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

34 Histopathology of Seed-Borne Infections2.9.3 HIBISCUS AND GOSSYPIUMThe outer integument is three-layered in H. ficulneus and four- to six-layered inGossypium (Figure 2.14A, B, H). It is nonmultiplicative. The inner integument isinitially three-layered and multiplicative, becoming seven- to nine-layered in Hibiscus(Figure 2.14C, D) and eight- to fifteen-layered in Gossypium (Figure 2.14H). Boththe integuments form the seed coat. The cells of the outer epidermis of the outerintegument elongate tangentially and lose their contents, but hair initials remainsquarish and finally undergo radial enlargement into hairs (Figure 2.14H). Stomata(Figure 2.14F, G) are found in the epidermis of the entire organism, but they areaggregated more in the chalaza. The middle layers are almost squeezed in Hibiscus,but they are differentiated into an outer zone of pigmented cells and an inner zone ofcolorless cells in Gossypium (Figure 2.14I). The cells in the inner epidermis elongateradially, lose their contents, and develop thickenings on the inner tangential and radialwalls in H. ficuleuns (Figure 2.14E), but they remain thin-walled in Gossypium.The inner integument differentiates into four zones. The cells of the outerepidermis enlarge radially and become thick-walled, forming the main mechanicallayer (Figure 2.14E, I). The cells of four or five subdermal layers enlarge, becomethick-walled, and accumulate tanniferous contents. The remaining middle layersremain thin-walled, lose their contents, and are partially digested, forming thecolorless zone (Figure 2.14C, D, and I). The cells of the inner epidermis stretchtangentially and accumulate tanniferous contents in Hibiscus (Kumar and Singh,1990), but the inner epidermis develops characteristic thickenings in Gossypium,called the fringe layer (Joshi, Wadhwani, and Johri, 1967).The development of the seed coat in Gossypium is similar to Hibiscus, but thelint and fuzz hairs are distributed all over the surface of the seed.2.9.4 CUCURBITA AND SECHIUMThe ovules are bitegmic, and the two- to four-layered inner integument (Figure2.15A, F) is absorbed during ontogeny. The seed coat develops from the outerintegument, which shows localized multiplication. By two successive periclinaldivisions the outer epidermis forms three layers, which are designated e, e≤, and e¢,from outside to inside (Figure 2.15B, G). The second division does not take placethroughout the surface in Sechium. The subepidermal layer e is, therefore, discontinuousin Sechium (Figure 2.15H). The cells in e divide only anticlinally to keeppace with the growing seed and later on enlarge tangentially and become thickwalled(sclerotic), forming the main mechanical layer in Cucurbita. The cells of eand e≤ divide periclinally and the outermost derivatives enlarge radially, formingseed epidermis. The remaining layers become thick-walled and lignified, formingthe hypodermis (Figure 2.15C to E).The cells of the ovular hypodermis divide once or twice periclinally, enlarge,become stellate, and develop prominent air spaces to form aerenchyma. They becomethick-walled and lignified in Cucurbita. The cells of other layers enlarge and those onthe outer side develop air spaces while those on the inner side remain compact, forminga chlorenchymatous or parenchymatous zone. This tissue at maturity is separated fromthe outer hard coat and forms the inner coat (Singh and Dathan, 1972).