Apomixis in Pennisetum: An Ultrastructural Study

Ultrastructural studies on sexual embryo sacs in angiosperms are widespread, but hardly any information is available at this level for asexual (apomictic) embryo sacs. Sexual and asexual embryo sacs of Pennisetum are compared in detail to reveal similarities and/or differences in cell number, their cytoplasmic content, in the distribution of wall projections and the embryo sac boundaries. At this level, the two types of embryo sac are remarkably similar, and, apart from small differences in wall structure, this confirms the view that the chief contrasts concern nuclear number and function. We note the "plasticity" of nucellar cells that generate the asexual embryo sacs and consider the likely usefulness of bringing together different approaches (ultrastructural, physiological, and genetic) in the study of important rangeland grasses.