P. Maheshwari classified the embryo sac on the basis of number of megaspore nuclei participating in embryo sac formation into following types : (1) Monosporic embryo sac : Only one megaspore nucleus forms embryo sac e.g., Polygonum, Oenothera. (2) Bisporic embryo sac : Two megaspore nuclei take part in development of embryo sac e.g., Allium, Endymion. (3) Tetrasporic embryo sac : All the four megaspore nuclei take part in development of embryo sac e.g., Adoxa, Plumbago, Dursa, Fritillaria, Penaea, Plumbagella, Peperomia. Development of Monosporic Embryo sac (Polygonum Type) The normal type of embryo sac development has been studied in Polygonum by Strasburger. Since this embryo sac develops from one megaspore, that is why this is monosporic embryo sac. It develops from chalazal functional megaspore (4th from micropyle). Nucleus of functional megaspore divides by three mitotic divisions to form 8 nuclei, out of which (i) Three of them get organised as cells at micropylar end forming egg apparatus. One is egg cell (n) and two are synergids (n) or co-operative cells. (ii) Three cells at chalazal end form antipodals (n) or vegetative cells of female gametophyte. (iii) Two nuclei (one from each pole) in the centre are called polar nuclei (n) As the embryo sac matures, these polar nuclei get fused to form a secondary nucleus (2n) or definitive nucleus. This constitutes an embryo sac as 7-celled and 8-nucleated structure.  |
11/5/2009
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