Download Consortium for Educational Communication

Consortium for
Educational
Communication
Module on
General Characters of Gymnosperms
By
Syed Akhlaq Geelani
Lecturer in Botany
Mobile No.9622841493
E-mail:[email protected]
Consortium for Educational Communication
Text
The word gymnosperm has been derived from a
Greek words gymnos meaning naked and sperma meaning
seeds. Thus the gymnosperms are group a of plants with
naked seeds. The word gymnosperm was first used by
Theophrastus in 300 B.C. in his book “Enquiry into Plants”.
Theophrastus included all the plants with naked seeds
under gymnosperms. They have been generally placed
in the division Spermatophyta, along with angiosperms.
Gymnosperms show close affinity with pteridophytes on
one hand and with the angiosperms on the other. Thus the
gymnosperms act as a bridge between pteridophytes and
angiosperms. The gymnosperms originated during the late
Paleozoic era but flourished well during Mesozoic era. Hence
the Mesozoic era is called as the age of gymnosperms.
However, most of the members of this group have now
become extinct and less than a hundred genera are known
today. Some gymnosperms like Ginkgo biloba, and species
of Cycas and Metasequoia are called living fossils, as these
plants are found both in living and fossil forms, and the
number of fossil forms is much more than the living forms.
There are about 70 genera with 725 living
species of gymnosperms in the world today. In India, there
are 16 genera with about 53 species. These genera are Cycas
from Cycadales; Abies, Cedrus, Cephalotaxus, Cupressus,
Juniperus, Larix, Picea, Pinus, Podocarpus, Taxus and Tsuga
from Coniferales; Ephedra from Ephedrales; and Gnetum from
Gnetales. In addition, Ginkgo biloba, a species from ChinaJapan, is also grown in certain parts of India.
Occurrence and Distribution
1.Gymnosperms are found both in tropical and temperate
regions; some species occur in arctic regions. The living
gymnosperms comprise 70 genera and 725 species.
Of these, majority belong to conifers or cone- bearing
plants. The conifers are mostly distributed in the cooler
regions of Europe, Asia and North America. They are
dominant in upper timber-line zone of mountain ranges
including the Himalayas.
Consortium for Educational Communication
2.The living gymnosperms are widely distributed in the
cold climates where snow, rather than rain, is the source
of water. The only native gymnosperms which thrive in
warm areas are the cycads.
3. In India, mostly the conifers thrive well in hilly areas.
Very few gymnosperms, except some cycads, grow in
plains. Cycas circinalis grows wild in some southern
States. Some gymnosperms are grown as ornamental
plants in gardens, e.g. species of Araucaria.
Habit
1.Living gymnosperms are mostly perennial, xerophytic,
mostly evergreen, arboreal and woody plants. They grow
as woody trees, bushy shrubs or rarely as climbers, e.g.
Gnetales. None of them are herbs or annuals.
2.Gymnosperms vary in their size from small herbs,
e.g. (Zamia pygmaea) to large gigantic trees. Sequoia
sempervirens, a gymnosperm, is the tallest living plant
in the world, reaching a height of about 125 meters,
and girth of about 30 meters. It is also known as
coast red-wood of California. Taxodium mucronatum
and Sequoiadendron giganticum with a girth of 125
feet and 80 feet, respectively, are the other gigantic
gymnosperms. On the contrary Zamia pygmaea,
with an underground tuberous stem, is the smallest
gymnosperm, about 25 cm in height.
External morphology
1. Most of the gymnosperms are tall, evergreen trees or
shrubs. There are no herbs. Certain species of Gnetum
are twiners.
2.The plant body is sporophyte and differentiated into root,
stem and leaves.
3.Plants posses well-developed tap root system. Root often
shows symbiotic association with algae, e.g. Cycas, or
with fungi e.g. Pinus (Fig.1). In Cycas, coralloid roots are
Consortium for Educational Communication
present in addition to the normal roots.
