Gymnosperms
•
The gymnosperms (gymnos : naked, sperma : seeds) are
plants in which the ovules are not enclosed by any ovary wall and remain
exposed, both before and after fertilisation. The seeds that develop post-fertilisation,
are not covered, i.e., are naked.
• Gymnosperms include medium-sized trees or tall
trees and shrubs. One of the gymnosperms, the giant redwood tree Sequoia is one
of the tallest tree species.
• The roots are generally tap roots. Roots in
some genera have fungal association in the form of mycorrhiza (Pinus), while in some others (Cycas
(zamia)) small specialised roots called coralloid roots are associated with N2-
fixing cyanobacteria. The stems are unbranched (Cycas)
or branched (Pinus, Cedrus).
The leaves may be simple or compound.
Gymnosperms: (a) Cycas (b) Pinus (c) Ginkgo
• In Cycas the pinnate
leaves persist for a few years. The leaves in gymnosperms are well-adapted to
withstand extremes of temperature, humidity and wind. In conifers, the
needle-like leaves reduce the surface area. Their thick cuticle and sunken
stomata also help to reduce water loss.
• Gymnosperms are seed-bearing plants that lack
the combination of specialized features that characterize the flowering plants.
The name gymnosperm, means naked seed. (e.g.
gymnasium); that is, the seeds are not enclosed within fruits.
• The gymnosperms are heterosporous;
they produce haploid microspores and megaspores.
• The two kinds of spores are produced within
sporangia that are borne on sporophylls which are
arranged spirally along an axis to form lax or compact strobili or cones. The
strobili bearing microsporophylls and microsporangia
are called microsporangiate or male strobili.
• The microspores develop into a male gametophytic generation which is highly reduced and is
confined to only a limited number of cells. This reduced gametophyte is called
a pollen grain.
• The development of pollen grains take place
within the microsporangia. The cones bearing megasporophylls
with ovules or megasporangia are called macrosporangiate or female strobili. The male or female
cones or strobili may be borne on the same tree (Pinus)
or on different trees (Cycas).
• Pre-fertilized endosperm is characteristic of
Gymnosperms.
• The megaspore mother cell is differentiated
from one of the cells of the nucellus. The nucellus is protected by envelopes and the composite
structure is called an ovule. The ovules are borne on megasporophylls
which may be clustered to form the female cones.
• The megaspore mother cell divides meiotically to form four megaspores. One of the megaspores
enclosed within the megasporangium (nucellus) develops into a multicellular female gametophyte
that bears two or more archegonia or female sex organs. The multicellular
female gametophyte is also retained within megasporangium.
• In gymnosperms, the male and the female
gametophytes do not have an independent free-living existence. They remain
within the sporangia retained on the sporophytes.
• The pollen grain is released from the
microsporangium. They are carried in air currents and come in contact with the
opening of the ovules borne on megasporophylls. The
pollen tube carrying the male gametes grows towards archegonia in the ovules
and discharge their contents near the mouth of the archegonia. Following
fertilisation, zygote develops into an embryo and the ovules into seeds. These
seeds are not covered.
• Gynoecium, the female reproductive whorl of
flower, consists of carpels (megasporophylls).
• A
carpel is differentiated into 3 parts-
Ø stigama
Ø style
Ø ovary
•
When gynoecium is sterile or underdeveloped, it is called pistillode.
Stamen is a modified leaf or microsporophyll. Bennettites
is the Fossil member.
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