The Changes in the Tapetum of Pinus banksiana Accompanying Formation and Maturation of the Pollen

As meiosis is completed, and following the synthesis of lipidduring meiotic prophase, the tapetum begins to form precursorsof sporopollenin. These accumulate in cisternae of the endoplasmicreticulum, resembling large dictyosome vesicles. They are releasedfrom the tapetal protoplasts intact, but rupture in the loculus.The liberated precursors polymerize either on lipid dropletsin the expanded tapetal walls, forming the orbicules, or onthe lipid layer surrounding the loculus, forming the secondcomponent of the peritapetal membrane. On rupture of the callosewall condensation also proceeds on the walls of the meiospores,already coated with a thin layer of sporopollenin synthesizedby the spore itself. The tapetal protoplasts expand considerablyduring synthesis of the precursors. Wide channels also formbetween the protoplasts, and the nuclei undergo irregular divisions. Ribosomes are conspicuous in the tapetal cytoplasm during thesporopollenin synthesis, but protein levels are low. It is proposedthat protein is exported to the loculus and untimately incorporatedinto the developing microspores. In the final phase of microsporogenesis the tapetum fragments,and parts move into the loculus.Protein levels in the tapetumare now high, possible indicating the massive synthesis of hydrolaseswhich accomplish the dissolution of the tissue. Removal of thelipid component of the peritapetal membrane precedes the desiccationof the anther. The surfaces of the mature pollen lack organizedor irregular deposits of tapetal debris.