Download Text Like all other living organisms, land plants are also believed to

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Like all other living organisms, land plants are
also believed to have originated from their aquatic
ancestors. In this evolutionary transition from aquatic
to terrestrial habitats, a group of plants commonly
known
as
vascular
plants
(or
Tracheaophytes)
developed an efficient conducting system, consisting
of xylem and phloem. The evolution of conducting (or
vascular) system solved the requirement of water and
food transport throughout the plant body – an
essential requirement for an organism to grow on
land. Also, the cell wall in the vascular plants
synthesizes lignin – a complex polymer of phenolic
compounds - which provides rigidity to the plant body
to reach great heights.
Presently, a number of extinct and living groups
of the vascular plants are recognized under different
classification schemes. Traditionally, the three broad
groups
recognized
under
vascular
plants
are:
Pteridophytes, Gymnosperms and Angiosperms. The
Pteridophytes are the vascular plants which lack
seeds. Unlike the gametophyte in Bryophytes, the
sporophyte
Pteridophytes.
is
the
dominant
generation
in
The sporophytic plant body is well-differentiated
into true root, stem and leaves, which provide a
system well-suited to the demands of life on land.
Pteridophytes occur both in plains and mountains, as
well as in cold, moist and shady places. They also
occur in humid and tropical climates and usually grow
on soil, rocks, in ponds, and as epiphytes on other
plants. Many Pteridophytes, especially ferns, rely on
symbiotic association of the mycorrhizal fungi.
Pteridophytes play some important roles. Some
ferns, such as fiddleheads of bracken (Pteridium
aquilinum), ostrich fern (Matteuccia struthiopteris),
and cinnamon fern (Osmunda cinnamomea) are used
for food. Polypodium glycyrrhiza (licorice fern) roots
are chewed for their pleasant flavor. Dryopteris filixmas (male fern) is used as a vermifuge. Tree ferns
are used as building material in some tropical areas.
Many ferns are grown in horticulture as landscape
plants, for cut foliage and as houseplants, e.g. the
Boston fern (Nephrolepis exaltata), Bird's Nest Fern
(Asplenium nidus). Rumohra adiantiformis (floral fern)
is extensively used in the florist trade. Microsorum
pteropus (Java fern) is one of the most popular
freshwater aquarium plants. Selaginella lepidophylla
(resurrection plant) is sold as a novelty item because
it can revive from a dry condition to a fresh green one
with just few drops of water. It is also used in herbal
medicine to cure cold and throat infections.
The very small, floating mosquito fern (Azolla)
are used as a biological fertilizer in the rice fields due
to their ability to fix atmospheric nitrogen. Ferns, such
as Pteris vittata (brake fern) have been studied and
found to be useful in the removal of heavy metals,
especially arsenic, from the soil. The important fossil
fuel coal consists of the remains of primitive plants,
including ferns.
Several ferns are noxious weeds or invasive
species, including Japanese climbing fern (Lygodium
japonicum), mosquito fern and sensitive fern (Onoclea
sensibilis). Giant water fern (Salvinia molesta) is one
of the world's worst aquatic weeds.
(I) General characters of pteridophytes
The plant body of pteridophytes comprises of two
distinct
phases
during
their
life
cycle:
diploid
sporophyte and haploid gametophyte. The diploid
sporophyte is the dominant phase in the life cycle, the
sporophyte being independent of gametophyte.
The sporophytic plant body
The
sporophyte
herbaceous
and
is
(diploid
phase)
differentiated
into
is
generally
true
roots
(adventitious), stem and leaves. The leaves may be
small microphyllous or large macrophyllous (fronds).
They
are
chlorophyllous
and
autotrophic.
All
vegetative parts possess vascular tissues organized
into vascular bundles or steles. So, pteridophytes are
the first vascular plants in evolution of Plant Kingdom.
