Histopathology of Seed-Borne Infections - Applied Research Center ...

Reproductive Structures and Seed Formation 252.8.1 ENDOSPERMThe primary endosperm nucleus divides mitotically, and depending on the mannerof wall formation during its development, three main types of endosperms arerecognized. (1) If the first few divisions are not followed by wall formation andnuclei lie free in the cytoplasm, the endosperm is of the nuclear type (Figure 2.9A,B). (2) If the first and subsequent nuclear divisions are followed by cell wallformation, the endosperm is of the cellular type (Figure 2.9C, D). (3) It is in theintermediate type, in which the first nuclear division is followed by cell wallformation, that two unequal chambers are usually formed. The micropylar chamberis larger, and its nucleus divides repeatedly without wall formation. In the smallchalazal chamber, the nucleus may undergo a few free nuclear divisions or it mayremain undivided. This endosperm is of the helobial type. During further development,cell wall formation takes place in the micropylar chamber. Nuclearendosperms also ultimately become cellular. The endosperm development is of thenuclear type in most of the food plants (Brassicaceae, Malvaceae, Tiliaceae,Fabaceae, Cucurbitaceae, Apiaceae, Solanaceae, Asteraceae, and Poaceae) and cellularin Sesamum (Pedaliaceae).The female gametophyte remains sac-like, and food material is translocated intoit from the surrounding tissue. In several families, portions of endosperm in thechalazal and/or micropylar regions undergo tubular elongation to form endospermhaustoria, which function to draw nutrition. Endosperm haustoria have been reportedin the nuclear as well as the cellular types of endosperms. Among the families offood plants, some genera of Fabaceae, Cucurbitaceae, and Pedaliaceae (sesame)develop haustoria. In Cucurbitaceae and Fabaceae, a chalazal, tubular, and coenocyticor cellular haustorium with dense cytoplasm, has been reported. The coenocytichaustoria in Cucurbitaceae (Chopra, 1955; Singh, 1957) and Fabaceae (Rau, 1953),when studied in living materials, show protoplasmic streaming. Dute and Peterson(1992), who studied the endosperm development in soybean, have reported thedevelopment of wall ingrowths in the chalazal endosperm haustorium, providingevidence that the haustorium functions to absorb nutrients from the surroundingnucellar tissue that shows lysis. Johansson and Walles (1993) report wall ingrowthsalong the whole embryo sac boundary in faba bean (Figure 2.12A).The endosperm tissue is unique to angiosperms. It is rich in food materials, andin many plants almost the entire endosperm is absorbed by the developing embryo(Fabaceae and Cucurbitaceae), while in others it is persistent (Poaceae, Solanaceae,Euphorbiaceae, Sesamum, Linum, Malvaceae). However, little is known regardingthe time when the embryo begins to utilize the endosperm for its nutrition. Recentstudies have shown that the endosperm, during the early stages of development,needs adequate nutrients for its growth, and only in late embryogeny does it havea pool of reserve materials, which are utilized for the growth of embryo (Newcomb,1973; Yeung and Clutter, 1978).