Electron microscopy shows that the tapetal cells that surround the developing pollen grain in the anther have a highly active secretory system containing lipophilic globules. These globules are believed to contain sporopollenin precursors. Tracer experiments have shown that phenylalanine is a major precursor, but other carbon sources also contribute. The biosynthetic pathway for phenylpropanoid is very active in tapetal cells, supporting the idea that its products are needed for sporopollenin synthesis. Chemical inhibitors of pollen development and many male sterile mutants have effects on the secretion of these globules by the tapetal cells.