Short Note - Structure of Ovule (Megasporangium)

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 the caruncle at the micropylar end.

A very peculiar condition is reported in opuntia where the funicle becomes very long and completely surrounds the ovule, thus looking like a third integument. In parasites like Santalum (Sandal wood) and Loranthus, there is no integument. The integuments arise from the chalaza which constitutes the basal part of the nucellus. They grow upward in close contact with the nucellus and enclose the latter completely, leaving only a small opening, the micropyle at the apex. An embryo sac is embedded in the nucellus towards the micropylar end.

The embryo sac as will be seen later bears the embryo and is the most important part of the ovule. In a mature ovule, the embryo sac bears towards the micropylar end three protoplasts constituting the egg apparatus one is the egg or oosphere while the other two are the synergids or help cells. At the chalazal end are three antipodal cells enclosed in the cell wall. In the center of the embryo sac lies the secondary nucleus.

The vascular supply of the ovule is supplied by a vascular strand that runs through the funicle and extends upwards, but rarely beyond chalaza.

In some cases as in Gmynosperms, the vascular bundles enter the integuments or even the nucellus also. Sometimes the integuments are green due to the presence of chlorophyll e.g. Amaryllis and bear stomata e.g. Nerium and Gossypium.

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