Question

what is polyembryony ? mention its types.

Answer 1

The phenomenon of occurrence of more than one embryo in a seed is called polyembryony. Polyembryony may arise by
(a) Cleavage of proembryo: Cleavage and proliferation of zygote or its derivatives leading to the establishment of separate embryonal primordia is quite common in orchids.
(b) Embryos from cells of the embryo sac other than the egg: The most common source of additional embryos are the synergids. Depending on whether it arises from fertilized synergid or unfertilized synergid, the embryo may be diploid or haploid. It is seen in Poa, Aristolochia, Argemone, etc.
(c) More than one embryo sac in the same ovule: Multiple embryo sacs in an ovule may arise from nucellar cells, derivatives of two or more megaspore mother cells, derivatives of the same megaspore mother cell. It is seen in Citrus, Casuarina, Pennisetum, etc.
(d) Activation of some sporophytic cells of the ovule: The embryos arising from the maternal sporophytic tissue (outside the embryo sac) are called adventive embryos. The only maternal tissues which are known to form adventive embryos are the nucellus and the integuments. It is seen in Mangifera, Citrus, Opuntia, etc.