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

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Abstract
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 in a diploaperturate configuration predominate. Such apertures range from diploporate to diplobrevidemicolpate to diplosyn- and diploparasyndemicolpate types. Compound apertures are rare, restricted to Oryctanthus and particular species of Cladocolea and Struthanthus. The (3-)4-5 colpate apertures of Ixococtus are unique in the family. Sculpturing in basically uniform with sculpturing elements ranging from perforations to shallow ridges and/or striato-rugulae. Ultrastructurally, the basic ektexine structure is composed of a thin, perforate tectum, granular/columellate interstitium, and a thick continuous foot layer usually twice as thick as the tectum plus interstitium. Pollen data suggest two basic groupings of small-flowered genera: Group I, composed of Dendropemon, Phthirusa pro parte, and Oryctanthus: and Group II, containing Phthirusa pro parte. Cladocolea, Struthanthus, Maracanthus and Oryctina. Among Group I genera, Dendropemon is closely linked to Phthirusa 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 Ixocactus 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.