at that time specimens of this species
with sporangia were not known) represents the gametophyte of the bigger R..major. This
supported the “Homologus Theory” of alternation of generations. However Pant’s
hypothesis has been abandoned since specimens of R. gwynne-vaughanii with sporangia are
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known. Taylor & Taylor (1993) discovered some well preserved fossils of gametophytes
from the Rhynie Chert deposits. A fossil described as Lyonophyton rhyniensis by Remy &
Remy has a small axis that terminates in a shallow, bowl-shaped structure (approximately
5mm in dia.), antheridia are distributed over the entire upper surface and many contain
preserved coiled sperms, stomata are present on the lower surface of the gametophyte. In
the centre are dark, elongate cells interpreted as conducting strand but the nature of the
tubes (tracheids or not) is questionable. Features of the epidermal cells and conducting
elements suggest affinities between Lynophyton rhyniensis and Aglaophyton major. There
are many more such fossil gametophytes that are morphologically similar to one another
and also appear similar to the gametophytes of Marchantiales that produce upright
antheridophores and archegoniophores from a prostrate thallus. However the gametophytes
of the Rhynie Chert have so far not been found to be attached to a thallus-like structure. If
A. major really has two free living generations as visualized above, it would be an important
solution of the life cycle of these ancient vascular plants.
Trimerophytales:
Plants have centrach xylem and masses of terminal sporangia that dehisce longitudinally.
The axes are profusely branched, pseudo-monopodially on the main axis and dichotomously
on the laterals. The well known Psilophyton princeps belongs to this order. But the fossil
described under this name by Dawson became a dumping ground for several similar plants
that basically had a proximal spiny axis with a distal fertile zone of lateral sporangia.
Edwards (1989) reconsidered this entire group and showed that it was a conglomerate of
many species and really encompassed a number of taxa and is no longer valid in its original
sense. Ten genera are included in this order.
Zosterophyllales:
The order is further divided into two families- Zosterophyllaceae and Gosslingiaceae. The
Zosterophyllales are with lateral sporangia having a distal dehiscence and exarch vascular
strands.
Fourteen genera are included in this order.
PSILOTOPSIDA
Two living genera, Psilotum and Tmesipteris of primitive Pteridophytes are included here.
Psilotum: (Greek psilos- bare or naked). Psilotum is a cosmopolitan genus. Two species P.
nudum and P. flaccidum are generally recognized. The ‘whisk fern’ (P.nudum) (Fig. 5) is a
cosmopolitan species of the tropics but can also be found in the warm temperate regions. In
India it is found in the warm central, south, west and east India. In the Himalaya it has been
reported from many places. It grows either epiphytically or on the ground or in rock
crevices. The other species, P. flaccidum with pendant flat, aerial branches, usually grows
as an epiphyte. It is with a restricted distribution. This rare plant is found in Jamaica,
Mexico and a few islands in the Pacific.
14
Fig. 5 : Psilotum nudum
General features: Rhizome dichotomously branched bearing rhizoids. No roots. Gemmae
often present. Aerial axes about 20cm (up to 1m) tall, regularly dichotomously branched,
green, bearing scaly microphyllous leaves. The vascular strand to the leaves stops at the
base of the leaves. Higher up on the plant, the leaves are replaced by fertile appendages,
which form a loose strobilus. The nature of the fertile appendage has been the subject of a
lot of discussion and controversy. It has been termed as a fertile appendage or as a
sporophyll, or as a strobilus, or as a very short lateral branch. The most favoured view and
the one accepted here, considers it as a very short lateral branch with a short stalk with two
leaf-like structures traversed centrally throughout by a vein which is connected with the
stele of the stem or the main axis, The trace extends into the base of the sporangial partition
and divides into three branches. In the center three laterally fused sporangia ( resulting in a
triloccular sporangium) are present Some consider the three lobed sporangium to represent
a single partitioned sporangium while others consider it to be three single sporangia which
are laterally fused. This latter view is accepted here. The development of the sporangium is
of the eusporangiate type (i.e. the entire sporangium develops from a group of sporangial
initials). The superficial cells divide by periclinal divisions into an inner and outer cell. In
subsequent development, the sporogenous tissue arises from the inner products of the initial
periclinal divisions, and most of the sporangial wall arises from the outer. The wall of an
eusporangiate sporangium is always more than one layer of cells in thick.
15
Stem: The erect aerial stems are almost circular (as seen in a transverse section) as they
emerge from the soil but change in shape towards the distal region, are often pentagonal
towards the first dichotomy and in the most distal portions become triangular. Epidermis
heavily cutinized but interrupted here and there with depressed stomata and guard cells,
Cortex massive, differentiated into three zones, outer 1-3 layered, photosynthetic (since the
leaves are scale-like and do not perform their photosynthetic function the green stem takes
over this activity); middle sclerenchymatous, 1-4 layered; and inner composed of thin many
layered parenchymatous cells; endodermis with casparian thickenings follows. In the center
is a sclerenchymatous pith; stele is an actinostele with prominent xylem rays (appearing like
a star); thin walled cells not clearly separable into phloem and pericycle surround the
xylem. In the transitional region at the base of the aerial axes the xylem increases in
amount, becomes medullated splitting into a variable number of separate strands. The stele
also changes at different levels and in different individuals. It may be triarch or tetrarch at
soil level; of much greater diameter and pentarch to octarch approaching the first
dichotomy; and tetrarch, triarch, or diarch in the most distal portions. The stele has been
interpreted as a protostele (actinostele) by some and as a siphonostele by others. If the pith
is considered as composed of sclerenchymatous cells then the stele is a siphonostele, but if
these sclerenchymatous cells belong to the xylem then it is a protostele.
16
Rhizome: Varies according to the size; thin portions (less than 1mm diameter are composed
entirely of parenchymatous cells); cortex undifferentiated, contain mycorhizal fungi, stele is
a protostele, xylem is surrounded by phloem followed by the pericycle and the endodermis.
Leaves: The thin scale-like leaves or appendages are spirally arranged, composed entirely of
photosynthetic tissues and covered by an epidermis. They contribute little to the nutrition of
the plant since they are without veins (vascular supply) and have no stomata.
Reproduction: The spores of Psilotum are slow to germinate. In nature gametophytes are
subterranean, more or less cylindrical, rarely more than 2mm in diameter, irregularly
branched or forked, and studded with numerous rhizoids. They lack chlorophyll and are
saprophytic. In a transverse section there is an epidermis, 3-4 layers of sub epidermis, rich
in starch, central tissue packed with mycorhizal fungus.
Beyond this there is little internal differentiation. Occasionally tracheids may be present in
the center. In form they resemble the rhizome and are very difficult to distinguish from
young rhizomes. They thus support the “Homologous Theory” of alternation of generations
according to which sporophyte and gametophyte are merely different manifestations of a
single plant body. Gametophytes are bisexual; antheridia are large and slightly emergent.
The jacket of the antheridium is composed of 10-12 cells in a single layer; at the apex is an
opercular cell. The jacket encloses multicilliate sperms. Archegonia have a small neck
composed of 4 rows of neck cells with 5 cells in each row, probably two neck canal cells, a
venter canal cell and an egg. At maturity all cells except the egg break down forming a fluid
which oozes out of the opening of the archegonia. This fluid not only helps to attract the
sperms but also acts as a filter to prevent sperms of other taxa coming in and fertilizing the
egg. The sperms swim to the archegonia and fertilize the egg.
Embryogenesis: In the fertilized egg or the zygote the first wall is transverse to the long
axis of the archegonia, forming a hypobasal and an epibasal cell. The hypobasal cell
through further divisions gives rise to a foot. The cells invade the prothallus tissue with
haustorial processes as in Anthoceros. The epibasal cell, divides by a vertical wall, followed
by a transverse wall, resulting in the formation of a quadrant. By repeated divisions a
cylindrical structure, the embryonic rhizome, is formed. No root primordia are present. The
rhizome develops rhizoids and becomes infected with a fungus as it emerges from the
gametophyte. Some of the branche tips become negatively geotropic and produce erect
aerial axes. Around this time the embryonic stem usually separates from the foot (which
remains within the gametophyte).
Cytology: P. nudum has two cytological races. A diploid with a chromosome number n=52;
and a tetraploid with n=104. (In certain gametophytes of the tetraploid tracheids are
present).
Summary of the features of Psilotum:
(i)
Rootless
(ii)
Aerial axes and rhizome dichotomously branched.
(iii)
Nutrition is partly saprophytic and partly subterranean
(iv)
A loose strobilus
(v)
Morphological nature of fertile appendages debatable.
(vi)
Gametophyte is subterranean, partly saprophytic.
(vii)
Development of embryo of the primitive type.
17
Tmesipteris: This genus is restricted in its distribution and 15 species extending from the
Philippines south to Australia, New Zealand and New Caledonia and eastwards to Samoa,
Fiji, Society Island, Marquesas Island and Vanuatu; T. elongata from New Zealand and
Australia; T. norfolkensis from Norfolk Islands; T. oblanceolata, T. oblanceolata subsp.
linearifolia and T.viellardi New Caledonia; T. oblangifolia from Vanuatu, Marquesas Island
and Philippines Islands; T. lanceolata, T. obliqua, T. ovata, T. parva, and T. truncata all
from Australia; T. sigmatifolia from New Zealand and New Caledonia; T. solomonensis
18
from British Solomon Islands; T. tannensis from New Zealand, Australia, Tasmania and the
Polynesian islands and T. vanuatensis from Vanuatu. These plants grow as terrestrials or as
erect or pendulous epiphytes.
Rhizome rootless, leafless, dichotomously branched, bearing rhizoids; some branches form
aerial shoots, which are simple or undergo one or several dichotomies depending upon the
species; near the base the aerial axes bear minute scale-like leaves, that are very similar to
the leaves of Psilotum, higher up and up to apex leaves simple, 1-2cm long, broadly
lanceolate, flattened in a vertical plane, with a mucronate tip, supplied throughout by an
unbranched vein which is connected to the stele of the stem by a trace, the vein stops short
of the mucronate leaf tip and does not enter it, leaf base is strongly decurrent resulting in an
angular stem (in a transverse section), bilaterally symmetrical (instead of dorsiventral).
Aerial branches near the tip are fertile, here the leaves bear sporangia forming a loose
strobilus with a short stalk, two leaf-like structures and in the center is present a biloccular
sporangia, which is believed to be the result of two sporangia having fused laterally to form
a synangium. The synangium has a many layered wall and two locculi with spores inside. In
abnormal cases the sporangial stalk may be two or three times dichotomously divided
bearing a sporangium at each bifurcation. There may thus be two or three biloccular
sporangia borne occasionally on short stalks. The vascular strand supplying the fertile
appendages branches into three, one to each of the leaves and one to the sporangial region.
This latter branch again branches into three in the septum between the two sporangia. The
development of the sporangium, embryogenesis and the anatomy of the rhizome is as in
Psilotum. In the transition region the xylem splits into a variable number of strands which
are mesarch (in Psilotum they are exarch. In the aerial stem the endodermis is not well
marked.
The outer cortex contains chlorophyll; epidermis is without stomata and is with a thick
cuticle. Stomata are restricted to the leaves and the decurrent leaf bases where they are
fairly abundant. Leaves are also heavily cutinized. Chromosome number n= 102-105; 204210.
Affinities of the Psilotopsida: The members of the Psilotopsida are characterized by a
marked primitiveness and have many peculiar features which make them a distinct group.
