ULTRASTRUCTURE OF PARKIA POLYADS (MIMOSOIDEAE: LEGUMINOSAE)

Pollen of 16 species of Parkia (ca. 30 spp.) was examined in the light, scanning and transmission electron microscopes (LM, SEM and TEM). Pollen is shed as large (61-236 μm), globose, 16–32-grained polyads that remain united following acetolysis. Thin sections reveal that polyad cohesion is maintained through fusion and/or adhesion and/or appression of adjacent ektexines in lateral and/or proximal and/or distal regions. In lateral and proximal regions the ektexine is often embedded in a dense granular endexinous matrix. Individual grains of the polyad are clearly outlined in most species but are completely obscured in several New World species. Sculpturing varies both in pattern and type of element. Nonuniformly sculptured polyads, in which the sculpturing of individual grains differs between the peripheral and central portions of the distal face, occur only among Old World species. Uniformly sculptured polyads occur among both New and Old World taxa. Two main sculpturing types are present among Parkia species: verrucate and tectate-perforate, the latter particularly well developed in the reticulate polyads of the neotropical P. ulei and P. multijuga p.p. Ultrastructurally, all Parkia polyads exhibit ektexine and endexine continuous around the individual grain. Distal ektexine is prominent and clearly stratified, usually exhibiting a thick, perforate tectum, columellate interstitium, and a thin, locally discontinuous foot layer. Lateral and proximal ektexine is significantly thinner and less structured. Individual grains within the polyad possess three to four peripheral-distal apertures usually at the points of contact between adjacent grains. Proximal and central grain apertures occur in most, but not all, species. Polyad characters suggest a particularly close relationship among the New World species P. decussata, P. discolor, P. gigantocarpa, P. igneiflora, and P. nitida. Parkia ulei, though sculpturally closest to P. multijuga p.p., is ultrastructurally most similar to the African P. biglobosa. The large tectate-perforate-fossulate polyads of P. pendula and P. platyparkia are unique in the New World.