Biology XII:2 Sexual rep in flowering plants(5 Post Fertilization)

Post-Fertilisation Events It includes development of endosperm and embryo, and maturation of ovules into seeds and ovaries into fruits. Formation of Endosperm The endosperm develops before the embryo because the cells of the endosperm provide nutrition to the developing embryo. The primary endosperm nucleus repeatedly divides to give rise to free nuclei. This stage of development is called free nuclear endosperm. Cell wall formation occurs next, resulting in a cellular endosperm. The endosperm may be either fully consumed by the growing embryo (as in pea and beans) or retained in the mature seed (as in coconut and castor). Development of Embryo The embryo develops at the micropylar end of the embryo sac where the zygote is situated. The zygote gives rise first to the pro-embryo, and then to the globular, heart-shaped, mature embryo. A typical dicot embryo consists of an embryonal axis and two cotyledons. The portion of the embryonal axis above the level of cotyledons is called epicotyl. It contains the plumule (shoot tip). The portion below the axis is called hypocotyl. It contains the radicle (root tip). The root tip is covered by the root cap. In a monocot embryo, there is only one cotyledon. In grass, it is known as the scutellum, and is situated at one side of the embryonal axis. At its lower end, the embryonal axis has the radicle and the root cap enclosed in the coleorrhiza. The epicotyl lies above the level of the scutellum, and has the shoot apex and leaf primordia enclosed in hollow structures called coleoptiles.