Many botanists believe that Psilotum with a very simple morphology, nearly dichotomous
branching, a protostele, lack of leaves and simple sporangia, is a survivor of the very
primitive lineage of plants like those of the Rhynie Chert. However these early vascular
plants became extinct in the Devonian while members of the Psilotopsida are known only in
the present age. The structures of the sporangium in the two groups are also very different
and are borne in a completely different position. Further while the Psilopsida left us with
well preserved records the members of the Psilotopsida have shown no aptitude for
fossilization. Results of recent molecular analyses suggest strongly that Psilotum is related
with the most primitive ferns. However this conclusion is still debatable on morphological
grounds. If Psilotum is related most closely to ferns, several interesting questions emerge
and have often been asked: Are their morphological homologies that are possible between
Psilotum and the supposed related fern taxa? What developmental genetic changes
occurred that led to the distinct morphological changes? If the relationships suggested by
the morphological phylogenies are believed to be correct, what could be the morphological
consequences of adding leaves to the simple axes of early Devonian plants? Psilotum may
offer a tool for such thinking. However they are not so significant phylogenetically as to
warrant their inclusion amongst the ferns. According to many Pteridologists, the only
conclusion that one can draw is that Psilotum and Tmesipteris are isolated survivors of an
ancient conservative group, of whose paleontological history nothing is known.
19
SPHENOPSIDA
An assemblage of trees or herbaceous plants, stems ribbed, leaves whorled; homo- or
heterosporus.
Classification: Four orders are recognized:
1. Hyeniales- All Fossils
Two Families:
(i)
Protohyeniaceae
(ii)
Hyeniaceae
2. Sphenophyllales- All Fossils
Two Families:
(i)
Sphenophyllaceae
(ii)
Cheirostrobaceae
3. Calamitales- All Fossils
Two Families:
(i)
Asterocalamitaceae
(ii)
Calamitaceae
4. Equisetales
Family Equisetaceae- Fossils, but currently represented by the sole living genus
Equisetum.
Hyeniales: They lack the general features typical of the Sphenopsida, and are considered to
be early representatives (Mid-Devonian) of the group. Their taxonomical position is
uncertain but characters place them close to the Calamites and Equisetum and yet suggest
the Rhyniopsida.
Protohyenia: The oldest known member of the order and probably contemporary with
Rhyniopsida and early members of the Lycopsida. It is suspected that these remains are
really portions of Pseudusporchnos a member of Pteropsida. Another fossil Calamophyton
which was earlier included in this order has been shifted to the Pteropsida (Cladoxylales) on
the basis of a similar internal anatomy.
Thus of the three genera that were earlier believed to be members of Hyeniales, only Hyenia
remains. Three species are known: H. elegans, H. sphenophylloides and H. vogtii.
H. elegans : Middle Devonian; rhizome thick, horizontal; plants up to 30cm tall, sterile
stem bore whorls of forking appendages which were forked two or more times, leaf-like,
alternating in successive nodes. It is difficult to decide whether these should be regarded as
leaves or branches but they functioned as leaves. Fertile axis forked once, bore whorls of
sporangiophores, which were similar to leaves except that the two segments were reflexed
and usually terminated into 2 or 3 sporangia each. Stem of Hyenia was not jointed. In the
other species, H. vogti aerial axes were branched and clothed with whorled leaf-like forked
appendages. Some later studies suggest that Hyenia should also be transferred to the
Pteropsida as in some new specimens there is no evidence of whorled lateral appendages.
These were spirally arranged and due to crowding, resulted in apparent verticels.
20
Sphenophyllales: Members were not a dominant group as they were not more than a meter
high. They were herbaceous, low growing plants even creepers, and occurred from upper
Devonian through early
Triassic. While remains of stems and leaves are referred to the genus Sphenophyllum,
remains of cones are referred to various genera like Sphenophyllostachys, Bowmanites,
Eviostachya etc.
Sphenophyllum: More than fifty species are known. Plants probably scandent, perennial,
had no physiognomy i.e. not dominant, weak, dependent for support; leaves in whorls at
nodes, 6-18, usually in multiples of three at each node. Stem slender, delicate, jointed,
longitudinally ribbed but ribs non-alternating, in spite of secondary growth not more than
1cm in diameter, irregularly branched, probably prostrate on ground. [the plant probably
looked like the present day Angiosperm, Galium sp., Rubiaceae]. Some consider it to have
been a lax climber while others consider it to have been an aquatic plant. Leaves borne
directly above, not alternating with those of adjoining nodes, showed a large range of
structure, leaves were narrow at the base and broad in the distal region, margin variable,
some being deeply cleft or slightly notched, others entire and deltate. Each leaf received a
single vascular bundle, which dichotomized very regularly within the lamina. Some species
were markedly heterophyllous, with the deeply cleft leaves near the base, while the entire
ones were borne higher up on lateral branches, an arrangement that suggests that the former
might represent juvenile leaves. Another suggestion was that Sphenophyllum was
submerged in the lower part. But the discovery of shoots with dissected leaves above simple
entire leaves, negates the second suggestion. The anatomy of the stem was very peculiar. In
the center was a triangular solid primary wood, with protoxylem at the three corners in an
exarch position. The protoxylem tended to break down to form the carinal canal as in S.
insigne, of the lower Carboniferous) but this rarely happened in the species from the upper
Carboniferous. Outside the primary wood, a vascular cambium gave rise to secondary
wood, first between the protoxylem, and then later extending all around. Larger stems had a
considerable thickness of cork on the outside, formed from a deep seated phellogen. Roots
resembled the stem in internal structure except that they were diarch.
Remains are found in the Artic zone, where no vegetation now grows, indicating
environmental conditions in past were entirely different from what they are now!
Cones: A small number of cones, referred to the genera Sphenophyllostachys or
Bowmanites have been found attached to the parent plant. Others found detached are also
placed in these genera on the basis of their similarity. A number of other genera of cones are
also referred to the Sphenophyllales. The cones show a considerable diversity in structure.
The spores in some species show a great difference in size, perhaps an incipient type of
heterospory; other species may have been homosporous.
Calamitales: The giant tree-like members flourished from the upper Devonian, reached
peak development in Carboniferous, declined in the Permian, and probably became extinct
in the Triassic. In the Carboniferous, along with the Lepidodendrales, Sigillaria and
members of the Cycadofilicales, they formed dominant forests of the world. Trees of
considerable height (20-30m tall, and 1meter in girth). Stem articulated; profuse lateral
branching as in Equisetum.
[It is strange that the present-day environmentalists blame man for the changes occurring
in nature. They claim that pollution from motor vehicles, factories, industry, refrigerators
etc. are responsible for the fast disappearing species of plants and animals. The green
house effect! But in ancient times there were no such things and yet these plants and
animals disappeared from the surface of the earth- lock stock and barrel! Why and how did
21
they go? Was it due to the fact that the earth at that time was warm and most of the animals
at that time were cold blooded and as the ice age dawned these got eliminated to be
replaced by those that were better suited and adapted to the low temperatures? The fauna
and flora of those times simply disappeared! Another reason for this could be that the
genetic-make up of these forms of life could not evolve further to suit the changing
environmental conditions and therefore they were eliminated and replaced by better forms
of life, and not because of pollution. as we now blame for the disappearance of some
present day species. Nature always desires change. Nothing is permanent on this earth
except the name of God (by whatever name you may call him) and change, for whatever
reasons]
Asterocalamites (Archaeocalamites) is the oldest member of the group with a strongly
ridged and grooved woody stem, the grooves continuing through successive nodes. Leaves
large, whorled at nodes and dichotomously forked many times. Along the slender branches
at intervals, were present the fertile regions. Sporangiophores peltate, superimposed, each
bearing 4 reflexed sporangia.
Calamites: A number of species are included. This giant “horse-tail” was tree-like in habit.
The aerial portion arose from a large rhizome and was up to 30m in height and 30cm in
diameter, trunks hollow in the center except in the region of the nodes where an entire tissue
is present. A siphonostele segregated into collateral bundles in a ring. The endarch
protoxylem disintegrates into a carinal canal (as in Equisetum). The structure of the cortex
resembles that of the present day Equisetum. Secondary growth was present, but no annual
rings are observed, indicating that the climate in which these trees grew was more or less
homogenous throughout the year. Petrified Calamitean roots, Asteromyelon, were
adventitious, probably arising from underground and aerial axes. Leaves unbranched,
whorled, 4-60, with a tendency to be cupped upwards. In most species they were free to the
base, but in a few they showed some degree of fusion into a sheath. Each leaf had a vascular
bundle. Cones were more elongated than in Equisetum and were terminal on branches. They
were variable in size as well as in the manner of attachment. In some species they occur
singly at nodes, in others form terminal groups, rarely occur on specialized branches. Some
species were heterosporous.
Equisetales:
Found in the Upper Devonian, best in Carboniferous from whence its decline started, until
in Triassic it was represented by a few genera. The order contains a single surviving family
Equisetaceae and a sole living genus Equisetum with 15 species of almost world-wide
distribution and confined to N. Temperate regions, though some are met with in Tropics
also, except Australia. From India four species are known: E. arvense, E. diffusum, E.
palustre and E. ramosissimum. Another species E. debile often reported from India, has
been merged in E. ramosissimum as it is supposed to be not much different. Another two
species, E. giganteum (an American species) and E. hymeale have been wrongly reported
and are not in India. Species of Equisetum are usually known as “horse tails” or “pipes” or
“scouring rushes”. Due to the deposition of silica in epidermal cells of the aerial stems they
are used to clean, polish and shine utensils.
Plants small to large, terrestrial, usually grow in wet or marshy places or in open, sunny
sand banks along rivers and margins of lakes; rhizome underground, branched, creeping,
extensive, bears roots and tubers (in some species) often deeply subterranean, divided into
nodes and internodes; scales present at nodes are fused to form a sheath. From nodes extra
axillary branches arise, usually annually to give rise to aerial shoots which are built on the
same pattern as the rhizome. These are characterized by a jointed appearance, ridges and
22
grooves are present which alternate in successive internodes; leaves in verticels, alternating
in successive internodes, their number same as ridges, small, whorled, scale-like, nonphotosynthetic (photosynthetic function taken over by stem), fused to form a nodal sheath.
Chromosome number n=108.
23
Stem anatomy: Outline variable, thrown into ridges and grooves, epidermal cells with heavy
incrustation of silica, stomata present in the region of grooves, below each ridge is present a
group of sclerenchyma cells, below grooves is present collenchyma. Stele of collateral
vascular bundles. Each bundle is present below a ridge, due to disintegration of protoxylem
in the vascular bundle a carinal canal is formed below the ridges. In the cortex, below each
ridge are present cavities or the vallecular canals. Thus the vallecular canals and the carinal
canals alternate. Distribution of endodermis is different in different species. In some only an
outer ring of endodermis is present a little beyond the vascular bundle (E. arvense). In
others there is an outer endodermis and an inner endodermis. In still others, each bundle is
surrounded by its own endodermis. Pith is hollow forming the central canal, but in the
region of node is present a diaphragm tissue.
Root: Di- or tetra arch. Growth of stem and root is by a single tetrahedral apical cell.
Cone or Strobilus: The relationship of the cone to the vegetative branches varies in various
species. In some species the fertile axis is produced in the beginning of the growing season.
It is unbranched and after having produced the spores it dies and the plant produces the
vegetative branches (as in E. arvense). In such cases there are distinct fertile (unbranched,
non-chlorophyllous) and sterile branches; in other cases the fertile unbranched nonchlorophyllous axis after having produced the cones turns chlorophyllous, vegetative and
branched (E. palustre). In yet other species the vegetative axis are produced first and
towards the end of the growing season the cones are borne at the tips of the branches (E.
hyemale).
Structure of the Cone: The central axis of the cone is divided into internodes and nodes, on
nodes are arranged in vertices or whorls a number of sporangiophores that alternate in
successive nodes. No bracts or leaves are present in between. The sporangiophore consists
of a central stalk and a broad, peltate, hexagonal disc-like structure (cap) which bears on its
under surface 5-10 pouch-like pendant sporangia. Each sporangiophore is supplied by a
vein which branches distally. When young the discs are tightly packed up but at maturity
the internodes elongate slightly and separate the sporangiophores.
The stalk also grows at a right angle so that the sporangia are approximately perpendicular
to the surface of the soil.
Development of Sporangia: Eusporangiate type. A single superficial cell (adjacent 5-10
cells may also add to the sporogenous tissue) on the peltate disc acts as sporangial initials.
