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

Structure of Seeds 51is found in association with seed coats with pores, whereas the closed type is foundwith those without pores. Kulik and Yaklich (1991) found that hyphae of Phomopsisphaseoli were present on the seed coat and hilum, and their penetration via the opentype of micropyle was far more prevalent than in seeds of cultivars with closedmicropyles.3.2.6 HILUMThe size, shape, and position of the hilum with respect to the micropyle and chalazaconstitute important features. The hilum is generally considered to be a scar leftafter separation of the seed from the funiculus. Recent studies have shown that itevolves during seed development. Partial seed abscission is observed during hilumdevelopment (Pamplin, 1963; Baker and Mebrahtu, 1990). The scar is not the resultof mechanical separation, but rather caused by the organization of an abscissionlayer. Hilum size varies from insignificant to quite prominent. In the latter, the cellorganization in the hilar region may differ from that of the seed coat. The cells maybe homogeneous or show tissue differentiation, as in fabaceous seed (Baker andMebrahtu, 1990). The broad surface of hilum lacks the usual cuticle and may havefissures. Hilar fissure is characteristic in the seeds of Fabaceae.3.2.7 RAPHEIn several types of ovules and seeds, the funiculus is adnate to the ovule surface.The abscission of seed occurs in the free part of the funiculus. The adnate partremains as a longitudinal ridge in seed and is called the raphe. Structurally, it usuallyresembles the seed coat, but sometimes it is distinctive.3.2.8 SEED APPENDAGESSeeds have various types of appendages, such as aril (Figure 3.2E, F), caruncle(Figure 3.2C, D), strophiole, wings (Figure 3.2A, B), and hairs. The aril is a soft,succulent local outgrowth of seeds of varied origin. The aril primordium may developfrom any part of the ovule (e.g., funicle, raphe, and chalaza), the placenta, or thecarpel. It may enclose the seed more or less completely as in Passiflora (Figure3.2F) or it may form a localized outgrowth in Turnera ulmifolia (Figure 3.2E) andothers. For detailed information on seed appendages in general and aril in particular,the reader should refer to Kapil, Bor, and Bouman (1980).Caruncle (Figure 3.2C, D) is a small, disclike appendage, the attachment andgrowth of which are limited to the exostome rim. The micropyle may be seen in thecenter (Ricinus and Euphorbia). Strophioles are glandular or spongy, with proliferationlimited only to the raphal region (Chelidonium majus).Wings (Figure 3.2A, B) are flattened extensions of seed having optimal strengthand a minimal biomass. Seed wings are often correlated with seed dispersal. Corner(1976) believes that the seed wings are local outgrowths of the testa. They may beperipheral or restricted to the raphe, chalaza, antiraphe, hilum, or funicle. Seed wingsare rarely provided with vascular bundles, whereas fruit wings have a well-developedvascular supply.