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

Reproductive Structures and Seed Formation 21outer integument; (3) on the outer side of the inner integument; (4) on the inside ofthe inner integument; and (5) on the nucellus. In unitegmic crassinucellar ovules,the cuticles for the single integument and the nucellus are present. In unitegmic andtenuinucellar ovules, however, since cells of the nucellus are disorganized early, thecuticle may also be lost.2.5.4 SPECIAL STRUCTURES IN OVULESThe ovule may have the development of some structures that continue differentiationin the developing seed. Epistase is organized at the micropylar end of the nucellus.It is a caplike structure formed from the nucellar epidermis or its derivatives. Itscells become cutinized.The nucellar cells on the chalazal side form a hypostase (van Tiegham, 1901).In literature the term hypostase has been used in a rather loose sense. Its featuresare diverse in different taxa. The cells are rich in cytoplasm, or accumulate tanninlikesubstances, or become cutinized, callosic, lignified, or suberised. More than onetype of cell may differentiate to form a hypostase as in Carica (Dathan and Singh,1970) and Passiflora (Dathan and Singh, 1973). Multiple functions have been attributedto the hypostase. It acts as connecting tissue between the vascular supply andthe embryo sac, facilitating the transport of nutrition (Johansen, 1928; Venkata Rao,1953; Tilton, 1980). When the cells are thick-walled, the hypostase forms a barrieror protective tissue outside the embryo sac.Another important feature reported in ovules of many families of dicotyledonsand monocotyledons is the differentiation of an integumentary tapetum or endothelium.In tenuinucellar ovules, the nucellus disorganizes early and the megasporemother cell or the female gametophyte is surrounded by the inner epidermis of theintegument. Its cells enlarge radially and become densely cytoplasmic (Figure 2.7G,J). The endothelium is common in unitegmic ovules and rarely formed in bitegmicovules (Linaceae). In the latter, the cells of the inner epidermis of the innerintegument form the endothelium. The endothelial cells are separated from theembryo sac by a cuticle whose thickness varies at different sites and alters duringseed development (Erdelska, 1975). It is suggested that the endothelium translocatesnutrients, derived from the integumentary tissue to the embryo sac (Esser, 1963;Masand and Kapil, 1966).2.6 DEVELOPMENT AND STRUCTURE OF FEMALEGAMETOPHYTEUsually one, rarely more, archesporial cells differentiate in the hypodermis of thenucellus due to their large size, prominent nucleus, and dense cytoplasm (Figure2.7C). With or without cutting of a parietal cell, the female archesporium functionsas a megaspore mother cell (Figure 2.7D). The megaspore mother cell undergoesmeiosis, forming four megaspores (Figure 2.7E, F). The three micropylar megasporesdegenerate and the chalazal megaspore functions (Figure 2.7G). The nucleus undergoesthree mitotic divisions to form eight nuclei (Figure 2.7G to I). Three of thenuclei at the micropylar end form the egg apparatus, an egg and two synergids, while