Notes And Study Material for Exam Papers

Development of Endosperm It begins before that of the embryo, arises from the primary endosperm nucleus (also known as triple fusion nucleus) formed as a result of the triple fusion of three nuclei i.e. two polar nuclei and a male nucleus. The 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 center so that the nuclei are pushed to the periphery. Later on, unless the endosperm is absorbed by the embryo in the free nuclear stage, wall formation takes place starting from the periphery towards the center of the 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 center of the embryo sac as it is not completely filled by the endospermic tissue. This type of endosperm development s

An ovule (megasporangium) may arise from the placenta at the base of the ovary or on the inner surface of the ovary. It is borne on a distinct stalk, the funicle which is attached to the body of the ovule at a point called the hilum. The central and somewhat conical part of the ovule is the nucellus. It consists of a mass of parenchymatous cells. It represents the metasporangium proper and is invested by two-layered integuments in monocotyledons and primitive dicotyledons (Archicliamydeae) but by single-layered integument in the higher dicotyledonous families (metachlamydeae); very rarely unitegmic and bitegmic ovules occur in the same family. In many cases, single integument has resulted due to the fusion of two separate primordials or by elimination of one of the two integuments. The third integument in the form of aril is found in plants like Asphodehus and Trianthema, while Ricinus and several other plants of the family, Euphorbiaceae, show an integumentary proliferation called