FINE STRUCTURE OF MISTLETOE POLLEN. III. LARGE-FLOWERED NEOTROPICAL LORANTHACEAE AND THEIR AUSTRALIAN RELATIVES

Pollen of all large-flowered neotropical loranthaceous genera and related Australian taxa (Nuytsia, Atkinsonia) were examined in the light, scanning, and transmission electron microscopes. Trilobate, deeply concave, peroblate or oblate pollen shapes are basic within the complex; triangular, slight convex, suboblate shapes are more derived features. The non-fixiform pollen of Atkinsonia is unique within the family. Most of the large-flowered genera possess exclusively syncolpate aperture types. However, among Psittacanthus species tricolpate, diplosynrugate and diplorugate apertures are also present, representing derived types for the complex. The inaperturate pollen of Atkinsonia appears to have developed independently within the family. Pollen sculpturing is typically non-uniform, i.e., there are pronounced sculpturing differences in polar and equatorial regions. Uniformly sculptured pollen is restricted to Atkinsonia, Ligaria and some species of Psittacanthus. Ultrastructurally, most exine modifications have occurred in the equatoral ektexine. Here the basic organization ranges from essentially columellaless to columellate, the latter the more derived condition. Endexine is typically thick and stratified in polar areas, thin and lamellate in peripheral and apertural regions. Similar pollen morphologies of Nuytsia and Gaiadendron support the idea of a transoceanic evolutionary connection between the Old and New Worlds. Pollen characters show Gaiadendron to be the most primitive and Psittacanthus the most advanced among the large-flowered neotropical genera.