The growth of the pollen tube wall of Oenothera organensis results from the insertion of bodies, composed principally of fibrils, synthesized in cytoplasm. This material for the early growth of the tube wall is derived from double membraned inclusions, present in the pollen on release from the anther. The wall of the later tube is derived from the products of dictyosome vesicles, believed to be non-cellulosic glucans. Some of these vesicles, which are formed in very large numbers on germination of the pollen grain, form an association with banked complexes of mitochondria, and it is proposed that this association indicates that phosphorylation of glucan precursors occurs within the vesicles, rather than in the dictyosome. The mechanism of tube wall growth following incompatible cross-pollinations is identical with that following compatible crosses. The early incompatible tube, however, contains far lower levels of free carbohydrates. Among these carbohydrates must be the precursors required for glucan synthesis, for the tube wall is laid down only thinly at this stage. Once the incompatible tube has passed through the outer layer of the stigmatic cells and their secretion, growth similar to that of a compatible tube starts. Subsequent growth of the tube appears to depend upon the amount of reserve material that passes through the region of low carbohydrate content. While the early interactions between the pollen and stigma are far from understood, it is clear that the self-incompatibility system acts principally during growth of the tube in the viscous fluid coating the outer stigmatic cells.