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

Reproductive Structures and Seed Formation 11spaces over-arched by the cuticle. The mesophyll is only a few cells thick, exceptin flowers with fleshy petals. The cuticle is commonly striated (Martens, 1934). Thefragrance of flowers is produced by volatile substances, mostly essential oils, occurringin the epidermal cells. In some plants, such as Lupinus and Narcissus, thefragrance originates in special glands called osmophors.2.3 FERTILE APPENDAGESThe stamens and carpels, male and female appendages, respectively, are highlyspecialized and form the seat of the development of male and female gametophytes,pollen and embryo sac, in angiosperms. The development and structure of male andfemale gametophytes have been studied in numerous plants of dicotyledons andmonocotyledons. Detailed information on this topic can be found in the publicationsof Schnarf (1929, 1931), Maheshwari (1950, 1963), Davis (1966), Johri (1984), andJohri, Ambegaokar, and Shrivastava (1992).2.3.1 STAMENA typical stamen consists of the filament bearing a two-lobed and four-loculed anther(Figure 2.3A, B). The two lobes of the anther are separated by a sterile tissue, calledthe connective. The single vascular supply of stamen traverses the filament and mayend at the base of the anther or may extend into the tissue of the connective. Thevascular bundle is not connected by any vascular element with the sporogenoustissue. The ground tissue of the connective and filament consists of parenchymatouscells. The epidermis is cutinized and may have stomata.The fertile region of the anther comprises four microsporangia, locules or pollensacs, two per anther lobe. During development and initial organization, the pollensacs of an anther lobe are distinct (Figure 2.3B), but at maturity due to confluence,these become one.2.3.1.1 Development of Microsporangium and MicrosporogenesisThe development of microsporangium is fairly uniform in angiosperms. The malearchesporium differentiates as a row of cells in the hypodermal region in four corners.The archesporial cells divide periclinally. The outer derivatives divide anticlinallyand periclinally forming anther wall layers other than the epidermis. Depending onthe pattern of divisions in the primary parietal layer, four types of anther walls arerecognized (Davis, 1966). The anther wall usually consists of an epidermis, anendothecium, one or two middle layers, and a tapetum (Figure 2.3C, D). Tapetum,the innermost wall layer that forms a jacket around the sporogenous tissue, is ofdual origin, partly formed by the innermost derivatives of parietal layer and partlyby the parenchymatous cells, adjacent to the sporogenous tissue of the connective(Periasamy and Swamy, 1966). Its cells become rich in cell contents and functionto nourish the developing sporogenous cells. Depending on its mode of function,the tapetum is secretory or amoeboid in nature. In the latter, the cells lose cell walls,and contents migrate in between the sporogenous cells (Figure 2.3G, H).