The first division is periclinal. Further periclinal and anticlinal divisions in the outer cell
form a multi-layered jacket (wall) the inner cells of which break down to form the tapetum.
The inner cells by further divisions in all directions give rise to the sporogenous tissue
which develops into the spore-mother-cells. Some of these abort and add to the tapetum for
the nourishment of the developing spores. The spore-mother-cells undergo meiosis
(reduction division) to produce four spores. The sporangia dehisce longitudinally.
Gametophyte: Each spore has a three layered wall, intine, exine and a perispore (or perine)
which at maturity splits into four strap-shaped bands or elaters each with flattened ends
forming spoon-like structures. The elaters are free from the spore-wall except at a point and
are tightly coiled around the spore. Their coiling or uncoiling depends upon the humidity.
They help in the dispersal of the spore, acting like a parachute, and also help in finding a
suitable substratum for germinating due to their hygroscopic nature. The spores are green
(have chlorophyll) and therefore their viability is very low, not more than two weeks. They
24
germinate immediately. The gametophytes are circular in outline, 2-3cm in diameter, cakeshaped with a continuous meristem along the margin on the outside. Unicellular rhizoids
anchor the gametophyte to the soil.
25
Vertical section of gametophyte: A lower compact region of 2-6 layers of cells with no air
spaces is seen, full of starch. This is the storage region. Upper region is of upright,
numerous, branched lobes (lamellae), which are photosynthetic, 1-cell in thickness at base.
The gametophytes are unisexual or bisexual but protogynous. Sex organs appear at the base
26
of the lobes. A single superficial cell acts as the antheridial initial. It divides into an outer
and an inner cell. The outer cell forms the wall of the upper portion of the antheridium; the
inner by successive divisions forms the spermatogenous tissue. The antheridium is
embedded and its development is similar to eusporangiate-type. Sperms are relatively large,
multicilliate, with 2-3 coils. The archegonia are borne near the base of the lamellae and
develop from a superficial cell and are of the usual stereotype. Each archegonia has a short,
projecting, upright neck with four rows of cells and four cells in each row. In the center are
either one or two boot-shaped neck-canal-cells, formed by longitudinal divisions of a single
cell, a venter canal cell and an egg.
Embryogenesis: The fertilization of the egg results in the formation of the zygote and marks
the end of the gametophytic generation which began with the formation of the spores. The
first wall is transverse to the long axis of the archegonia followed by a vertical wall,
forming a quadrant. Of the two epibasal (towards the neck of the archegonia) cells, one
gives rise to the cotyledon and the other to the stem. The two hypobasal cells may form the
foot and the root develops later, or only one develops into the foot and the other the root.
The embryonic root grows through the gametophyte into the soil, thus establishing the
independence of the young sporophyte. The gametophyte may persist for some time after
sporophyte development.
LYCOPSIDA
Classification:
1. Order Protolepidodendrales (All fossils)
Contains three families:
(i)
(ii)
(iii)
Drepanophycaceae: Baragwanthia Drepanophycus
Asteroxylaceae: Asteroxylon.
Protolepidodendraceae; Protolepidodendron.
2. Order Lycopodiales
A single family Lycopodiaceae: Lycopodium (in the earlier conservative sense),
Phylloglossum.
3. Order Lepidodendrales: All fossils
Contains four families:
(i)
(ii)
(iii)
(iv)
Lepidodendraceae: Lepidodendron.
Bothrodendraceae: Bothrodendron.
Sigillariaceae: Sigillaria.
Pleuromeiaceae; Pleuromeia.
4. Isoetales:
Family Isoetaceae: Isoetes; Nathorstiana (fossil).
5. Selaginellales: Family Selaginellaceae: Selaginella; Selaginellites (Fossil)
(The Lycopsida has only four living genera: Lycopodium, Phylloglossum, Isoetes and
Selaginella. All others are fossils. Add to these the other three living genera Psilotum,
Tmesipteris and Equisetum, the Pteridophytes (other than members of the Pteropsida,
i.e. the ferns) are represented by only seven living genera).
Alternately, the Lycopsida can be divided into two sub groups :
A. Homosporae (Isosporae or eligulatae): includes orders nos. 1 and 2 above.
27
B. Heterosporae (ligulatae): includes the remaining orders.
Protoplepidodendrales: Drepanophycaceae.
Baragwanathia was once considered to be the oldest known vascular plant as the rocks in
which its remains were found were thought to be Silurian but are now believed to be Lower
Devonian. Ligules were absent. (The discovery of Cooksonia, late Silurian, has replaced
Baragwanathia as the earliest known vascular plant). Baragtwanathia was herbaceous, with
spirally arranged microphylls. Sporangia reniform, occurred in zones, probably
homosporous. Drepanophycus was very wide-spread, and is known to have survived
through middle Devonian into the Upper Devonian.
Asteroxylaceae:
Asteroxylon mackiei is one of the earliest known Lycopods from the Rhynie Chert with
microphyllous leaves; rhizome creeping, naked, developed repeatedly bi-furcating root-like
structures (these were not true roots as the root-cap was absent), could reach up to 20cm
deep; aerial axes up to 40cm high, forked with one branch of the dichotomy not developing
as much as the other branch resulting in the formation of lateral branches, both branches
bore leaves, up to 5mm long, spirally arranged; sporangia scattered in the axils of leaves,
shortly stalked, kidney-shaped. Stele is a star-shaped protostele (actinostele). In earlier
descriptions a naked dichotomously forked fertile axes with terminal pear-shaped sporangia
has always been found in close association but never in organic connection with the
vegetative portion. These were believed to be the fertile axis of Asteroxylon. Due to the
terminal sporangia Asteroxylon was placed along with earlier members in the Psilophytales
based on Psilophyton princeps. Later after sporangia were found associated with the leaves
and so it was shifted to the Lycopodiales.
Protolepidodendraceae:
Protolepidodendron herbaceous, rhizomes densely clothed with spirally arranged leaves,
aerial axes erect, dichotomously forked, protostele; leaves forked; sporangia ovoid, borne
on the adaxial surface.
Lycopodiales: Lycopodiaceae:
The subdivision of the family has been a matter of considerable disagreement. Some
recognize two families of the living members others subdivide the genus Lycopodium into
12 sections ascribed to three subgenera, while still others recognize 11 of the subsections as
genera. Many adopt a little conservative approach and recognize 5 or 7 genera. However
four genera (Phylloglossum, Huperzia, Lycopodiella and Lycopodium) are generally
recognized. Here only two genera Lycopodium and Phylloglossum are recognised. Fossils
of Lycopodiaceae are poorly preserved and cannot be referred with certainty to the living
members. Species of the genus Lycopodium (and Selaginella) have been known to man
from very early times and are generally referred as “Club Mosses”.
Lycopodium: Members of the genus are chiefly confined to the Tropical and Subtropical
regions. They are usually met with in humid and moist forests, though stray species may be
found in temperate zones or even at high altitudes. From India 17-27 species have been
recorded. In India there are two main centers of distribution, East India and South India.
Some species are also found in central and western
India. In the west Himalaya, L.selago has been reported as far west as Pakistan (Swat) and
Kashmir in India. Seven species are known from Uttranchal (L.hamiltonii, L.petiolatum,
L.pulcherrimum, L.selago, L. cernuum, L.annotinum, and L japonicum). The genus
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possesses high chromosome numbers based on x=17. The gametic number varies between
n=78, 104, 108, 110, 136, 165 etc. Several natural hybrids are also known.
Various species have different habitats. Some are terrestrial, in which stem may be erect as
in L.cernuum, and the high altitude small-sized L.selago; or terrestrial, with an
indeterminate creeping or climbing main prostrate stem from which arise determinate erect
or ascending branches as in L. clavatum; or they may be epiphytic as in L.squarrosum and
L.phlegmaria with beautiful pendant branches. A small epiphytic species is L.setaceum.
(All these are Indian species).
Stem: dichotomously branched, (in primitive species), rarely with lateral branching. Leaves
simple, small, linear, somewhat flat, and oval, isophyllous or anisophyllous, homophyllous
or heterophyllous, microphyllous, spirally arranged, with a few exceptions in
L.complanatum and L. volubile where they are arranged in four rows. Each leaf is supplied
by a single vascular strand which may not reach the tip
Anatomy: Stem: The cortex in most species is sclerenchymatous, in others it may be
entirely parenchymatous, while in still others it may be differentiated into three zones, a
middle parenchymatous with an outer and inner sclerenchymatous. Endodermis is quite
clear. Pericycle may be one or many layered.
Stelar system: Species can be classified on the basis of the structure of the stele:
(i)
Lycopodium selago Type: Here xylem is scandent but continuous in center, solid
with no admixture of parenchyma. Number of arms is variable in different
species, even in different regions of the same individual. Arms are flattened or
extended at the periphery where protoxylem is exarch. Phloem lies in between
the arms of xylem.
(ii)
L. clavatum Type: Here the central cylinder contains several transversely placed
xylem plates alternating with phloem plates. It is a derivative of L.selago type by
the progressive intrusion of phloem masses so as to break down the continuity of
the xylem masses. Protoxylem is still exarch. Intermediate intrusions are met
with in other species of the genus.
(iii)
L. cernuum Type: Xylem and phloem are more uniformly distributed. The
phloem is distributed in patches amongst xylem. A condition prior to this is met
with in L.phlegmarium where there are large patches of phloem in between
xylem strands. The leaf trace is single and given off from the periphery of xylem
cylinder without causing any disturbance in the stellar cylinder (cladiosiphonic
leaf trace; in phyllosiphonic type the leaf trace leaves behind a corresponding
gap in the stem stele).
Root: the xylem is crescent-shaped in a cross section, or the stele is similar to the stem
stele. The xylem may be hexa to decarch. Xylem is continuous in the cortex.
Vegetative Reproduction: In some species it is through bulbils. These are simply condensed
branches, covered with thick fleshy leaves or arrested extremely unequal stem dichotomies.
Food material is stored in them. They fall down and give rise new plants.
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Fig. 11. The stele in various species of Lycopodium. A. L. selago Type; B. L. clavatum Type; C. L. cernum
Type. D. L. squarrosum Type.
Reproduction: Reproductive parts: In the primitive species the sporophylls are not different
from the sterile ones, and they do not form different cones. There is generally a loose
aggregation of sporophylls intermixed amongst sterile leaves, as in L. selago and L.
setaceum resulting in the formation of a loose strobilus. In other species the sporophylls
though not very different from sterile leaves are delimited at the apices of vegetative
branches to form primitive incipient cones and the sporophylls gradually pass downwards
into the sterile leaves (L.squarrosum). In other cases the sporophylls are highly different
from the sterile leaves forming distinct cones borne at the apices of the branches. The sterile
and fertile leaves have therefore different functions. Sterile are photosynthetic while fertile
ones are non-photosynthetic, but bear and protect the sporangia as in L.cernuum. In L.
clavatum the differentiationhas gone still further as cones are not borne directly on the
branches, but the cones are segregated from the shoots by intervening elongated branches
having scale-like leaves. These features show a gradual division of labour.
30
31
Structure of Sporangium: In the primitive species, sporangia are small in size but in species
like L.clavatum sporangia are massive. The sporangium is usually ‘kidney-shaped’ with a
short, thick stalk- the archaesporial pad. The wall of the sporangium is three or four layered,
innermost is tapetal. There is considerable variation in the manner in which the sporangia
are borne in relation to the sporophylls. In some the sporangia are in the angle between the
sporophylls and the cone axis i.e. it is axillary as in L.selago. In others the sporangium may
be borne on the adaxial surface of the sporophylls and may be described as ‘epiphyllous’ (L.
clavatum and L.cernuum).
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Gametophytic Generation: Two distinct types of gametophytes can be identified: (i)
Surface living gametophytes and (ii) Subterranean gametophyte. An intermediate type
between the two may also be present.
When spores germinate deep underground, prothalli are colourless, cylindrical and
branched. When prothalli grow near surface of soil they are compressed, flattened and discshaped, but when they grow on surface they are green and lobed.
