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

Reproductive Structures and Seed Formation 9or compound spikes (wheat, barley, pearl millet, and rice), provide additional protectionto flowers, but cause increased humidity around them. They may also providea conducive environment for the development of pathogens that succeed in invadingthem.The flower is a shoot of determinate growth. The distal end of the axis is swollenor cup-shaped, forming a thalamus or receptacle, which bears laterally floral organs.A typical flower of dicotyledons has four types of organs comprising two whorls ofsterile appendages, the sepals and petals collectively called the calyx and corolla,respectively, and two whorls of fertile appendages, the stamens and carpels, collectivelytermed the androecium and gynoecium. The appendages are arranged on thereceptacle in succession spirally or in whorls. Many variations occur in size, shape,color, and number of members in each whorl and in their organization. Usually inmonocotyledons, there is only one whorl of sterile appendages, termed the perianth(onion and garlic). Flowers may be bisexual or unisexual. In the unisexual conditionstaminate and pistillate flowers are monoecious, or borne on one plant, as in maize,cucumber, pumpkin, and squash. If the staminate and pistillate flowers occur ondifferent plants, as in papaya, they are dioecious.In a flower, if the floral appendages are inserted successively one above the otherbelow the gynoecium (carpel), the flower is hypogynous and the ovary is superior(Figure 2.2A). When the receptacle becomes concave and surrounds the ovary or isfused with the ovary wall so that the sepals, petals, and stamens arise from the topof the ovary (Figure 2.2B), the flower is epigynous and the ovary is inferior as incoriander, cumin, carrot, cucumber, pumpkin, squash, sunflower, lettuce, apple, andpear. In an epigynous flower, the ovary is exposed to the environment from initiationuntil maturity, i.e., the formation of fruit.The flower in the grass family, Poaceae, is subsessile and bracteate; the drychaffy lemma, which is usually awned, is the bract. The dry membranous structureon the posterior side is known as the palea, which represents two fused posterolaterallysituated bracteoles. The palea encloses the flower and itself is enclosed by thelemma. The flower consists of lodicules (tepals), androecium, and gynoecium. Usuallythere are two fleshy, often hairy, lodicules situated in the anterolateral position.In Bambusa, there are six lodicules. Above the lodicules, there are usually threestamens and rarely more, six or less in Oryza and Bambusa, etc., two in Anthraxon,and one in Uniola. The gynoecium is tricarpellary and syncarpous and the ovary isunilocular with a single large ovule. The vascular anatomy has shown that theplacentation is parietal and that two of the three placentae are sterile. The ovarybears two laterally situated feathery styles and stigma.2.1.1 VASCULARIZATION OF FLOWERThe anatomical structures of the peduncle (axis of the inflorescence) and the pedicelare similar to that of the stem. The vascular cylinder may be continuous or split.The vascular bundles of the pedicel at a higher level break up and give out tracesto the floral appendages in succession in a hypogynous flower (Figure 2.2A). Thevascular traces meant for the floral appendages enter each unit either unbranched orafter undergoing further branching. Each sepal generally receives three traces, a