3. PLANT KINGDOM
ANGIOSPERMS
(FLOWERING PLANTS)
-
They are an exceptionally large group of
plants.
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They range in size from tiny, almost
microscopic Wolffia to tall trees of Eucalyptus (over 100
metres).
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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:
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Flower
is the reproductive structure.
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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.
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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.