33
Surface living Type: In some species the spores are green, thin–walled and germinate
without delay, on the surface of the ground. The gametophyte is green (except at base),
cylindrical or ovoid with a lobed or branching top, very small, 2 or 3mm long. The
gametophyte matures usually within one season and is short-lived.
Subterranean Type: In species with non-green spores germination may be delayed (from 3-5
or even 6-8 years). During this period the brown, thick-walled spores may be carried down
by the rain-water or insects, worms, or other small animals or they may get buried under
layers of humus. Gametophytes are non-green, underground, colorless, top-like or carrotlike or disc-like or tuberous, large, often 1or 2 cm long or wide. Gametophytes grow slowly
requiring several years (6-15 years) to mature, are long-lived, living even after maturity and
even nourishing young sporophyte for many years (eg. L. clavatum). After the spores
germinate and start dividing, the gametophytes may rest at the 5-celled stage for a year. Up
to this time there is no fungal association which occurs at this time for successful
development. L.selago shows variability. It produces a surface living gametophyte with a
photosynthetic upper region, and a lower fungal hypheal region. It may also produce a
completely subterranean gametophyte with fungal hyphae in the lower region but covered
all over with rhizoids. Archegonia and antheridia are restricted to upper parts of
gametophytes. In L. phlegmaria, the prothallus is very slender, branched, colourless and
underground. In L.cernuum the gametophyte has numerous photosynthetic lobes and is
partially photosynthetic. Sex organs: Prothalli are monoecious. Gametangial initials lie just
behind the apical meristem. In the underground types, antheridia and archegonia form
distinct patches and cover the entire crown or base of the lobes. In elongated types they are
present on a central cushion and are intermingled.
Development of antheridia is of eusporangiate type. They are embedded and produce a large
number of pear-shaped, biflagellate antherozoids. In archegonia only the venter is
embedded with the necks protruding as in other Pteridophytes. In some species the neck
may be long with 4-16 neck canal cells. In L.cernuum the neck is short with 2-3 neck canal
cells. The gametophytes of some epiphytic species multiply by fragmentation and
reproductive bodies of two types (minute types that rest before development and larger ones
that grow at once) are formed.
Embryogenesis: The first wall in the fertilized egg is transverse to the long axis of the
archegonium (endoscopic). The upper small cell forms the suspensor cell. The lower cell
divides by vertical walls resulting in two tiers of four cells each. The upper or hypobasal
cell gives rise to the foot or may remain small. Of the four lower cells, two form the stem
and the other two the first leaves. The foot becomes bulbous and is an absorptive organ,
getting its nourishment from the prothallus. The axis of the embryo bends so that the stem
apex points upwards. In L.selago the hypobasal region remains small. In L.cernuum, the
hypobasal region remains small and apparently does not serve as the absorptive organ. The
epibasal region through further divisions is converted into a massive tuberous structure-the
protocorm. It is provided with rhizoidal hairs and mycorhizal fungus and grows slowly. On
this protocorm develop leaf-like structures-the prophylls. At the summit of the protocorm
develops a stem apex from which a normal shoot develops. The protocorm stage may
persist for a long time.
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Phylloglossum drumondii: Restricted in distribution. Endemic to Australia and New
Zealand. A tiny inconspicuous plant about 5-6 cm high with an ovoid underground tuberous
portion (protocorm) that puts forth a whorl of 4-20 cylindrical, green leaves. At the end of
the growing season, a short stalk is formed from or near the apex of the plant which bears a
small young tuber for vegetative reproduction. At maturity robust plants produce an
elongated naked stem bearing a small terminal cone. All parts except the tuber die at the end
of the growing season. The structure of the cone is similar to that in L. cernuum. The
sporophylls are small in size and each possesses a single sporangium. Chromosome number
2n=502-510.
Lepidodendrales: Lepidodendraceae:
Lepidodendrales includes over 200 species. First appeared in the Lower Carboniferous and
reached their peak development in the Upper Carboniferous. They formed swamp forests in
which members of the Lepidodendraceae, Bothrodendraceae and Sigillariaceae were codominant with the Calamitales. They formed trees up to 40 meters or more tall, had stout
35
trunks, some with a crown of branches, others hardly branching at all, but all had a similar
type of underground portion, collectively known as Stigmarian axes. Members of the
Lepidodendraceae and Bothrodendraceae were similar in external appearance. However
members of Sigillariaceae were sparingly branched only once or twice at the apex or not
branched at all, but were amongst the largest trees. Pleuromeia (Pleuromiaceae) was a
much smaller plant from the Triassic rocks. It was similar to the present day Isoetes.
Isoetales: Isoteaceae:
The family contains a single genus Isoetes. In 1954 Stylites was discovered from the Andes
region of Peru at an altitude of 4750 meters round the margins of a lake. However S.
andicola (Isoetes andicola) has been shown by several workers to exhibit the end range of
variation shown by other South American species of Isoetes and has therefore been merged
in that genus but kept distinct as subgenus Stylites under Isoetes. The genus Isoetes
(Quillworts) is cosmopolitan in distribution with around 130 species (200-250 according to
some). A considerable amount of endemism is present due to ecological isolation. From
India 14 species are reported, although some would like to recognize only a single species,
I. coromandelina from India considering the rest to be merely variants of this species, a
very wrong conclusion! Plants are terrestrial or aquatic, either seasonal or evergreen
perennials, resembling a monocot, 5-50 cm tall, with a thick condensed corm-like stem that
is usually unbranched, but may usually show 1 or 2 dichotomies. Dichotomously branched
roots usually present. The crown of leaves is close together due to the slow growth of the
stem and all leaves are potential sporophylls. Sporangia are of two type’s micro sporangia
(that produce microspores) and mega sporangia (produce megaspores). [Isoetes like many
other Pteridophytes, is heterosporous, producing two types of spores that differ not only in
size (microspores are smaller; megaspores are larger) but also in their function
(microspores germinate to form a gametophyte which is generally endosporic i.e. the entire
gametophyte develops within the walls of the microspore except the rhizoids that protrude
outside and bear only antheridia; megaspores on the other hand germinate to produce
exosporic gametophytes that bear only the archegonia]. Chromosome number: Diploids,
tetraploids and hexaploids based on x=11 are known; a single species based on x=10 is also
known. The subgenus Stylites has a more massive, elongate, often dichotomously branched
stem with unbranched roots arising from a single lateral groove running the length of the
stem.
Some fossils recorded from the Triassic and later deposits (Isoetites) are attributable to this
family as they resemble Isoetes.
Selaginellales: Selaginellaceae:
Fossil records of this order are found as far back as the Lower Carboniferous. Selaginella is
the sole living genus of the family with about 750 species world-wide, but chiefly
distributed in the tropical and subtropical regions. Also occur in the temperate regions. They
generally grow in humid localities in shade like the forest-floors of rain forests. A few
species are xerophytic. Some are very small (like mosses) others are vine-like, with stem
growing to several meters.
The genus can be divided into two groups:
A. Homeophyllum – A small section with 50 species, leaves isophyllous, spirally
arranged. eg S.spinulosa which is monostelic; S. selaginoides has endarch stele and
a limited amount of secondary growth in the hypocotyl region. This is the only
record of secondary growth in the genus
36
B. Heterophyllum: symmetry dorsiventral, leaves anisophyllous, arranged in four rows,
two rows of small leaves attached to the upper sides and two larger lateral ones. In
the fertile region all leaves are isophyllous; cones 4-angled, monostelic or polystelic.
Some recognize three genera, Selaginella, Lycopodites and Didiclis. However these three
genera are not sharply delimited morphologically and in spite of the large number of species
only a single genus is recognized since Selaginella is a well characterized taxonomic unit, is
universally known, practical and a useful genus, not needing to be split.. From India around
62 species have been reported but some “new” species have been merged into the parent
species due to improper data and inadequate single locality collections. Around 15 species
are known from the Himalaya. Some of the common Himalayan species are: S. adunca, S.
bryopteris, S. caulescens, S. chrysocaulos, S. chrysorrhizos, S. involvens, S. monospora, S.
pallidisma etc. A xerophytic species, S. adunca, grows between altitudes of 500-1000m and
naturally perinates through an underground rhizome. S. bryopteris also inhabits xerophytic
dry localities. It is a resurrection plant. The branches role up inwards forming a ball-like
structure which drifts and is blown to distant places by wind. On the return of favourable
conditions or when moisture is available the branches simply spread outwards. This
property of the plant is also made use of by quacks who to prove the efficacy of their
“medicine” sprinkle a little bit of it on the plant which turns green due to the water in the
“medicine” to the delight of the gullible road-side onlookers who fall an easy prey to the sly
and crooked quacks.
S.caulescens: It is believed to be an iridescent plant. It has an ascending habit and the
branches spread like the feather of a peacock. Rhizophores are absent. Mainly from the
eastern Himalaya.
S. chrysocaulos: common between 500-200m altitudes. It has an interesting manner of
perenation and vegetative reproduction by forming surface buds. At the end of the growing
season, the branch ends show repeated dichotomy. A rhizophore is given off which bears
more roots that attach to the soil.. Leaves are green, closely compacted, relatively broader
but shorter than ordinary leaves. Upon return of favorable conditions these buds simply
grow out to form the shoots.
S. chrysorrhizos: common, but grows at slightly higher altitudes (above 2000m) in the west
Himalaya. It has characteristic underground tubers. Some branches from the lower region of
stem run underground and bear scale leaves, the stem swells at the apex forming an
underground tuber. Its internal structure is similar to that of the stem. The tuber possesses a
vascular cylinder and storage tissue. Externally covered by scale-leaves.
S. pallidisma: grows between 200-2500m altitude. Not so common. Forms extensive mats.
The branches are raised a little above the surface by the rather robust rhizophores. The
apices of the branches swell and form tuberoid outgrowths and perenation is through these
dormant apices.
Plants terrestrial, very occasionally epiphytic; stem erect or prostrate, with a subsidiary
branching system, basal portion not differentiated into a distinct rhizome. In most of the
species shoots are dorsiventrally prostrate. Main stem wide-creeping, often much branched,
of indefinite growth, or short-creeping and then becoming erect often with a distinct
unbranched region below. Leaves spirally arranged, distant on main stem but contiguous on
the branches, ligulate (ligule believed to protect the growing point, but this is probably an
enigma), those on basal creeping portions often distantly arranged, either isomorphic (no
differentiation between the large and small leaves) or dimorphic, arranged in four rows, two
upper small and two lower large, platystichous, supplied by a single unbranched vein;
strobili terminal on the branches, tetrastichous, compact, three patterns in the arrangement
37
of sporangia in each strobilus have been noticed, (i) strobili having a basal zone of mega
sporangia with an upper zone of micro sporangia; (ii) strobili having two rows of mega
sporangia and two rows of micro sporangia; (iii) strobili are entirely megasporangiate;
sometimes the strobili may be mixed; mega- and micro sporangia are borne in the axils of
the leaves (sporophylls); sporophylls resemble the vegetative leaves but sometimes the
mega sporophylls are slightly larger than the micro sporophylls; spirally arranged or in four
rows; heterosporous. Chromosome number: four base numbers exist, x=7, 8, 9 and 10.
Diploids, triploids and tetraploids are known.
Anatomy:
Stem: Very variable in different species and sometimes variable in the same species in
different parts i.e. creeping rhizome may be different from erect branches. The following
variables are found:
(i)
Selaginella spinulosa Type: Here in the ascending branches there is a solid
protostele with a number of angles, at the periphery protoxylem elements are
present, phloem and pericycle follow, endodermis is in the form of trabaculae
and has the typical “casparian strips” or thickenings on the radial walls. The
rhizome has a solid protostele with the protoxylem in the center. Cortex is
entirely parenchymatous. In S. krausiana the ribbon-shaped stele splits into two
due to the branch trace.
