Sporoderm development inNymphaea mexicana (Nymphaeaceae)

Mature pollen grains ofNymphaea mexicana have a verrucate proximal surface, a psilate distal surface and an anazonasulculus (encircling-sulcate aperture). The developmental events of microspores and tapetal cells were observed with TEM and SEM. Radially oriented substructural elements are seen in the microspore surface coating ofNymphaea mexicana from the early tetrad stage through the whole exine development. These elements, being the structural units of the microspore surface matrix (glycocalyx), are associated with sporopollenin precursor accumulation. In young free microspores, radially oriented elements are observed at both proximal and distal poles as a palisade between the endexine and plasmalemma.—Several points are discussed: (1) the initial and mature forms of exine substructure elements; (2) the significance of exine substructure for realisation of morphogenetic processes; (3) the ways by which verrucate and psilate sculpture patterns are developed.