Sunday, July 26, 2020

Plant Kingdom - Notes | Class 11 | Part 4: Gymnosperms

3. PLANT KINGDOM

GYMNOSPERMS

-    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).

-    They include medium-sized trees or tall trees and shrubs. Sequoia (giant redwood) is the tallest tree species.

-    The roots are generally tap roots.

-    Roots in some genera have fungal association in the form of mycorrhiza (E.g. Pinus).

-    In plants like Cycas, small specialized roots (coralloid roots) are associated with N2- fixing cyanobacteria.

-    Stems are unbranched (Cycas) or branched (Pinus, Cedrus).

-    Leaves are simple or compound. They are well-adapted to withstand extreme temperature, humidity and wind.

-    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:


-    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.

-    Sporophylls bear sporangia in which spores are produced.


-    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.


-    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.

-    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.

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