Page 3 - LN-CH-2[2]
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It is a large diploid cell, dense cytoplasm with prominent nucleus.
The MMC undergo meiotic division resulting four haploid megaspores
Female gametophyte:
Out of four megaspores, one megaspore is functional and other three degenerates.
The functional megaspore developed into the female gametophyte.
Female gametophyte is known as the embryo sac.
Development of embryo sac from a single megaspore is called asmonosporic type of embryo sac.
The nucleus of the functional megaspore divided by mitotic division to form two nuclei which move to the
opposite pole, 2-nucleated embryo sac.
Two successive mitotic division leads to formation of 4-nucleate and later 8-nucleate stages of the embryo
sac.
All mitotic divisions are free nuclear type; karyokinesis is not followed by cytokinesis.
Six of the eight nuclei are surrounded by cell walls and organized into cells.
Three cells are grouped together at the micropylar end, constitute the egg apparatus.
The egg apparatus, in turn consists of two synergids and one egg cell.
Synergids have special filiform apparatus, which play an important role in guiding the entry of pollen tube
into the synergids.
Three cells arranged towards chalazal end are called antipodal cells.
The large central cell has two polar nuclei.
A typical angiosperm embryo sac at maturity is 8- nucleated and 7-celled.
Pollination:
Transfer of pollen grains from the anther to the stigma of a pistil is termed as pollination.
Both male and female gametes are non-motile.
Kinds of pollination:
Autogamy:
Pollination within same flower.
In open and exposed anthers and stigma autogamy is rare.
Viola, Oxalis and Commelina produce two types of flowers:
o Chasmogamous: exposed anther and stigma
o Cleistogamous: closed anther and stigma.
Cleistogamous flower is invariably autogamous and assured seed set even in the absence of the
pollinator.
Geitonogamy:
Pollination between two flowers of the same plant.
Pollination by pollinating agent.
Genetically similar to the autogamy.
