Sunday, July 26, 2020

Plant Kingdom - Notes | Class 11 | Part 5: Angiosperms (Flowering Plants)

3. PLANT KINGDOM

ANGIOSPERMS (FLOWERING PLANTS)

-    They are an exceptionally large group of plants.

-    They range in size from tiny, almost microscopic Wolffia to tall trees of Eucalyptus (over 100 metres).

-    They include 2 classes: Dicotyledons & Monocotyledons.

o  Dicotyledons: Have 2 cotyledons in seeds, reticulate venations in leaves and tetramerous or pentamerous flowers (4 or 5 members in each floral whorl).

o  Monocotyledons: Have only one cotyledon, parallel venation in leaves and trimerous flowers (3 members in each floral whorl).


REPRODUCTION:


-    Flower is the reproductive structure.

-    Male sex organ in a flower is the stamen. Each stamen consists of a filament with an anther at the tip. Within the anthers, the pollen mother cell divides by meiosis to produce microspores which matures into pollen grains.

-    Female sex organ in a flower is the pistil. It consists of a swollen ovary at its base, a long slender style & stigma. Ovary contains ovules. An ovule has a megaspore mother cell that undergoes meiosis to form 4 haploid megaspores. 3 of them degenerate and one divides to form embryo sac.

-    Each embryo-sac has a 3-celled egg apparatus (one egg cell & two synergids), 3 antipodal cells & 2 polar nuclei. The polar nuclei eventually fuse to produce a diploid secondary nucleus.

-  Pollen grains dispersed from anthers are carried by wind or other agencies to the stigma of pistil. It is called pollination.

-    Pollen grains germinate on the stigma and the resulting pollen tubes grow through the tissues of stigma and style and reach the ovule.

-  Pollen tubes enter the embryo-sac where 2 male gametes are discharged. One male gamete fuses with egg cell to form zygote (syngamy). The other male gamete fuses with diploid secondary nucleus to produce triploid primary endosperm nucleus (PEN). Because of the involvement of two fusions, this event is called double fertilisation. It is an event unique to angiosperms.

-  The zygote develops into an embryo (with one or two cotyledons). The PEN develops into endosperm which provides nourishment to the developing embryo.

-    Synergids & antipodals degenerate after fertilization.

-    During these events, the ovules develop into seeds and the ovaries develop into fruit.

-    The seeds are enclosed by fruits.

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