Abstract
Twelve native United States species have been examined. The pollen grains could be divided into five pollen types based on sexine ornamentation, aperture structure, sexine/nexine ratio and sexine 1/sexine 2 ratio. Within these types it was often possible to distinguish the grains of the different species. Two groups of types have been recognized based on the similarities in ornamentation. One group, consisting of the Phyllanthus amarus type, the P. tenellus type and the P. urinaria type, is characterized by the Tilia structure, whereas the second group of types, comprising the P. caroliniensis type and the P. liebmannianus type are microreticulate. The postulation that different subgenera show different pollen types is true in general, but there is one exception: P. niruri, taxonomically belonging to the subgenus Phyllanthus, has pollen grains of the P. caroliniensis type, a type also comprising species from the subgenus Isoclades.
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