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

20 Histopathology of Seed-Borne Infectionslong funicle almost completely surrounds it, the ovule is called the circinotropoustype.The ovules are also classified as crassinucellar and tenuinucellar. In the former,parietal layers exist between the nucellar epidermis and the female gametophyte(Figure 2.7A), whereas in the latter the nucellar epidermis alone covers the femalegametophyte (Figure 2.7B, E). The tenuinucellar ovules are characteristic of sympetalae(Lycopersicon, Solanum, Sesamum, Helianthus, and Lactuca) while otherdicotyledons (Gossypium, Hibiscus, Glycine, Vigna, Phaseolus, Pisum, Cucurbita,and Cucumis) and monocotyledons (Triticum, Hordeum, Zea, Oryza, and Allium)have crassinucellar ovules.The ovules have one (Figure 2.7B) or two integuments (Figure 2.7A). In bitegmicovules, the inner integument is largely of dermal origin except in Euphorbiaceae(Bor and Bouman, 1974) and Malvaceae (Joshi, Wadhwani, and Johri, 1967; Kumarand Singh, 1990), but the outer integument or the single integument is of subdermalor dermal origin. The integument arises as a rimlike outgrowth and grows to enclosethe nucellus to various extents, having an opening at the apex, the micropyle. Themicropyle may be formed by both the integuments or by the outer or inner integumentalone. Electron microscopy has shown that the micropyle in Beta vulgaris containsa fibrillar periodic acid-Schiff positive substance and is often covered by a thin sheetor hymen (Olesen and Bruun, 1990).2.5.2 VASCULAR SUPPLY OF OVULEThe ovule in angiosperms commonly receives a single vascular bundle from theventral carpellary vein (Figure 2.6A), rarely two or more in Sechium and Sicyos(Puri, 1954; Singh, 1965). The vascular supply ends as such or after fanning out inthe chalaza (Figure 2.6B, C). The ovular supply may extend in the outer (Cucurbitaceaeand Fabaceae) (Figure 2.6D, E) or the single integument (Asteraceae andConvolvulaceae) on antiraphe side and rarely in the inner integument as in Euphorbiaceae(Figure 2.6F, G). Integumentary vascularization is rare in monocotyledons.Large ovules usually have a developed vascular supply, whereas small ones showreductions. Very small ovules as in Orchidaceae have no trace of vascular supply.The integumentary vascular supply may be unbranched, or branched in some cases,forming a network of bundles (Kuhn, 1928). Vascular elements, xylem and phloem,may become differentiated in later developmental stages, or it may not take place,and the vascular supply consists of procambial strands only.Nucellar tracheids are known in the Asclepiadaceae, Capparidaceae, Casuarinaceae,Amantiferae, Liliaceae, and Olacaceae. The tracheids are annular or spiral,very small, slender, and usually isolated or in small clusters.2.5.3 CUTICLES IN OVULECuticles are reported to be present in ovules from early stages of development. Theovule primordium bears a cuticle. After the development of the integuments in abitegmic and crassinucellar ovule, as many as five cuticular layers may be identified:(1) on the outside of the outer integument and the funiculus; (2) on the inside of the