ULTRASTRUCTURAL AND CYTOCHEMICAL ASPECTS ON THE STAMINAL TAPETUM OF CALOTROPIS-PROCERA (ASCLEPIADACEAE)

Ultrastructural transformations and cytochemical modifications of the tapetum were studied in the course of pollinium ontogeny of Calotropis procera (Ait.) Ait. (Asclepiadaceae). The tapetum is formed during the premeiotic stage and maintains its integrity just before anthesis. During this period, binucleated tapetum cells are characterized by: 1) the low development of the endoplasmic reticulum and Golgi apparatus; 2) the lack of visible and persistent storage of metabolites; 3) a dense cytoplasm, rich in ribosomes and spotted with many vacuoles; 4) the transitory development of vacuolated spherules (nucleoloids) near the nucleus; 5) an abundant population of polymorphous plastids in which many lipid droplets are formed; 6) finally, in microspore stage, the secretion on its inner face of important deposits of glucidic and proteic substances. No orbicule is formed. Shortly before anthesis, the protoplasm of tapetal cells is degenerated resulting in the formation of lipid droplets. However the cell walls remain intact. The particular feature of this tapetum is discussed comparatively with secretory and plasmodial tapetums.