(ii)
S. chrysocaulos Type: This is characteristic of those species that are with a
dorsiventral stem. Stele is single. Xylem is flattened in the form of a ribbon,
solid, two marginal protoxylem groups which receive leaf traces from dorsal and
ventral side of stem, xylem is surrounded by phloem, 1-3 layers of pericycle and
the trabecular endodermis.
(iii)
S. uncinata Type: This is a slightly advanced type. The uppermost branches
contain a stele as in S. chrysocaulos. In lower portions of well developed stem, a
dorsal cord of xylem separates from the main stele with its own phloem but both
dorsal cord and main stele are surrounded by a common pericycle on the outside.
The stele is still single.
(iv)
S. willdenowii Type: The species has a stout stem and is climbing in nature.
Rhizome is monostelic type and has a ribbon-shaped xylem. Higher up in the
aerial stem are three steles but within the same pericycle. In very stout stems five
steles may be present.
(v)
S. lyalli Type: (also in S. pectninata). Leaves are homophyllous (isophyllous)
and in 4 rows. The creeping axis (rhizome) has a siphonostele, then a
polycyclic solenostele, the outer ring possessing exarch protoxylem, surrounded
by phloem, pericycle and endodermis. In the center is another stele with solid
xylem. When a branch trace is given off it forms gaps and the outer stele gets
split into four primary steles, while in the center there are 12-13 smaller steles;
leaf traces are given off from the outer stele.
38
39
Leaf: Lower and upper epidermal cells contain chloroplasts. Mesophyll is undifferentiated
and composed of photosynthetic parenchyma with intercellular spaces. A single concentric
bundle is present. Stomata generally confined to the abaxial surface only, either scattered or
localized near the vein region, apex or margin of the leaf.
Roots: Roots may arise directly from the base of the stem (S.spinulosa). In other species
there are present peculiar smooth structures (rhizophores) that arise from the under surface
of branches, at the region of forking. When stout they support the aerial stem. Stele
monarch to tetrarch with protoxylem on one side. Phloem is in the form of a horse-shoe. In
S. krausiana the structure is like that of the stem with centerach phloem. The rhizophore can
also develop into a typical leafy stem under certain conditions. Some morphologists
describe the roots to be borne on the rhizophores, while others describe the rhizophores as
changing into roots upon touching the ground. The rhizophores may branch dichotomously
or apparently, monopodially. Their morphology is debatable. Some consider them as a stem
due to: (i) Exogenous origin (ii) No root-cap (iii) Readily converted into shoots on suitable
cultivation. However others consider them to be a root due to: (i) Internal structure is
40
generally like the roots (ii) They are usually naked, do not bear leaves. Since the
rhizophores do not fit into either the category of roots or stem, but exhibit some of the
characters of each, some botanists consider them to be special structures “Sui-generis”
Sporangium: The mature sporangium has a wall of three layers, the innermost acts as a
persistent tapetum. In the microsporangium there are numerous trilete microspores, about
100 per microsporangium, small, 20-60um in diameter; in megasporangium there are only
four megaspores. Occasionally variations exist. Sometimes the number of megaspores vary
from 6-42, or in some other species (S.rupestris) the number of megaspores may be
reduced to one, megaspores trilete, large 200-600um in diameter, exine or the outer wall
layer variously ornamented in both type of spores.
Development of Sporangium: Eusporangiate type. There are two bands of archaesporial
cells with four cells in each band. A wall is laid in each cell resulting in an outer and an
inner cell. Outer cell by further periclinal divisions forms a wall of two cells. The inner cells
form the sporogenous tissue, while the peripheral layer behaves as the tapetum. In micro
sporangia almost all mother cells produce spore tetrads. But in mega sporangia, only one
mother cell functions, the rest degenerate and provide nourishment to the functional mother
cells that undergoes meiosis to produce four megaspores.
Gametophytes: Male: Each microspore has an intine and an exine. Development of male
gametophyte is endosporic. The first division cuts off a small peripheral prothallial
vegetative cell and a large antheridial initial. The prothallial cell is the sole representative of
the prothallus and undergoes no further divisions and takes no further part in the
development of the gametophyte. Further divisions are
restricted to the antheridial initial. It divides by a vertical wall to give rise to two halves.
Further walls are laid down as shown in figures 1-8. The fully developed gametophyte has a
vegetative cell and two antheridia. The antheridium matures in the microspores that have
been shed. At maturity the walls collapse and biflagellate spermatozoids wriggle out of the
ruptured microspore wall. The stage at which the microspore is shed varies in various
species. In S.chrysocaulos, S. chrysorhizos, S. pallidisma and S. monospora the microspore
is shed after the formation of the prothallial cell. In S.adunca, and S.caulescens the
microspore is shed after development of spermatogenous tissue. In most other cases the
microspore is shed at the 13-celled stage.
Female Gametophyte: The female gametophyte begins to develop even before the
megaspore has attained its size. The nucleus divides by free nuclear divisions and forms a
large central vacuole. The nuclei are placed in a peripheral lining of cytoplasm. Further
rapid nuclear divisions take place in the apical region followed by formation of walls
around the individual nuclei. This results in the formation of a small tissue – the generative
tissue. The basal walls bordering the vacuole region thicken to form a sort of a diaphragm.
The lower vacuole region is the storage region. Archegonia begin to develop at the apex of
the generative tissue. Meanwhile the megaspore wall cracks, exposing the gametophytic
tissue to light, which turns green. Due to further growth, humps of tissue may be formed.
Rhizoids also make their appearance. Each archegonia has a small neck of four rows of
neck cells with two cells in eachrow and with a single neck canal cell. After fertilization
there is development of an irregular, large celled tissue in the lower region. The cells here
are multi nucleate. In four species, S.chrysocaulos, S.chrysorhizos, S.pallidisma and S.
monospora, megaspores are ejected from the mega sporangium when it is at free nuclear
stage, in S.adunca and S.caulscens after female gametophyte has been formed and
sometimes even when archegonia have been developed. In S. rupestris fertilization occurs
41
when megaspore is still lodged within the mega sporangium. Some species are apomictic
(embryo formed without fertilization).
Embryology: The zygote (fertilized egg) divides by a transverse wall. The upper cell forms
the suspensor which pushes the lower cell into the storage tissue. The embryonic cell then
divides by a vertical wall into two equal cells. The second wall strikes the first wall
obliquely and separates a small triangular cell at the apex. The two larger cells after further
42
divisions give rise to a cotyledon each. The foot arises from some of he cells of the leaf
segment adjacent to the suspensor, by the growth of the foot the axis of the stem turns at
right angles to the suspensor. The primary root arises later from the cells of the leaf segment
close to the suspensor.
Fossil Members: Treated under Selaginites, discovered from upper carboniferous of France.
In some species structure as in modern Selaginella particularly Selaginellites primaevans. It
resembles Selaginella spinulosa. Leaves are spirally arranged and there are four
megaspores.
FILICOPSIDA
This group represents the dominant vegetation amongst the Pteridophytes, just as the
Conifers amongst the Gymnosperms. In the number of genera and species, members of the
Filicopsida (or the ferns) are second only to the angiosperms amongst the terrestrial plants.
Opinions are very divergent as to the number of families 31-64 (or even more), genera 300443 and species 9,000-15000 species. The numbers vary because some groups are still
poorly studied and the species-concept is a matter of debate. From India 1000-1500 species
are usually recognized. Essentially ferns as other Pteridophytes inhabit Tropical Forests,
(Monsoon forests and Tropical rain forests), Wet Lands, Temperate Lands, Arid Zones and
the “Land of Gods and Myths” or Mountain Summits and Polar Regions. Ferns are
extremely diverse in habit, form and reproductive habits. In size they vary from minute
structures only two to three millimeters tall to huge trees. Amongst the smallest ferns on
land in India is Anogramme leptophylla, with Azolla pinnata being the smallest aquatic fern.
Some ferns may be twining climbers while others float on the surface of water. Many ferns
grow as epiphytes or as lithophytes. The fossil history of ferns extends as far back as the
Devonian period of the Paleozoic.
Importance of ferns: Ferns are not of much use to man directly. These days some species of
Azolla (A.. oinnata – the mosquito fern) are being used as green manure since the lower
lobes of leaves house a nitrogen-fixing alga. Some ferns play a role in ecological
succession. A few are used for decoration. Now a days fern fronds are being
indiscriminately used in bouquets. The lovely fronds of these plants are being ruthlessly
collected from the wild and sacrificed in a manner, that it may spell dooms-day for them. It
appears their beauty has become a curse for them like the ‘horns of the stag’. Ferns do not
have a long history as cultivated plants although they are becoming popular in horticulture.
The fibrous mass of roots encasing the stems of the tree ferns has found great favor as a
growing medium for orchids resulting in their mass destruction. Large fronds are used in
thatching, providing bedding for farm animals, and for packing purposes. Species of
Pteridium are also used to dye wool and silk. A few ferns are edible. Ferns are also claimed
to be useful in medicine. The medicinal use of ferns covers a wide spectrum. Fevers, worm
infections, chest complaints, blood disorders, snake bites, asthma, insanity, liver and kidney
stones, diseases of the spleen, rickets, diseases of the genital organs etc. are supposedly
treatable by a variety of ferns. The ancient philosophy was that nature had given a plant a
particular shape to its leaves, flowers, fruits or seeds to help man to know which ailment a
plant could cure or for which organ it was useful. Thus Adiantum lunulatum (A.philippense)
is believed to be useful for mental disorders (often considered related and affected by the
moon) since the fern has half-moon shaped pinna. A major significance of ferns is in
biological research.
Some strange and interesting myths are also related to ferns. In earlier times ferns were
believed to possess magical powers and mystical properties. In medieval times the stem of
the European fern Dryopteris filix-mas (also found in Kashmir, which is its southern most
43
boundary) was used in love philters. Its crosiers were believed to give protection against
sorcery and the evil eye. An oil was extracted from its rhizome which gave rise to a very
popular perfume. Species of the Bracken, (Pteridium) were believed to be bestowed with
magical powers. Its stem contains starch which is used for brewing beer. Many more
interesting facts of this carcinogenic fern are known. The maiden-hair fern (Adiantum
species) are believed to have powers to promote growth of hair, and are often used as hair
conditioners. More interesting anecdotes about ferns can be found in Camus et al. (1991).
The typical fern is a perennial sporophyte with roots, stems and spirally arranged
megaphyllous leaves. The fern leaf including the stipe (petiole) and lamina (blade) is known
as the frond. When young the frond is circinately coiled and is known as a fiddlehead or
crosiers. The lamina may be simple or variously divided pinnately or palmately. For
reproduction the plant produces spores in a sporangium. The sporangia may be aggregated
together in a sorus or may not be aggregated. Stele is variable, protostele or solenostele or
dictyostele or polycyclic stele. Sporangium consists of a stalk, a body which has a series of
thick-walled elastic cells (annulus) which function in opening and closing the sporangium.
Spores are released through a stomium or opening. Spores hetero- or homo sporous,,
bilateral monolete or tetrahedral trilete; spores germinate to form aa exosporic
(homosporous ferns) or endosporic (heterosporous ferns), gametophytes of homosporous
ferns are small, internally undifferentiated structure (prothallus), antheridia produce a
number of multiflagellate spermatozoids.
Classification:
The following criteria are used in fern classification:
(i)
External morphology of the shoot: Stem erect or ascending or prostrate; thick or
thin; branched or unbranched; branched dichotomously or laterally.
(ii)
The apical meristem and its further segmentation to give rise to the various plant
parts. In general there is present an apical cell.
(iii)
Architecture and venation: Leaves may be thin and filmy or thick; simple or
variously divided, pinnately or palmately, once or many times; venation may be
simple or dichotomous or reticulate or reticulate with free endings of vein lets.
(iv)
Vascular anatomy of stem and Leaf: As the stellar system does not fluctuate with
small environmental or even major environmental conditions, it is an important
character. In fossil plants it is usually preserved and therefore a very valuable
basis for classification.