The sporophyte performs vegetative as well as
asexual reproduction. Vegetative propagation takes
place by buds that develop on the rhizome, or by
fragmentation of rhizome. Asexual reproduction takes
place
by
means
of
spores
produced
inside
the
sporangia. The sporangia are borne on lower surface,
or in axils, of fertile leaves called sporophylls. The
sporangia are borne singly, or in groups called sori.
Plants may be homosporous (i.e., they produce only
one type of spores), or heterosporous (i.e., produce
two different types of spores - smaller microspores
and larger megaspores). Within the sporangia are
produced haploid spores by meiosis. The spores, in
turn, germinate to produce a haploid gametophyte.
The gametophytic plant body
A haploid spore germinates and develops into the
gametophyte. It is a small and simple structure with
short
life
span,
and
is
called
as
prothallus.
Homosporous species produce bisexual gametophytes.
Heterosporous
species
produce
unisexual
gametophytes - microspore germinates to produce
male gametophyte, and the megaspore produces
female gametophyte. Gametophytes show the ventral
and dorsal differentiation. The gametophyte is usually
photosynthetic (except in heterosporous members)
and reproduces sexually by oogamy. The male sex
organs, called antheridia, produce sperms by mitosis.
Female sex organs, called archegonia, produce egg by
mitosis.
Both
the
sex
organs
(antheridia
and
archegonia) are multicellular, with sterile jacket, but
without stalks. Fertilization occurs in presence of
water and takes place in the venter of archegonium.
The diploid zygote develops into embryo in the
archegonial venter. The embryo grows by mitosis into
a sporophyte, thus completing the life-cycle.
Therefore,
the
life-cycle
in
pteridophytes
is
diplohaplontic, i.e. it shows heteromorphic alternation
of generations between sporophyte (diploid) and
gametophyte (haploid), which are independent of
each other.
(II) Classification of pteridophytes
Several systems of classification for pteridophytes
have been proposed. Eames (1936), Tippo (1942) and
others placed all the vascular plants (pteridophytes
and
spermatophytes)
Tracheaophyta
parallel
in
a
with
single
Division,
Thallophyta
and
Bryophyta. They segregated Tracheaophyta into four
sub-divisions:
Psilopsida
(psilophytes),
Lycopsida
(club mosses, spike mosses, quillworts), Sphenopsida
(equisetophytes)
Gymnospermae
and
and
Pteropsida
Angiospermae).
(Filicinae,
Bold
(1957)
abandoned the term Tracheaophyta as a Division.
Instead, he classified all the vascular cryptogams into
four
Divisions:
Arthophyta
and
Psilophyta,
Microphyllophyta,
Pterophyta.
Following
this,
Zimmermann (1959) divided pteridophytes into five
Divisons:
Psilophyta,
Lycophyta,
Sphenophyta,
Noeggerathiophyta and Pterophyta. One of the most
recent systems of classification for the extant ferns is
that of Smith et al., 2006, which is based on both the
morphological and molecular data. Despite these
different systems of classifications proposed from time
to time, majority of the pteridologists at present
broadly recognize seven groups for those plants
previously grouped together as pteridophytes under
the Kingdom Plantae. These are:
1. Rhyniophytes
2. Lycophytes
3. Ophioglossoid ferns
4. Psilophytes
5. Equisetophytes
6. Marattioid ferns
7. Pterophytes
III. Diagnostic features of pteridophyte groups
1. Rhyniophytes
• The rhyniophytes, represented by only the extinct
fossil plants, were among the first vascular land
plants
•
It includes the genus Rhynia, a well-known
vascular plant from the early Devonian, ca. 410360 million years ago.
• The
sporophytes
consisted
of
dichotomously
branched axes that bore terminal sporangia.
• They lacked both roots and a leaf-bearing shoot
system: these two features evolved later within
the Lycophytes.
• The stems of rhyniopytes were protostelic, in
which the protoxylem (i.e. first-formed xylem)
was at the centre (i.e. centrarch).
2. Lycophytes
• The lycophytes (or lycopods) are a lineage of
plants that diverged after the rhyniophytes.
