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Gymnosperms

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Gymnosperms - Lesson Summary

Gymnosperms are plants in which the ovules are not enclosed by an ovary wall and remain exposed before and after fertilisation, and before developing into seeds.

The stems of a gymnosperm can be branched or unbranched and the needle-like leaves, thick cuticle and sunken stomata on the leaves reduce the rate of water loss. The reproductive structure of a gymnosperm is called a strobilus or a cone and gymnosperms have both male and female strobili.

The strobili are seen on the same tree, as in Pinus or on different trees, as in Cycas. The male strobilus contains microsporophylls, which bear microsporangia that produces haploid microspores. A few microspores develop into male gametes called pollen grains and the rest degenerate. The female strobilus is a cluster of megasporophylls, which bears ovules containing the megasporangium. The megasporangium, surrounded by a layer of envelopes produces haploid megaspores and a megaspore mother cell. The megaspore mother cell undergoes meiosis to give rise to four haploid megaspores.

One of these spores develops into a multicellular female gametophyte. The female gametophyte further bears two or three female sex organs called archegonia, which develop inside the ovule.

The pollen grains released from the microsporangium are carried by air to reach the micropyle. A pollen tube sprouts from the pollen grain and grows towards the archegonium, where it discharges the male gametes. These gametes fuse with the egg to form a zygote. This process is called fertilisation. The zygote develops into an embryo and the ovule into a seed post-fertilisation. 

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