Notes And Study Material for Exam Papers

Development of endosperm begins before that of embryo. It arises from the primary endosperm nucleus (also known as triple fusion nucleus) formed as a result of triple fusion of three nuclei i.e. two polar nuclei and a male nucleus. Primary endosperm nucleus is triploid (3N). It undergoes a series of divisions resulting in the formation of many free nuclei lying within the embryo sac. A vacuole appears in the centre so that the nuclei are pushed to the periphery. Later on unless the endosperm is absorbed by the embryo in free nuclear stage, wall formation takes place starting from the periphery towards the centre of embryo sac and by further centripetal growth, the embryo sac is filled with a compact cellular tissue around the embryo without any intercellular spaces. This tissue is endosperm occasionally as in coconut a vacuole persists in the centre of the embryo sac as it is not completely filled by the endospermic tissue. This type of endosperm development since it involves the ...

In several genera of angiosperms more than one viable embryo in the seed has been observed. It is more frequent in Gymnosperms than in angiosperms. This condition arises in various ways: (i) The oospore produces more than one embryo during development as a result of splitting. This is known as cleavage polyembryony and is very common in gymnosperms (e.g. Pinus) and angiosperms e.g. Allium, citrus and some orchids. (ii) An ovule may contain more than one functional megaspore producing as many embryo sacs and oospheres which on fertilization give rise to more than one embryo in the seed. (ii) In addition to the normal embryo produced from a fertilized oosphere, embryo may also be formed from the synergids (e.g. Dandelion) or rarely the antipodals with or without fertilization. (iv) Additional embryos may be produced as a result of sporophytic budding by the division of the cells of the nucellus (e.g. lemon) or integument (e.g. onion). Parthenocarpy: As a rule...