(v)
Dermal Appendages: The apex of the stem and young leaves or all or some plant
parts are covered by different types of dermal appendages. The dermal
appendages are of three types i.e. hairs, scales and fibrils. Hairs are usually one
cell wide and one or many cells long, simple or stellate, variously coloured, soft
or hard, the apical cell (if hair is pluricellular) may be acicular or knob-shaped or
hooked, etc. Scales are usually a 1-celled thick plate of parenchumatous cells
and may be peltate or basifixed, of various colour, shape and size, margin also
variable, entire or serrate or crenate etc. with glandular cells, distribution on
plant variable, cell-walls thin or thick, cell cavity empty or filled. In between the
scales and hair are the fibrils. On a plant either all types of dermal appendages
are present or any two or any one or none. The dermal appendages are valuable
criteria in fern classification.
(vi)
Position and structure of the sorus: Sporangia may or may not be aggregated to
form sori. The position and shape of the sorus is variable. The sorus may be
44
round, oval, kidney-shaped, linear, hippocrepiform etc. The sori may be
marginal or adaxial. All sporangia within a sorus may be at the same stage of
development (Simplices) or there is a gradual initiation of sporangia (Gradatae)
or the sorus may be of mixed sporangia at various stages of development
(Mixtae).
Depending upon the nature of the sorus and stage of sporangial development the
ferns are divided as: Simplices, Gradatae and Mixtae. The Simplices are
primitive and extend to the Paleozoic, the Gradatae with an oblique annulus,
occupy a middle position and have been discovered in the Mesozoic. But in the
Mixtae annulus is present but in a vertical fashion. In Simplices the size and area
of the receptacle is fixed and only a fixed number of sporangia can be borne on it
with all of them being at the same stage of development. They mature at the
same time. The number of sporangia per receptacle varies between 10-12. In
some cases the sporangia are borne in two series. In such cases the internal space
within the radial sorus gets filled by overlapping sporangia. Overcrowding
results and there is no space for the proper dehiscence of sporangia and
consequently some of them do not dehisce. This mechanical inefficiency can be
solved by the elongation of the receptacle on which the sporangia are borne at
different times and are at different stages of development (Gradatae). Moreover
there is no simultaneous drainage of food by the developing spores. The spores
are released in a staggered fashion and even if the environmental conditions are
not favorable when the spores are initially released; this problem is solved by a
constant crop of spores over a given length of time. This is an evolutionary
advanced feature for survival. The next step in the evolution of the sorus is the
mixed sorus (Mixtae).
(vii)
Indusial protection: Some sori are naked others have an indusium for the
protection of young developing sporangia. The shape and form of the indusium,
method of attachment are variable. The indusium may be formed as an
epidermal outgrowth covering receptacle and sporangia (true indusium) or it
may merely be the in turning of the margin of the leaf to protect the sporangia
(pseudo indusium or false indusium).
(viii)
Characters of the Sporangium: Eusporangiate and massive or Leptosporangiate
and small, if leptosporangiate whether stalk is thick or 1 or 3-celled.
(ix)
Spore output within a sporangium and the nature and form of sporangia. In
massive sporangia (eusporangiate type) belonging to the primitive ferns sporeoutput is large. In the more advanced types (leptosporangiate) the number of
spores per sporangium is gradually reduced and in the higher leptosporangiate
homosporous ferns only 64 spores are present in a sporangium (only in sexually
reproducing ones; in apogamous species only 32 spores per sporangium are
present). In shape the spores may be tetrahedral or bilateral. Spore size and its
ornamentation also help in distinguishing taxa.
(x)
Type of Prothallus and mode of Spore germination: The prothallus may be
subterranean and brown in some cases or it may be surface living and green. It
may be branched or cordate or of various shapes and size. Spore germination
may be of the Equatorial Type or Polar Type and the prothallial development
may be of: Osmunda type or Marratia or Adiantum or Drynaria or Ceratopteris
or Kaulina or Aspidium Type (Nayar & Kaur (1971).
45
(xi)
Position and structure of Sex organs: The antheridia may be massive, embedded
and with the jacket composed of many cells (in eusporangiate types) or small,
superficial and with the jacket of only three cells (in homosporous
leptosporangiates).
(xii)
Embryogenesis: In primitive ferns a suspensor cell is formed but it is absent in
the more advanced types. Further development of the zygote is variable
depending upon whether the stem is erect or not.
(xiii)
Cytology: Cytology has afforded an additional tool in the classification of ferns.
Classification:
Many systems of classification exist. A workable system, in which there are neither a large
nor a few number of families is proposed:
Two major groups are recognized: (i) Eusporangiate (ii) Leptosporangiate. This clear cut
demarcation is marred by the Osmundaceae which possesses intermediate features between
these two groups.
EUSPORANGIATE:
1. OPHIOGLOSSACEAE: The families Botrychiaceae and Helminthostachyaceae are
merged into Ophioglossaceae after the discovery by Sun et al. in 2001 of a genus
Mankyua from Korea which has characters intermediate between Ophioglossum and
Helminthostachys. It has a ternately divided lamina but the fertile spike (sporophore)
is linear and fleshy, branched at base; sporangia sunken in a fleshy sporophore,
arranged in two rows, and marginal in position somewhat similar to Ophioglossum.
2. MARRATIACEAE : The families Angiopteridaceae, Christeniaceae and Danaeacea
are merged.
Angiopteris, Christensenia and Marratia.
LEPTOSPORANGIATAE: The following families are recognized:
1. OSMUNDACEAE: Osmunda and Todea.
2. PLAGIOGYRIACEAE: Plagiogyria.
3. GLEICHENIACEAE: Gleichenia and Dicranopteris.
4. GRAMMITACEAE: Ctenopteris, Grammitis, Prosaptia, Scleroglossum and
Xiphopteris
5. LOXOGRAMMACEAE: Loxogramme
6. POLYPODIACEAE: About 30 genera.
7. SCHIZAEACEAE: Actinostachys, Anemia, Lygodium, and Schizaea.
8. SINOPTERIDACEAE: Cheilanthes, Doryopteris, Notholaena and Pellaea.
9. CRYPTOGRAMMACEAE: Cryptogramma and Onychium.
10. .ACTINIOPTERIDACEAE: Actinopteris.
11. .HYPOLEPIDACEAE: Hypolepis.
12. .PTERIDiACEAE: Pteridium.
13. PTERIDACEAE: Pteris.
14. ACROSTICHACEAE: Acrostichum.
46
15. ADIANTACEAE: Adiantum.
16. HEMINONITIDACEAE: Anogramma, Coniogramme, Hemionitis, Idiogramma,
Pityrogramma, nd Syngramme
17. VITTARIACEAE: Vittaria.
18. TAENITIDACEAE: Taenitis.
19. .PARKERIACEAE: Ceratopteris.
20. MARSILEACEAE: Marsilea.
21. HYMENOPHYLLACEAE: Hymenophyllum, Trichomanes.
22. .DICKSONIACEAE: Cibotium.
23. CYATHEACEAE: Alsophilla (Cyathea), Sphaeropteris
24. MONACHORACEAE: Monochosorum.
25. DENNSTAEDTIACEAE: Dennstaedtia, Emodiopteris, Histiopteris and Microlepia.
26. LINDSAEACEAE: Lindsaeaceae and Sphenomeris.
27. ASPLENIACEAE: Asplenium and Sinephropteris.
28. THELYPTERIDACEAE: Thelypteris.(the splinter genera are merged into a single
genus)
29. CLEACEAE: Onoclea.
30. ATHYRIACEAE: Arhyrium, Cystopteris, Deparia, Dictyodroma, Diplazium,
Dryoathyrium and Gymnocarpium, Woodsia.
31. HYPODEMATIACEAE: Hypodematium.
32. PERANEMATACEAE: Acrophorus, Lithostegia and Peranema.
33. DRYOPTERIDACEAE: Arachniodes, Cyrtomium. Dryopteris, Nothoperanema and
Polystichum.
34. BOLBITIDACEAE: Bolbitis and.Egenolfia
35. TECTARIACEAE: Tectaria.
36. .ELAPHOGLOSSACEAE: Elaphoglossum.
37. NEPHROLEPIDACEAE: Nephrolepis.
38. .OLEANDRACEAE: Oleandra.
39. DAVALLIACEAE: Araiostegia, Davallia, Humata Leucostegia and Paradavallodes.
40. BLECHNACEAE: Blechnum and Woodwardia.
41. GYMNOGRAMMITIDACEAE: Gymnogrammatis
42. STENOCHLAENACEAE: Stenochlaena.
43. SALVINIACEAE: Salvinia, Azolla.
(Only names of Indian genera are given as examples).
47
PTERIDACEAE:
The circumscription of the family Pteridaceae varies from only a single genus Pteris or 63
which then includes diverse elements of ferns. Most of these are now recognized as separate
families.
Pteris:
Plants are usually terrestrial or epilithic. Rhizome is short, erect or short-creeping, slender to
massive, scaly generally towards apex, scales basifixed, brown or black, linear lanceolate,
margin entire or variously lobed. Solenostelic or dictyostelic or sometimes polycyclic.
Stipes (petioles) arise close together, well developed, adaxially grooved, colour straw or
brown, generally scaly at base, naked upwards, vascular bundles either one (in a cross
section ‘U’ or ‘V’ or omega-shaped), or less often 2 C-shaped vascular bundles, or in large
species sometimes with two larger adaxial and a semi-circle of smaller abaxial ones.; rachis
(mid-rib) grooved on upper side, sometimes winged. Lamina rarely simple, 1-2 rarely up to
5-pinnate, ultimate divisions often pinnately lobed or pinnatifid, lamina occasionally pedate
or palmate, never finely dissected, texture herbaceous or coriaceous, glabrous or less often
hairy rarely scaly when mature; lowest pair of pinnae often forked near the base on
basiscopic side resulting in a long pinnule with its lobes similar to other pinnae, however
there are many species that do not show such a forking of the lowermost pinnae (or pinnule)
eg. the very common Pteris vittata, P. excelsa etc., terminal pinna similar to lateral ones;
leaf axis rarely winged, adaxially grooved, the grooves with few exceptions continuous on
axes of different order; larger divisions sessile or short (rarely)–stalked, very often of an
elongate-oblong to lanceolate-linear,usually symmetric, upper divisions lobed-confluent.
Stomata polocytic and anomocytic. Veins free or anastomosing to form a series of narrow
areolae along the costa or costules; areolae with or without included veinlets; costa and
costules usually with short setae on the upper surface, Sori marginal (actually submarginal
as the leaf margin curls back to form the indusium), linear; pseudoindusia formed by
reflexed pinna margin. Spores brown, tetrahedral trilete rarely bilateral monolete, nonperinate, exine smooth or tuberculate or verrucose, the two surfaces have different types of
ornamentation, a prominent collar-like ridge girdles the spores,. perine absent. Chromosome
base number x=29.
Gametophyte Generation:
The spore, which is the first stage of the gametophyte generation, is a living cell, protected
by a thick wall and is the main source of population dispersal. The spores of Pteris have an
exine with a species-specific ornamentation and are believed to retain viability long after
shedding but germinate readily under appropriate moist conditions. Germination is preceded
by swelling of the contents of the spore by absorption of water and the spore opens. A small
colourless protuberance the rhizoidal initial emerges out of the spore opening and is soon
followed by another protuberance full of chloroplasts, the protonemal filament initial.
Through cross-walls perpendicular to the long axis of the filament, a uniseriate protonemal
filament composed of barrel-shaped 2-7 chlorophyllous cells bearing one or a few rhizoids
at the base, is formed. The terminal cell of the protonemal filament undergoes divisions by
vertical and transverse walls, at right angles to each other resulting in a group of four cells,
which by further divisions in two planes form an anterior plate of cells leading to the
formation of the spatula stage. An abrupt change in the plane of cell divisions results due to
formation of a prothallial plate (meristem). The stage at which the meristem is formed
differs in various species. In some cases a wedge-shaped ephemeral apical meristematic cell
may precede the characteristic pluricellular meristem. Depending upon the position of the
meristem and when it is established, the young prothallus is asymmetric (lopsided) for a
48
longer or shorter period. Further growth leads to the formation of a cordate prothallus with a
central cushion 3-5 or 5-10 cells thick. It is surface-living, green and photosynthetic. The
shape of the adult prothallus varies in various species of the genus Pteris. It is taperingbilobed type (narrowed gradually towards the base) in P. excelsa, P. pseudoquadriaurita
and P. stenophylla) but broad-winged with an almost posterior end in P.vittata and P.
wallichiana.
