MEGASPOROGENESIS AND EMBRYO SAC DEVELOPMENT IN ANGIOSPERMS ANGIOSPERMS

MEGASPOROGENESIS AND EMBRYO SAC DEVELOPMENT IN ANGIOSPERMS

ANGIOSPERMS • Angiosperms are flowering plants. • In a flower , stamen is the male reproductive part and gynoecium /pistil is the female reproductive part. • During sexual reproduction, pollen grains are produced inside the anther. Gynoecium has three parts- Ovary, style and Stigma. Female gametophyte development occurs inside the ovule.

GYNOECIUM, CARPEL, OVULE

MEGASPOROGENESIS: • The process of formation of megaspore from megaspore mother cell is called Megasporogenesis. • The process of megasporogenesis occurs inside the nucellus of the ovule. • Ovular primordium is composed of a group of meristematic cells. • Single celled archesporial cell differentiates near micropylar end. • Archesporial cell divide into outer parietal cell and megasporocyte (Megaspore mother cell) • Megaspore mother cell undergo meiotic division to form four haploid megaspores. • Out of four megaspores, three at the micropylar end degenerates and one remain functional

MONOSPORIC EMBRYO SAC DEVELOPMENT • Single functional megaspore develops into 7 celled , 8 nucleate embryo sac. • The nucleus of single functional megaspore, undergoes three successive mitotic divisions , to form 8 nucleate embryo sac (female gametophyte) • One nucleus from each pole migrate towards the centre (Polar nuclei) • Cell wall formation starts. Three nuclei, at the micropylar end , get organized into egg apparatus

FEMALE GAMETOPHYTE • Egg apparatus: Two synergids and one egg cell • Three cells at the chalazal end, constitute antipodal cells. • Two polar nuclei, forms diploid secondary nucleus in the central cell. • Thus embryo sac become 7 -celled, 8 nucleate in Angiosperms.

ELECTRON MICROSCOPIC VIEW

CHECK YOUR GRASP. . . ? ? ? 1. If the nucellar chromosome number of an angiosperm flower is 20, what will be the chromosome number of the following? a) Egg b)Antipodal cell c) Zygote d) Endosperm e)Central cell 2. How many embryo sacs are likely to be formed in the ovary of an angiosperm flower bearing 20 megaspore mother cells?

3. If the number of chromosomes in root cells is 14, what will be the number of chromosomes in synergid cells of an ovule? 4. If the PEN of an angiosperm plant has 24 chromosomes, the same in the MMC will be?

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