Typically, seeds are relatively dry structures as compared with other plant tissues, and in this condition, they are resistant to the ravages of time and their environment (Mitter, 1993). Characteristics of seeds, more than any other characters that the plant possess, require precise integration and co-ordination between different functions in order to have successful reproduction especially in arid zones. Seed is usually used in a functional sense, viz. as a unit of dissemination, a disseminule. In essence, seed is a miniature plant because it is responsible for its regeneration and ultimately for its reproduction success (Khurana and Singh, 2001). An angiospermic seed may appear very simple externally but possesses a complex ecophysiology for its further resumption of growth, primarily in germination. The seed is a highly dehydrated structure and is metabolically quiescent. The cotyledons serve as centres of absorption and storage, drawing nutritive material from the endosperm. The cotyledons of many plants function as primary photosynthetic organs. Each species of plant has its specific period of viability; a seed sown after the period of optimum viability may produce weak plants or may not germinate. In desert regions where rains are unpredictable and of limited duration, ephemeral plants have short life cycles and it is important for them to germinate only after a substantial fall of rain. Dormancy of seed, especially those of arid zone plants are biologically significant in spreading or delaying germination. Delayed germination is of great significance, especially in an arid zone, where the conditions are unflattering and unpleasant for normal growth and development of plants for long periods. Probably in no other habitat, a seed is subjected to such scrupulous environmental conditions as in the arid ecosystem where an acute shortage of water is a primary limitation in the germination of seeds.
Unit of dissemination, Seed viability, Dormancy of seed, Delayed germination
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