49
Archegonia are borne on cordate meristic prothalli invariably below the notch. Rhizoids are
also very abundant on the cushion. No trichomes or glands of any kind are present on the
prothalli of Pteris. Antheridia are produced from early stages of development of the
prothallus and these are present either on the central cushion below the archegonia or on the
wings. Three kinds of gametophytes are generally produced: (i) large-sized and generally
fast-maturing cordate prothalli with symmetrical wings, initially archegonial but later may
bear antheridia (ii) relatively smaller cordate prothalli, narrow below, usually protoandrous
and during subsequent archegonial phase hardly producing any fresh antheridia, (iii) slowgrowing ameristic prothalli exhibit arrested growth remain small, non meristematic and
irregularly shaped but loaded with antheridia.
Sex organs: Development of Antheridia: The ontogeny of the antheridium in ferns has been
a subject of controversy. In the antheridial initial three walls are formed successively and
according to the classical concept (before 1951) the first and third wall were described as
‘funnel-shaped’ and the second as hemispherical and periclinal lying over the first wall.
Davie in 1951 refuted the classical concept and considered successive walls to be transverse
initially and later bulge downwards or upwards. Stone (1962) thought the first and in some
cases, the third wall to be funnel-shaped. Much emphasis was laid on the axis of the nuclear
division and the differentiation in cytoplasm prior to cytokinensis which determined the
position in which each new cell wall is deposited and in interpreting the formation of a
funnel-shaped wall. Verma and Khullar (1965) considered the first and third walls to be
either straight transverse or slightly concave or deeply concave and retaining their shape
except for minor displacements. The second wall is hemispherical, lying over the first wall
with its convexity more or less running parallel to the periphery of the antheridium in order
to enclose the dense contents of the androgonial cell within
An antheridium inducing substance known as antheridogen or antheridiogen has been
isolated from the gametophytes (especially in Pteridium aquilinum) and from the culture
medium in which the gametophytes were grown. This substance stimulates and hastens the
precocious formation of antheridia by 3-4 weeks. Seven types of gibberellins had the same
effect as antheridiogen and so it is believed that the antheridiogen is a gibberellin. An
extract from predominantly archegonia bearing- prothalli exhibited more antheridium
inducing activity than an extract from juvenile, predominantly antheridium-bearing
prothalli.
Archegonia: A superficial cell on the cushion acts as the archegonial initial. It divides by a
periclinal wall into an outer smaller primary neck cell and an inner cell. The latter divides
again by a periclinal wall cutting off a central cell and a basal cell, thus forming a tier of
three cells. The primary neck cell divides by two longitudinal walls at right angles to each
other to form four cells. These are the initials for four tiers of neck cells. By further
successive periclinal divisions of the neck cells, an elongated neck is organized. The central
cell first cuts off a primary neck-canal cell and then divide again to form a venter canal cell
and an egg. The neck canal cell divides to form two neck canal nuclei. At maturity the
archegonium has a neck of four rows of cells with 4-5 cells in each tier; an egg, a venter
canal cells and two neck canal nuclei are present in the center. Before fertilization all cells
above the egg disintegrate. Spermatozoids are attracted by the species-specific fluid oozing
out of the opening of the archegonia, and fertilize the egg.
50
51
Embryogenesis: Although more than one egg on each prothallus is fertilized only one
develops into a young sporophyte, (A natural population control!). It is not known whether
it is the first formed or the subsequently developed archegonia that form the sporophyte.
The first division in the fertilized egg is by a cross-wall to the long axis of the archegonia
followed by another at right angles to the first resulting in the formation of a quadrant.
Differentiation into the organs of the young sporophyte may occur as early as the quadrant
or octant stage. The outer anterior part of the quadrant gives rise to the first leaf, the inner
anterior to the shoot apex, the inner posterior to the foot and the outer posterior to the first
root.
Departures from the normal sexual life cycle (as described above) can result in the
cytological and/or morphological alternation of generations being interrupted or modified.
Such examples include apogamy, apospory and vegetative reproduction. Apogamy and
52
apospory occur in ferns often as sporadic aberrations, compounded or combined with other
developmental errors.
Apogamy: (apo=without; gamy=marriage). In many homosporous ferns (about 10%) a
sporophyte is produced without gametic fusion (fertilization). This is known as apogamy.
Many species of the genus are apomictic and Pteris cretica, is the earliest and well known
example. In apomictic forms archegonia are not developed and any cell of the prothallus
directly gives rise to the sporophyte. In an apomictic life-cycle the genetic consequences of
sexuality, recurrent gene segregation and recombination are averted. In such cases the
function of meiosis in halving the number of chromosomes is neutralized by events during
the premitotic mitoses which remains incomplete at a particular stage, when a failure of
anaphase separation followed by the formation of restitution nuclei produce spore-mothercells with double the number of chromosomes. Such a shift in the breeding system could be
through a suitable mutation in a sexual taxon or due to the interaction of the two genomes of
a species combined in a hybrid brought together in cross fertilization. Apogamy can also be
induced in laboratory conditions by sugar or ethylene treatment.
Apospory: In apospory the gametophytic tissue is produced by the sporophyte without the
intervention of spores. The prothalli so produced are usually functional but may have a
somewhat different morphology from those of the same species produced through spores. It
occurs sporadically in ferns but can also be induced. Aposporous prothalli bear normal sex
organs and since they have the same chromosome number as the parental sporophyte tissue
they provide a means of inducing polyploidy.
Reproductive Biology:
Although homosporous ferns may have bisexual gametophytes, there is no evidence that
these gametophytes characteristically self-fertilize. It was earlier believed that due to the
close proximity of the gametangia on the prothallus, it underwent self-fertilization, where
the sperms from the antheridia swim to fertilize the egg. In contrast, electrophoretic data
indicate that inter gametophytic matings occur frequently. However studies have indicated
that many homosporous ferns possess mechanisms that promote out crossing in natural
populations. Noteworthy of these mechanisms is the production of the hormone
antheridiogen (discussed above), which results in the production of effectively unisexual
gametophytes, thus promoting intergametophytic mating. The sequence of gametangia
formation on mersitic prothalli reveal that some species of Pteris are either exclusively or
mostly inter gametophytic in their mating system. Intra and inter gametophytic mating or a
combination of both the mating systems may be found in the polyploids. Another interesting
aspect is the inability of monoecious prothalli, upon isolation, to bear a sporophyte.
Through genetic analysis the existence of a weak self-incompatibility system has been
demonstrated. Several factors including self-incompatibility and genetic load play a role in
determining the infertility of monoecious prothalli in isolation or in pair-wise combinations
to produce sporophyte. The fern breeding systems are diverse rather than stereotyped.
Several questions on the genetic and epigenetic controls of their sexuality remain to be
answered.
MARSILEACEAE
Three genera are contained in this family of aquatic ferns. They are very un fern like in
appearance. The Brazillian genus Regnellidium (named after A.F.Regnell) with a single
species R. diphyllum, also found in adjacent Argentina; Pilularia (pilula in Latin =a little
ball) with between three and six species, found in western South America, Australia and
New Zealand. P.minuta is amongst the smallest of all ferns. Lamina is absent and the leaves
consist only of the filiform petiole in Pilularia. In Regnellidium only two leaflets are
53
present while in Marsilea four leaflets are present. All members are aquatic plants and
heterosporous.
Marsilea (named after an Italian naturalist, F.L. Marsigli) or “The Four-leaved
Clover”.Occurs in Tropical and Temperate regions, and between 50-70 species are known
world-over. From India and between 10-15 species are known (the variable number is due
the non- recognition of some species due to their plastic nature under varying ecological
conditions and often very similar appearance). Marsilea minuta (Fig. 22) is the most
common species in India. Plants are aquatic or amphibious, growing in shallow ponds or
ditches. Some species especially from Australia are very drought resistant.
The rhizome grows either on the surface of the mud or slightly below it, is capable of
indefinite growth and may grow in all directions, is creeping, and dichotomously branched.
The fronds (leaves) arise from the upper side of the rhizome in two alternate opposite rows;
internodes may be short or long. On the underside of the rhizome one or more adventitious
roots are present at the nodes. The length of the stipe (petiole) varies with the water-level
where the plant grows. and has air-chambers, but when the same species grows on damp
soil or mud the petioles are shorter and more rigid. Lamina circinate when young; pinnate,
but as a result of two dichotomies arising in close succession to each other, it is divided into
four pinnae that float on the surface of water and are borne very close together. The four
leaflets do not actually arise at one locus but two of these are slightly higher than the others
and are inserted in alternate fashion. Pinnae are sometimes very small, with a variable
shape that varies from obovate to obcuneate to cuneate; margin variable, rarely toothed at
their distal margins, plants growing out of water have smaller pinnae with erosed margins,
stomata sunken, occur on both surfaces; veins close, forked, sparingly to amply
anastomosing forming a reticulum.
Anatomy: Rhizome has a solenostele (amphiphloic siphonostele) and consists of the xylem
surrounded by an inner and outer phloem followed by a single layer of pericycle which in
turn is covered by a single endodermal layer. The pith is either parenchymatous or
sclerenchymatous (in plants growing on soil). Large air-chambers are present in the cortex.
Roots are with a protostele, monarch or diarch. The stipe (petiole) has a ‘V’-shaped mass
of xylem with protoxylem exarch in position. The xylem is successively surrounded by
phloem and endodermis. The vascular trace of the petiole leaves a gap above the place of its
departure from the stem stele. A trans-section of the pinnae shows an upper and a lower
epidermis, both with sunken stomata; mesophyll is differentiated into palisade and spongy
areas.
Reproduction: Sporangia are borne in special structures with a more enhanced protection to
the spore bearing organs, the sori being borne inside a nut-like body known as a sorocarp
(sporocarp). Usually this structure is referred as a ‘sporocarp’. This body is now interpreted
as a deeply modified portion of the frond (one or two pinna) duly fused at the edges to form
a conceptacle inside which the sori are borne. This structure is not homologous with the
sporocarp of the Salviniaceae and therefore cannot be designated by the same name. The
term sorocarp was proposed for it by Pichi Sermolli (1977)
54
Fig. 22 Marsilea minuta. A Plant growing in nature on land. B. Sorocarp
55
.
The sorocarps are borne on the petiole (stipe) or on special stalks inserted at or near its base,
singly or in clusters (depending on the species), very rarely on a several times branched
system of stalks. The sorocarps are borne almost at right angles to the stalk, are subglobose
to ellipsoid or bean-shaped, the upper edge often concave, of two connate segments, each
with a row of several parallel sori. Near the point of insertion of the sorocarp one or two
tooth-like appendages are present. The sorocarp wall is two-celled in thickness. A
gelatinous ring lines the innermost wall layer. The vein of the petiole enters the sorocarp
body, arches upwards, bends backwards and runs almost parallel to the wall of the sorocarp
and is known as the dorsal cord. Many lateral veins perpendicular to the dorsal cord are
given off, alternately and each of them forks dichotomously about midway between its base
and apex (except the first and last). Towards the lower margin of the sorocarp, there is a
net-work of veins due to anastomoses of the lateral veins, as shown in figure. A ridge-like
receptacle develops at the region where each lateral forks. Receptacles on one half of the
sorocarp alternate with those of the other half. Each receptacle projects inwards from the
sorocarp wall and soon differentiates a row of sporangial initials along its summit. Other
sporangia are differentiated in a basipetalous succession along the flanks of a receptacle as
in a typical gradate sorus. Each sorus is covered with a delicate indusium and the indusia of
adjacent sori partially fuse together, so that each receptacle and its sporangia lie in a cavity.
