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in the seed of gymnosperms and most flowering plants, a tissue in which the nutrient substances necessary for the development of the embryo are deposited. In gymnosperms the endosperm is formed in the ovule during germination of the megaspore; it consequently becomes the female prothallus (gametophyte), with a haploid set of chromosomes. The archegonia in gynosperms develop from certain cells of the endosperm. In angiosperms the endosperm is formed after fertilization as a result of the merging of the spermatozoid with the secondary (diploid) nucleus of the embryo sac. In this case the cells of the endosperm are triploid. By uniting sets of chromosomes from both parent plants the endosperm becomes a physiologically active, viable nutrient tissue. It not only feeds the embryo but fosters embryonic growth and differentiation.
A distinction may be made between the nuclear, cellular, and helobial types of endosperm. In the first type the cell walls do not form immediately after formation of the nuclei. In cellular endosperms the cell walls form immediately after each division of the nuclei. Helobial endosperm is intermediate between the other two types. Endosperm is not equally developed in the mature seeds of flowering plants of different families. Thus, it is large in Gramineae, Solanaceae, and Umbelliferae, but in many other families, for example, Leguminosae, Compositae, and Rosaceae, the endosperm is poorly developed, and reserve nutrient matter is deposited in the embryo itself, mainly in the cotyledons. In many Orchidaceae the endosperm is not formed.
REFERENCESKhudiak, M. I. Endosperm pokrytosemennykh rastenii. Kiev, 1963.
Poddubnaia-Arnol’di, V. A. Tsitoembriologiia pokrytosemennykh rastenii. Moscow, 1976.