The Development of Patterning in the Alveolar Sexine of Cosmos Bipinnatus

SUMMARY Development of the sexine component of the pollen wall of Cosmos bipinnatus has been examined in detail. The pattern is first generated in the peripheral spore cytoplasm by ribosome‐coated tubules of membrane, themselves apparently ordered by microtubules. This arrangement of membranous tubules moves to the plasma membrane upon the surface of which are formed ridges of fibrillar material, themselves in register with the tubules beneath. This fibrillar material is laid down heavily in the sites destined to be occupied by spines. The patterning of the ridges is not analogous with that of the sexine, but of the alveoli contained within it. The early sexine itself is formed from precursors secreted by the spore cytoplasm, which encase the fibrils and polymerize. On release of the young spores from the tetrad, the fibrillar content of the sexine is lost, forming the network of alveoli characteristic of this type of pollen wall. The sexine also expands at this point and, since the nexine does not and the two layers are firmly attached to each other at the apertures, this growth results in the bowing out of the sexine in the non‐aperatural regions and, thereby, the formation of the cavea.