4.The stem is aerial, erects, woody, unbranched, as in
Cycas, or branched as in Pinus, Cedrus, etc. In some
genera like Pinus, the branches are of two types.
i). Long shoots or branches of unlimited growth
ii). Dwarf shoots or branches of limited growth.
5.The leaves are mostly xerophytic, evergreen, simple
or compound, and vary in size from small scaly leaf
to more than six feet long leaves as in Cycas. The
leaves of Pinus are of two types, i.e. scale leaves and
foliage leaves. Scale leaves are minute, membranous,
brown and deciduous. Foliage leaves are present on
dwarf shoots only, and they are known as foliage
spurs (Fig. 2). They are green, simple, needle-like
or pinnately compound. The leaves of Cycas exhibit
circinate vernation, a characteristic feature of ferns. The
arrangement of leaves may be opposite and decussate
as in Gnetum, whorled as in Cedrus, or spirally arranged
as in Taxus.
Internal structure
Stem
1.Anatomically the stem shows an epidermis, cortex and
stele. The epidermis is single-layered and very soon
replaced by periderm. Cortex is parenchymatous. Based
on the relative quantity of soft tissues and mechanical
tissues, gymnosperms are classified into manoxylic and
pycnoxylic types.
2.In manoxylic forms, as in Cycads, the wood is not
compact, and has parenchyma. In pycnoxylic forms
wood is compact, and relatively less amount of soft
tissue is present. The stele is surrounded by a typical
epidermis followed by a layer of pericycle.
3.The vascular system consists of a number of vascular
bundles arranged in the form of a ring, surrounding the
central pith.
Consortium for Educational Communication
4.The vascular bundles are collateral, endarch and open.
This type of stele is called eustele.
5.Phloem consists of only sieve tubes and phloem
parenchyma. Companion cells are absent.
6.Xylem consists of trachieds and xylem parenchyma
only. Vessels are absent except in Gnetales. The xylem
is either endarch or mesarch.
7.Secondary growth is seen in all the members. It is
usually normal. However, anomalous secondary growth
may be found in Cycas and Gnetum. Normal secondary
growth takes place due to the activity of fascicular
cambium, which produces the typical annual rings.
8.Parenchymatous pith is present in the central region of
the stem. In some cases mucilage ducts or resin canals
are found both in the pith and cortex (Fig.3).
Internal structure of Leaf
1.Anatomically, the leaf exhibits xerophytic characters.
The epidermis is always covered by a cuticle. The
stomata are sunken, and often present on pits.
2.The mesophyll may be differentiated into palisade and
spongy parenchyma as in Cycas or undifferentiated
with arm parenchyma as in
Pinus (Fig.4).
3.In some members, as in Cycas with a single-veined
leaf, a transfusion tissue is present to help in lateral
conduction, necessitated by the absence of side veins.
4.The stomata are sunken and often present on pits.
The stomata are either syndetocheilic or haplochelic.
5.In syndetocheilic type, guard cell and subsidary cells
originate from the same mother cell. In haplochelic
type, the guard cells and subsidiary cells originate
from the different mother cells. Haplocheilic
stomata are considered to be primitive compared to
syndetocheilic.
Consortium for Educational Communication
6.In Gnetum both types of stomata can be seen.
Stomata may be distributed on both sides of leaf as in
Pinus and Ginkgo, or confined to the lower side as in
Cycas.
Internal structure of root
1.Most of the gymnosperm roots show a typical
dicotyledonous root structure; the xylem is either diarch
or triarch. Cortex is mostly parenchymatous. As in the
stem, secondary growth is seen both in the cortex and
the stele.
Reproduction
1.Vegetative reproduction is very rare, and is seen in
some cycads where it is brought about by the formation
of bulbils.
2.Spore formation is the typical mode of reproduction in
gymnosperms.
3.All the gymnosperms are heterosporous, i.e. they
produce two different kinds of spores, the male
microspores and the female megaspores. The spores are
borne inside the sporangia.
4.The two types of sporangia are born on special leaflike structures, called sporophylls. The microsporangia
or pollen sacs are born on microsporophylls or
stamens, and the megasporangia or ovules are born on
megasporophylls or carpels (Fig.5).
