Fine Structure of Mistletoe Pollen VI. Small-Flowered Neotropical Loranthaceae

Pollen of small-flowered neotropical Loranthaceae (8 genera; ca.135 species) was examined in the light, scanning, and transmission electron microscopes.Pollen is typically medium-sized and oblate.Pollen amb is more variable than equatorial shape, ranging from trilobate deeply concave to circular.Both isopolar and heteropolar pollen grains occur within the complex, the latter restricted to species of Phthirusa and Struthanthus in which the apertures differ at each of the polar faces.Simple apertures arranged i in a diploaperturate configuration predominate.Such apertures ne from diploporate to to C und apertures are rare, restricted to Oryctanthus and particular species of Cladocolea and.| Sasha s Pun The (3-)4-5 colpate apertures of Ixocactus are unique in the famil ily.Sculpturing is -uniform with sculpturing ridges and/or striat e. Ultrastructurally, the basic ektexine structure IS composed of a thin, perforate tectum, prac ated nie interstitium, and a thick continuous foot layer usually twice as thick as the tectum plus interstitium.Pollen data suggest 2 Group I genera, dropemon is closely linked to PAthirusa through the species P. pyrifolia and P. platyclada.Oryctanthus is a highly derived genus with only remote ties to other Group I taxa.Among Group II genera, pollen data indicate a close relationship between Cladocolea and the Mexican species of Struthanthus.Pollen characters of /xocactus indicate no relationship with any small-flowered neotropical genus.Rather its pollen features are closer to those of the Eremolepidaceae and the African species of Viscum also shows a marked tendency towards unisexual flowers and dioecism.Only three of the eight genera exhibit strictly bisexual flowers (Oryctanue ?Department 3 t strictly bisexual Foca and others with exclusively unisexual flowers; three genera (Struthanthus excluding S. panamensis, Maracanthus, Oryctina) demonstrate only unisexual flowers.Small flower size, the tendency towards unisexual flowers and dioecism, and an x = 8 chromosome number (Barlow & Wiens, 1971) together suggest an independent evolution of this group unlike Old World and other New World Loranthaceae.