Exine structure in Cecropia L. (Moraceae) pollen grains

Abstract

Species of the genus Cecropia have very small pollen grains. The exine stratification and surface ornamentation are here elucidated by means of electron microscope observations. The ten species studied may be divided into two groups according to the distribution of spinules over the pollen grain surfaces: one group with uniform distribution and the other with spinules only at equatorial regions so that the polar areas are nearly smooth. From the surface inward, the exine presents supratectal spinules; a well formed tectum traversed by very small channels; short and fine bacula; a discontinuous nexine 1 and a continuous nexine 2, which is thicker around the pores. With respect to the differentiation of the exine structure, Cecropia exhibits a degree of evolution within the Moraceae family, which argues against any approximation of this genus to the Urticaceae family.