5.The sporophylls are usually aggregated in the form of
compact structures, called cones or strobili. Plants may
be monoecious or diocious with reference to strobili.
Very rarely bisporangiate strobili may be found.
6.Male cones are generally short-lived and smaller than
female cones. Microsporophylls may be flat, as in Cycas,
or peltate as in Pinus.
7.Female cones are usually long-lived. They sometimes
Consortium for Educational Communication
remain on the plants for several years till maturity and
ripening of the seeds. Megasporophylls may be foliar or
leaf-like and not aggregated in the form of cones as in
Cycas (Fig. 6), or cauline and aggregated in the form of
cones as in Pinus.
8.Microsporangia are born on the abaxial or
morphologically lower surface of microsporophylls. They
may be numerous and grouped into sori as in Cycas or
reduced to two as in Pinus.
9.In Cycads, the male cones are the largest among the
gymnosperms, and arise singly at the top of the plant.
Female cones are not formed, but megasporophylls are
loosely arranged around the stem apex.
10.
In Pinus, the male cones arise in clusters, at the
end of the long shoots, in the axil of scale leaves. Female
cones are large and are arranged in two or three pairs on
the long shoots (Fig.7).
11.
In Gnetum the reproductive structures are known
as inflorescence. The male inflorescence or male cone
consist a long axis, on which cup-shaped structure,
called cupules, are formed by the fusion of brackets. In
the axil of these cups, male flowers are born. The female
inflorescence or female cone also contains a long axis
with cupules. In the axil of these cupules, 4-10 female
flowers or ovules are arranged in a ring. Bisporangiate
strobili are also noticed in some species of Gnetum.
Ovule and female gametophyte
1.In gymnosperms, ovules are naked, and born on
megasporophylls. The ovules of Cycas are largest among
the plant kingdom. The ovule consists of central mass
of parenchymatous tissue called nucellus. The nucellus
is surrounded by integuments, which covers the whole
nucellus except at the top, where it leaves a small
passage called microphyle (Fig. 8).
Consortium for Educational Communication
Consortium for Educational Communication
2.The integument is differentiated into an outer fleshy
layer or sarcotesta, middle stony layer sclerotesta and
the inner fleshy layer or sarcotesta. The apex of the
nucellus develops into pollen chamber, where pollen
grains undergo period of rest after pollination.
3.The development of female gametophyte begins with the
differentiation of megaspore mother cell in the nucellus.
Its development is either monosporic or tetrasporic.
Megaspore mother cell undergoes meiosis to form
linear tetrad of four megaspores, out of which, upper
three cells degenerate, and the lower most remains as
functional megaspore.
4.The functional megaspore then develops into female
gametophyte. Archegonia are formed towards the
micropylar end of the female gametophyte. In Gnetum
archegonia are not formed. A few nuclei at the
microphylar end, of the female gametophyte act as egg,
which is an angiospermic character. The archegonia of
gymnosperms do not have neck canal cells and Venter
canal cells.
Microspore and male gametophyte
1.Pollen grains or microspores are unicellular. In Pinus
pollen grains are winged (Fig.9). Microspores develop
into male gametophyte. During the germination of
microspores, one or two prothelial cells are cut off at
one end, leaving large anthredial cell, which divides into
generative cell and larger tube cell. The generative cell
divides into stalk cell and body cell. Body cell produces
two male sperms. The tube cell forms the pollen tube,
which acts as carrier of sperms (Fig.10).
Formation of male gametophyte
Pollination
1.In gymnosperms, pollination is anemophilous. The
ovule secretes mucilaginous substance, which oozes out
from the micropyle of the ovule and forms a pollination
drop, which traps the pollen grains. The pollen grains
are drawn on due to drying of pollination drop. Further
changes take place in the pollen chamber. The pollen
tube grows through the tissue of the nucellus, until it
reaches the neck of the archegonia.
Fertilization
1.Gymnosperms do not require water for fertilization.