• The
roots
of
lycophytes
have
an
endarch
protoxylem.
• The stems have an exarch protoxylem, just
reverse of the roots.
• Lycophytes have a sporophytic leaf structuraltype known as lycophyll. These are characterized
by absence of a gap in the stem vasculature, and
have a single unbranched vein.
• The
lycophytes
comprise
of
several
extinct
genera, e.g. Lepidodendron, Sigillaria.
• The only lycophytes that survived to the present
are small herbaceous plants, usually grouped into
three families: Lycopodiaceae, Selaginellaceae,
and Isoetaceae.
• The Lycopodiaceae, which are commonly called
“club mosses”, are distinguished in having one
type of spore (i.e., homospory).
• The group contains about 300 species, distributed
among 5 genera. Although resembling mosses,
they are true vascular plants wherein the longlived phase is the sporophyte.
• Sporangia of the Lycopodiaceae develop laterally
in the axils of the specialized leaves, called
sporophylls.
• The sporophylls differ in size and shape from the
vegetative leaves and are aggregated into a
terminal shoot system.
•
This determinate reproductive shoot, consisting
of a terminal aggregate of sporophylls with
associated sporangia, is known as a strobilus or
cone.
• The
two
other
families,
Selaginellaceae
and
Isoetaceae, differ from Lycopodiaceae in being
heterosporous.
• These also differ in having a characteristic flap of
tissue, called ligule, on the upper surface of
leaves near the base.
• Heterospory, occurring in these 2 families, refers
to
the
production
of
two
types
of
spores:
microspores and megaspores.
•
The gametophytes of Selaginella and Isoetes are
endosporic, they develop entirely within original
spore wall.
• The Selaginellaceae contain ca. 700 species in
the single genus Selaginella, commonly called
spike-moss. These occur in two vegetative forms:
(a)
some
have
spirally
arranged
vegetative
leaves that are isomorphic, i. e. only of one size
and shape; (b) others have leaves that are
dimorphic, two lateral rows of larger leaves and
two dorsal rows of smaller leaves.
• The Isoetaceae consist of ca. 150 species in the
single genus Isoetes, commonly called quillwort
or Merlin’s grass. The plants consist of a cormlike stem that has roots attached below and
numerous needle-like leaves above. These are
small herbs with prostrate rhizomes that bear
true roots and upright shoots but lack secondary
growth.
• The
leaves
are
microphyllous
and
spirally
arranged on their shoots. The sporangia are
arranged in cones or along the shoots.
3. Ophioglossoid ferns
• The ophioglossoid ferns are unique in that each
leaf (i.e. frond) consists of a sterile segment,
which
contains
the
photosynthetic
blade
or
lamina, and a fertile segment.
• The most common genera are Ophioglossum,
commonly called adder’s tongue and Botrychium,
commonly called grape fern or moonwort.
• Whereas the species of Ophioglossum have a
simple undivided lamina and an unbranched
fertile segment, those of Botrychium have a
compound lamina and a branched segment.
• The roots are unusual in lacking both root
branches and root hairs.
•
The
gametophytes
are
non-photosynthetic,
subterranean and may contain mycorrhizal fungi.
4. Psilophytes
• The psilophytes (commonly called “whisk ferns”)
consist of only two living genera, with Psilotum
(two species) and Tmesipteris (10 species).
• Psilotum
occurs
in
tropical
and
subtropical
regions of the world, but Tmesipteris is restricted
to Australia and South Pacific Islands.
•
Psilotum nudum, the whisk broom, is the most
widespread species of the psilophytes.
• Psilophytes have an independent, dominant, freeliving sporophyte; the haploid gametophyte is
small, subterranean, free-living and may contain
mycorrhizal fungi.
•
The sporophyte consists of a horizontal rhizome
that gives rise to aerial, photosynthetic, generally
dichotomously branching stems.
• All the psilophytes are homosporous, producing
spores that are of the same size and shape.