Sporangial initials at the apex of the ridge-like receptacle develop into mega sporangia
while those lower down develop into micro sporangia. The development of the sporangia is
of the leptosporangiate type. Each sporangia produce between 32-64 spores. All spores
except one degenerate in the terminal sporangia. The size of the surviving spore increases
many times its original and is the megaspore. Thus each mega sporangium has a single
megaspore. All spores in the lateral sporangia mature, but are much smaller in size than the
megaspore. These mature into microspores. As the spores mature, the outer wall layers of
the sorocarp become stony and their cell walls thicken markedly. Annulus is absent in both
the mega and micro sporangia. The sorocarps persist for a long time even after the
vegetative parts have died. Spores are released by rotting or through injury of the sorocarp
wall. Water is absorbed by the gelatinous ring and the two halves of the sorocarp split apart.
The gelatinous ring bearing the sori, the sorophore, emerges. Later the indusium and
sporangial wall disintegrate to liberate the spores.
Gametophyte: Spores remain viable for a long period and germinate only after they are
released from the sorocarp. Megaspores are spheroidal to slightly ellipsoidal, with a small
hemispherical nipple-like protuberance, surface nearly plain, wall quite thick except in the
region of the protuberance. The nucleus which lies in the protuberance portion of the
megaspore, divides and a transverse wall is laid along the line of junction of the
protuberance resulting in the formation of a small cell and a large cell( which is the
remaining lower portion of the spore) in which no further nuclear division takes place.
Nuclear divisions take place in the small cell resulting in the formation of a basal and three
lateral segments. However the sequence in which these segments are formed is not fixed.
Sometimes it is the basal segment that is cut off first followed by the laterals and sometimes
two or three laterals are formed first before the basal segment. Repeated anticlinal divisions
in all four segments results in a vegetative tissue one cell in thickness. The central cell acts
as the archegonial initial which cuts off a primary cover cell and a central cell. Two
successive divisions in the primary cover cell results in quadrant of four cells which
function as the neck initials. A division in each of these four initials results in a short neck
composed of four rows of cells with two cells in each row. The central cell divides into a
small primary canal cell and a large venter cell. The primary neck canal cell may divide
(depending upon the species) to form two neck canal cells. The primary venter cell divides
56
into a small venter canal cell and a large egg. To facilitate fertilization a fluid is formed by
the disintegration of all cells above the egg. The female gametophyte is surrounded by a
broad, ovoid, gelatinous funnel-shaped envelope extending from the protuberal portion
upwards.
The microspores are much smaller than the megaspores and are globose. Initiation of
development of the male gametophyte takes place by the nuclear division to form two
unequal cells within the spore wall. The smaller cell is the prothallial cell which fails to
divide further and is the sole representative of the vegetative cells of the prothallus. The
larger cell divides into two, followed by further divisions to form the antheridium which is
covered by jacket cells that surround sixteen spermatogenous cells that metamorphose into
coiled multiflagellate sperms. The flagella are attached only to the broad posterior coils of
the sperms. The prothallial cell and the jacket cells have meanwhile disintegrated. The
microspore wall bursts and the protruding antheridia rupture, releasing the sperms.
Fertilization is effected by swarms of sperms swimming into the gelatinous envelope.
Although many sperms may reach the vicinity of the archegonium, only one enters the
cytoplasm of the egg and penetrates the egg nucleus resulting in the formation of a zygote
and marking the end of the gametophyte generation. The gametophyte is endosporic and its
development is very rapid.
Embryogenesis: Two divisions in the zygote at right angles to each other form four cells.
Two of these cells (epibasal or anterior half) develop into the cotyledon and stem, and the
other two (hypobasal or posterior half) into the foot and root. Further embryonic
development is also accompanied by the stimulation of the cell divisions in the adjoining
tissues resulting in the formation of a sheathing calyptra. Growth of the two or three-celled
thick calyptra keeps pace with that of the embryo for sometime but the rapid development
of the primary root and cotyledon outpace it resulting in them bursting through the calyptra.
(Fig. 23)
Polyploidy and Pteridophytes.
Polyploidy is one of the outstanding features of the homosporous Pteridophytes. Although
there is an overall high incidence of polyploidy in these plants, there is no uniformity in its
distribution among the various living genera of these plants, nor amongst their supposed
primitive or advanced genera nor of habitat etc. It is common in some genera and totally
absent in others. All types of polyploidy i.e. autoploidy, genomic alloploidy, segmental
alloploidy are represented. Polyploidy is frequently associated with increase in dimensions
of spores and stomata.
It was earlier believed that polyploidy was co-related with increase in latitude or altitude.
High levels of polyploidy were considered to be a response to low temperatures. Later it
was believed that there was a relationship between polyploidy and climate. Tropical areas
were supposed to harbor more polyploids as they afforded more congenial conditions for
the growth of the gametophytes which were thought to be the bottle neck for breeding and
speciation. The temperate lands were with fewer polyploids. With more data coming in
from all parts of the world it became clear that a tropical climate alone is not in itself a
guarantee of high polyploidy. It is now believed that the stimuli to polyploid production are
complex and can not be ascribed to a single factor alone. Several factors are involved for
the geographical distribution of polyploids like (a) the geological age of the area (b) climate
(c) the composition of the flora i.e. if it has more species that have a deposition towards
polyploidy it will show a higher percentage of polyploidy, etc.
57
High basic chromosome numbers characterize the living homosporous Pteridophytes. These
high numbers are considered to have arisen through repeated cycles of polyploidy during
the long evolutionary history of these plants. According to Verma (2004) this assumed
58
palaeopolyploid status of the present-day diploids coupled with the feature of their
potentially monoecious gametophytes gave birth to a hypothesis on the evolution of their
genetic system as based on polyploidy (duplicated sets of chromosomes) to offset the
homozygotizing consequence of presumed habitual gametophytic selfing, by storing
heterozygosity at homoeologus loci and its release via occasional to frequent meiotic
paring between homoeologous chromo0somes together with a genetic restriction on
chromosome pairing to form only bivalents. However the tenets of this hypothesis are not
conclusive. Evidence obtained from electrophoretic approaches revealed genetic diploidy of
the present day diploids. Even if the present day diploids originated through ancient
polyploid events, the present day high basic chromosome numbers appear to be completely
genetically and chromosomally diploidizied (Verma 2004). Despite extremely high
chromosome numbers, they are genetically diploid organisms that show no genetic evidence
for high levels of polyploidy. Genetic evidence from enzyme electrophoresis also does not
support the claim that homosporous Pteridophytes are highly polyploid organisms. Data
suggests that these homosporous plants are diploid organisms without large numbers of
duplicated genes. There is also no genetic evidence that these plants maintain
heterozygosity through homoeologus chromosome pairing. Instead homosporous
Pteridophytes appear to maintain genetic variation through various levels of out crossing. In
nature, the gametophytes of these plants frequently cross-fertilize. A variety of mating
systems from inbreeding through mixed to out crossing are found in found in Pteridophytes.
Gene-flow estimates indicate that high levels of inter population gene flow may occur in
species of homosporous Pteridophytes. It was earlier emphasized that dramatic differences
in polyploidy, breeding systems, selfing rates, and gene flow existed between homosporous
and heterosporous Pteridophytes. Available evidence indicates striking overall similarities
between these two groups. If homosporous ferns are truly diploid organisms initiated with
high chromosomes numbers, fundamental difference could exist in genome organization
between these homosporous and heterosporous plants. These plants may possess fewer
active genes per chromosome, lower levels of gene linkage and large amounts of inactive
DNA.
The possibility of wholesale genetic diploidization in homosporous Pteridophytes is an
enigma clothed in mystery. The extremely high basic chromosome numbers in the present
day species reflect ancient polyploid lineages with diploidizied genomes. Until recently, it
was assumed that polyploids were actually static entities because of the genetic inertia
inherent following gene duplication. Several questions need to be answered: Are polyploids
subject to the same kinds of pressures as primary species or can they accumulate mutations
more rapidly because they have duplicate genes for each critical function, or do
allopolyploids diverge at the polyploid level to become new and cryptic species, or are
polyploids turning to the diploid level through gene silencing or are polyploids speciating
through reciprocal gene silencing to form tertiary species? (Haufler 1996; Werth &
Windham 1991).
However the issue of high chromosome numbers of polyploid origin and the mechanisms
involved in the whole-sale diploidization of genomes or silencing of duplicated genes
remains unsolved. Evidences from karyotype analysis, multiple origin of polyploids,
reproductive biology of the gametophyte generation and the nuclear DNA amounts all hold
promise towards a better understanding of the evolutionary biology of Pteridophytes
(Verma 2004).
COMPARTMENTALIZATION OF THE PTERIDOPHYTIC GENOTYPE.
The life cycle of the Pteridophytes is one of the best known examples of “The Alternation
of Generations”. The two morphologically distinct generations, sporophyte and
59
gametophyte, are except for the sporophyte in its embryonic phase, entirely independent of
each other in homosporous Pteridophytes. What are the causal factors of the phase change?
This has baffled people. Mehra (1972) believed that the alternation of generations cannot be
visualized either within the concept of interpolation of a new generation in response to
subaerial conditions of life, or the concept of transformation, known as the homologous
theory which considered the sporophyte as a modified version of the gametophytic
generation. He considered the alternation of generation as a part of the normal
morphogenetic process of growth and development a result of genetic mutations which have
had a positive survival value due to obvious advantages which this change conferred on
their progenitors. Mehra proposed the hypothesis on the inter conversion of the
gametophyte and sporophyte generations. He visualized it as an expression of various
processes imprinted in an organism in the form of genes whose precise functioning in slabs
and blocks, simultaneously or successively, leads to the organization of specific tissues and
organs. The fern system is highly plastic, and it can be manipulated at will. The entire
genome may be represented by a straight line, compartmentalized into four gene-blocks,
each determining root, leaf, stem (rhizome) and gametophyte. In each case the master gene
initiated their activity first, with the subsequent genes expressing themselves next in a
sequential order resulting in the expression of an organ or a different generation. It is
difficult to initiate a sub-unit of a gene block bypassing its predetermined chain sequence. It
is difficult to initiate sporangia on the roots, or palisade cells on the gametophytes, or
antheridia and archegonia on the leaves. In the gametophyte only the gene block for the
gametophyte is active and the remaining gene blocks are repressed. However the
appearance of sporangia directly on the gametophytes, formation of gametophytes on
protoplasts extracted from juvenile leaves, and zygotes freed and cultured yielding
gametophyte-like tissue need investigation. In the light of the gene block hypothesis. Bell
(1989) identified the sites responsible for phase change (gametogensis and sporogensis).
The number of genomes in a particular situation hardly matters in respect of differentiation.
But whenever there is more than one, their respective gene blocks act synchronously in a
particular situation. The genome of ferns is compartmentalized, as is evidenced from
genetic load in the form of recessive lethal genes operative only in the sporophytic
generation (Verma 2005).
The Pteridophytes represent the most intriguing group of plants. They occupy the second
position in numbers and luxuriance of the present day forests. Unfortunately interest in
these vanishing plants is on the wane and fast dwindling, as is evidenced by very few
researchers in this group of mystic plants. Some questions raised in this chapter it is hoped
will initiate and trigger a revival of research interest in Pteridophytes.
ACKNOWLEDGMENTS
The author wishes to thank the following for various types of help/information during the
preparation of this Chapter. Mr. C.R.Fraser-Jenkins (London), Prof. S.C.Verma
(Chandigarh), Prof. Y.P.S.Pangtey (Nainital), Prof. N.Punetha (Pithoragarh), Dr.
S.S.Sharma (Solan) and Dr. H.C.Pande (Dehra Dun) and Prof. A.K.Bhatnagar (Delhi).
Their help is gratefully acknowledged. I would also like to thank Dr. Rekha Mittal for co
operating and extending the date of submission of the manuscript, due to my ill health.
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