2.At the time of fertilization, the tip of the pollen tube
bursts and releases the sperms. The sperms swim to the
neck of the archegonia with the help of cilia, and one
of the sperms fuses with the egg to form the zygote. In
Pinus and Gnetum the sperms are non-motile, which is
an advanced character.
3.This type of fertilization is called siphonogamy.
The young sporophyte
Consortium for Educational Communication
1.After fertilization the zygote is formed. In gymnosperms
the development of zygote is meroblastic, in which only
the basal (embryonal) part develops into an embryo
or embryos, while the upper (houstorial) and middle
(suspensorial) parts do not take part in the formation
of embryo. Embryo is straight with the exception
Bennettitales.
Embryogeny
1.In gymnosperms, except in Gnetum, the diploid nucleus
undergoes free nuclear division. In Cycas, as many as
1000, or more, nuclei are formed as a result of free
nuclear division. In Pinus, only four nuclei are formed,
after that the wall formation begins, resulting in the
formation of 16-celled proembryo.
Polyembryony
1.Polyembryony is the characteristic feature of
gymnosperms. In cycads, more than one archegonia
are fertilized, resulting in the formation of more than
one embryo. This phenomenon is known as simple
polyembryony. In conifers, polyembryony is due to the
separation of four embryonal cells, and each of them
develops into an embryo. This phenomenon is called
cleavage polyembryony. It occurs in Pinus, but only one
embryo attains maturity and the rest degenerate.
2.In gymnosperms, the embryo is differentiated into root,
stem and leaf. It is endoscopic, which means plumule is
directed away from micropyle and radical towards the
microphyle (Fig.11).
Seed
1.After fertilization, the ovule develops into seed. The
inner fleshy layer of ovule turns into tegmen, and middle
stony layer forms testa or the seed coat, which protect
inner embryo.
2.Embryo is developed at the end of long suspensor which
is pushed down into the food ladden endosperm that
develops before fertilization. Female gametophyte,
Consortium for Educational Communication
soon after it has absorbed the food from the nucellus,
becomes endosperm.
3.As there is no double fertilization or triple fusion, the
endosperm is a haploid tissue.
4.The whole of the nucellus, except a small portion,
is crushed. The remaining small part of the nucellus
persists in the seed in the form a narrow film, or cap,
known as perisperm.
5.Since there is no closed ovary, true fruits, like that of
angiosperms are not found in the gymnosperms. The
developed ovules remain naked.
6.Middle stony layer of the integument becomes
extremely hard and protects the female gametophyte
and embryo.
7.Cotyledons may be one or two, as in Cycas, or a whorl
of many, as in Pinus. They become green while still
enclosed within the seeds.
8.The seed generally undergoes a period of dormancy.
However, the seed of Ginkgo and Cycas germinates
without undergoing any period of rest.
9.During germination, the seed coat ruptures, radical
grows downwards and develops into primary tap root,
while the plumule grows upright and develops into
stem, whose growth is mostly unlimited except in
Welwitschia.
10.
Sporophytic generation is diploid and is dominant
and independent, while the gametophytic generation is
much reduced and dependent.
Affinities
Gymnosperms form a heterogeneous plant group with
several evolutionary trends which have been operative
since past. Evolutionary trends reveal that gymnosperms
form a bridge between pteridophytes and angiosperms.
Keeping in view their resemblance with pteridophytes and
angiosperms on one hand, and certain difference with these
Consortium for Educational Communication
groups on the other hand, they have been segregated as an
independent group of plants.
Resemblance with Pteridophytes
Gymnosperms resemble with pteridophytes in the
following features.
1.Both the plant groups show heterologous alternation
of generation. Sporophytic generation is dominant,
independent and large at maturity, while the
gametophytic generation shows progressive reduction
and dependence.
2.Sporophytes of both the plant groups are distinguished
into root, stem and leave, and possess well-marked
vascular system.
3.Many cycads resemble ferns in having large compound
leaves and circinate vernation.
4.Both heterosporous pteridophytes and gymnosperms
possess endosporic gametophytes.
