Abstract
Commonly found in sympetalous plants with unitegmic and tenuinucellate ovules, the integumentary tapetum exhibits great diversity in its distribution, morphology, cytology, differentiation, and behaviour. It is separated from the nucellus and embryo sac by layers of cuticle. The thickness, uniformity and continuity of cuticle is variable not only in diverse taxa but also at different places in the same species. The cuticular layers manifest interruptions, and the embryo sac wall bears certain ingrowths in the regions of these discontinuities. Ultrastructurally, the endothelial cells show characteristics of meristematic as well as secretory cells. Sometimes they develop wall projections and even contain multivesicular bodies. Large quantities of proteins, carbohydrates, ascorbic acid and some enzymes such as oxidative enzymes, amylases, proteases are also known to occur. Besides, a deposition of callose at the onset of pollination is recorded inPetunia. Proliferation of the integumentary tapetum in some hybrids results in seed abortion. It is believed that endothelium helps in coordinating growth in the ovule, channelizes nutrition to the embryo sac, and later performs the protective function.
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