• They lack true roots; only absorptive rhizoids
arise from the rhizome.
• The leaves of psilophytes are reduced, peg-like
structures and may lack a vascular strand in
which case they are termed enations.
• The sporangia are two-, or three-lobed which is
called as synangium. The synangia are yellowish
at maturity and are subtended by a forked
appendage.
5. Equisetophytes
• The equisetophytes (also called sphenophytes)
contain several genera of extinct plants, and a
single living genus, Equisetum, with 15 species.
• These
are
homosporous
herbs
without
any
secondary growth.
• The leaves are small, megaphyllous, with a single
trace of vascular tissue.
• Today, the only living equisetophytes are species
of
the
genus
Equisetum,
commonly
called
“horsetails”.
•
They have an extensive underground rhizome
system with adventitious roots; the rhizome gives
rise to erect, aerial shoots.
•
The ribbed stems contain epidermal cells that
are impregnated with silica.
• The stems are hollow with cross-walls called
septa at each node.
• The leaves are whorled and laterally fused to
form a sheath-like structure at the nodes.
• At the tip of some aerial stem are strobili or
cones, containing sporangia, which are pendant
from
a
stalked,
peltate-structure
called
the
sporangiophore.
• The spores are unique among the vascular plants
in
containing
appendages
choloroplasts
called
elators.
and
The
unusual
elators
are
hygroscopic and uncurl the spore body upon
drying, thus aiding in spore dispersal.
• The gametophyte is small (1 mm across) green
mass of parenchyma with no vascular tissue.
6. Marattioid ferns
• These are a group of six genera which were
traditionally
grouped
with
pterophytes
(i.e.
leptosporangiate ferns).
•
However,
unlike
true
ferns
(i.
e.,
leptosporangiate ferns), the sporangia are of
eusporangiate type.
• They have large pinnate or bipinnate leaves with
circinate vernation.
• The sporangia are located on the lower surface of
leaflet blades.
•
In some cases, the sporangia are fused into a
common structure called synangium.
• The gametophyte is photosynthetic.
7. Pterophytes
• The pterophytes (also known as Filicales or
Polypodiales or Pteridales) refer to what are
commonly known as leptosporangiate ferns.
• It contains the greatest diversity with ca. 11,000
living species.
• These occur in almost every habitat, terrestrial
and aquatic; the majority occurs in tropics but
some are found in the temperate mountain
regions.
• All are perennial and herbaceous.
• Although some attain the size of small trees
(upto 100 feet) – the so called “tree ferns” –
however they never have secondary growth.
• The sporophyte consists of a single axis, either an
upright shoot or a prostrate rhizome that bears
both true roots and megaphyllous leaves.
• The leaves contain vascular tissue and the veins
are dichotomously branched.
• The
leaves
are
called
as
fronds,
and
are
compound with a rachis and leaflets.
•
These are of various shapes and sizes, ranging
from less than 1 cm long in Azolla microphylla to
several meters long in the giant tree ferns.
•
Initially the fronds are in coiled form and expand
by unrolling from base to tip with new growth
produced at the apex.
• In addition to photosynthesis, the leaves also
serve as sporophylls.
• On the underside of the sporophylls are sori
(singular:
sorus), which
are
formed
by
the
clusters of sporangia.
•
On the outer rim of the sporangium is a single
row of specialized cells, collectively known as an
annulus, which helps in spore dispersal.
•
The sori may or may not be covered by a flap of
tissue called the indusium.
• The
gametophyte
is
small
heart-shaped
structure, covered with rhizoids on the lower
surface but without vascular tissue.
• The antheridia are borne among the rhizoids and
the archegonia occur close to the gametophyte
apex
• All the aquatic ferns are heterosporous. The
reproductive
structures
are
complex
generally organized into spherical sporocarps.
The sporocarps allow the sporangia and spores to
remain dormant for long periods of time – an
adaptation that enables them to survive when the
ponds dry up seasonally.
and