5.Male gametes of cycads (Cycas) and Ginkgo are motile
like that of pteridophytes.
6.Female sex organs are archegonia in both pteridophytes
and gymnosperms.
7.Male gametophytes of heterosporous pteridophytes and
majority of gymnosperms are highly reduced and posses
only two prothalial cells.
Consortium for Educational Communication
pteridophytes
Gymnosperms differ from pteridophytes in the
following features:
1.Gymnosperms are commonly large-sized trees, shrubs or
climbers. Pteridophytes on the other hand, are smaller in
size.
2.Gymnosperms posses tap roots whereas roots of
pteridophytes are adventitious.
3.Gymnosperms have eustelic organisation, whereas in
pteridophytes eustele is not found.
4.Gymnosperms undergo secondary growth, whereas
pteridophytes lack secondary growth.
5.In gymnosperms male and female gametophytes are
simple, non-green and dependent on sporophytes. They
are very much reduced in comparison to gametophytes
of pteridophytes.
6.In gymnosperms megaspore remains in
megasporangium, whereas in pteridophytes they are
shed from the sporangia and develop into independent
female prothallus.
7.In gymnosperms, neck canal cells and venter canal cells
are absent, whereas archegonia of pteridophytes are
provided with neck canal cells and venter canal cells.
8.Seed formation is not common in pteridophytes except
in Selaginella, whereas in gymnosperms seeds are
produced.
8.Endosperm develops before fertilization, as a result of
free nuclear division in the tissue of female gametophyte
in both gymnosperms and pteridophytes.
Affinities with angiosperms
9.Embryogeny is endoscopic in many pteridophytes and
majority in gymnosperms
Gymnosperms resemble with angiosperms in the
following features.
1.Plants of both the groups are woody and perennial.
Differences between gymnosperms and
2.The stele is eustelic both in gymnosperms and
angiosperms.
Consortium for Educational Communication
3.Both show secondary growth, except monocots.
4.Both are heterosporic and endosporic.
5.Megaspore is retained in megasporangium, resulting in
the formation of seed.
6.The male gametophytes are highly reduced.
7.Pollen tube acts as carrier of sperms.
8.The sperms are non-motile except in Cycas.
9.The plants show distinct alternation of generations.
Differences between gymnosperms and angiosperms
Gymnosperms differ from angiosperms in the following
features:
1.Majority of angiosperms are herbaceous, whereas
gymnosperms are mostly woody.
2.Angiosperms may be annuals, biennials or perennials,
whereas gymnosperms are mostly perennials.
3.In angiosperms, xylem possesses vessels and phloem
bears companion cells, whereas in gymnosperms (except
in Gnetales) xylem is devoid of vessels and phloem lacks
companion cells.
4.In gymnosperms cones are unisexual, whereas in
angiosperms the flowers are usually bisexual.
5.In gymnosperms ovules are naked, whereas in
angiosperms they are enclosed in the ovary of the
carpels.
6.Gymnosperms do not show double fertilization or
triple fusion, whereas in angiosperms it is a common
phenomenon.
Consortium for Educational Communication
7.Angiospermic seeds are enclosed within the ovaries that
develop into fruits, whereas in gymnosperms seeds are
exposed.
Classification of gymnosperms
Chamberlain (1934) divided the gymnosperms into two
large groups:
A.Cycadophyta: - In this group sporophylls are in cones.
It includes three groups; Cycadofilicales, Bennettitales
and Cycadales.
B.Coniferophyta:- This group included four orders;
Ginkgoales, Cordaitales, Coniferales and Gnetales.
Of these Cycadofilicales, Bennettitales and Cordaitales
are totally extinct. Cycadales, Ginkgoales, Coniferales and
Gnetales are living.
1.Arnold (1948) divided gymnosperms into three phyla.
a.Cycadophyta – including pteridospermales,
Cycadeoidales and Cycadales.
b.Coniferophyta – including Cordaitales, Ginkgoales,
Taxales and Coniferales.
c. Chlamydospermophyta- including Ephedrales and
Gnetales.