3. PLANT KINGDOM
GYMNOSPERMS
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Gymnosperms (gymnos: naked, sperma:
seeds) are plants in which the ovules are not enclosed by ovary wall and
remain exposed before and after fertilization. Seeds that develop post-fertilization
are not covered (naked).
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They include medium-sized trees or tall
trees and shrubs. Sequoia (giant redwood) is the tallest tree species.
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The roots are generally tap roots.
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Roots in some genera have fungal
association in the form of mycorrhiza (E.g. Pinus).
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In plants like Cycas, small specialized
roots (coralloid roots) are associated with N2- fixing
cyanobacteria.
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Stems are unbranched
(Cycas) or branched (Pinus, Cedrus).
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Leaves are simple or compound. They are
well-adapted to withstand extreme temperature, humidity and wind.
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In Cycas, the pinnate leaves
persist for a few years.
- In conifers (Pinus, Cedrus etc.),
the needle-like leaves reduce the surface area. Their thick cuticle and sunken
stomata also help to reduce water loss.
REPRODUCTION:
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Gymnosperms are heterosporous. They
produce haploid microspores and megaspores.
- Some leaves are modified into sporophylls.
They are compactly and spirally arranged along an axis to form lax or strobili
or cones.
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Sporophylls bear sporangia in which
spores are produced.
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Sporophylls are 2 types:
o
Microsporophylls: They
are arranged to male strobili (microsporangiate). They bear microsporangia.
The microspores develop into male gametophytes. It is highly reduced and
confined to only a limited number of cells. This gametophyte is called a pollen
grain. The pollen grains are developed within the microsporangia.
o
Megasporophylls: They
are arranged to female strobili (macrosporangiate). They bear megasporangia (ovules). Megasporangium
mainly consists of a body called nucellus. It is protected by envelopes.
The megaspore mother cell is differentiated from a cell of the nucellus. Megaspore
mother cell undergoes meiosis 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. The
multicellular female gametophyte is also retained within megasporangium.
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The male or female cones may be borne on
the same tree (Pinus) or on different trees (Cycas).
- Unlike bryophytes and pteridophytes, 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.
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The pollen grain released from the microsporangium
are carried in air currents and meet the opening of the ovules. The pollen tube
carrying the male gametes grows towards archegonia in the ovules and discharges
their contents near the mouth of the archegonia.
- After fertilization, zygote develops into an embryo and the ovules into seeds.
