Floral morphology and taxonomic relations among the genera of Amaranthaceae in the New World and the Hawaiian Islands

Twenty-three genera of Amaranthaceae occur in the New World. Two endemic genera occur in the Hawaiian Islands. Among the genera of the subfamily Amaranthoideae, Celosia, Cyathula and Achyranthes have their main distributions in the Old World; the two last-named genera are represented in the Americas only by widespread weeds. All the New World genera of the subfamily Gomphrenoideae are mainly or entirely restricted to this region. Characters of androecium and gynoecium are fundamental in the recognition of genera within the family. Androecia of different genera may be structurally and phylogenetically more similar than would appear from a cursory examination. It is suggested that the type of staminal tubes found in Pseudogomphrena and Froelichia can be derived from that in Alternanthera and Froelichiella by reduction of filament length and a fusion of pseudostaminodia with the filaments. The staminal tube in Gomphrena could result from a further decrease in distance between pseudostaminodia of the Pseudogomphrena type, and a deeper forking of the pseudostaminodia; each so-called apical filament lobe in Gomphrena would then be homologous with half a pseudostaminodium in Pseudogomphrena. Much of the variation in the androecia of these and other genera, as well as within genera such as Pfaffia, can be explained as the combined results of coalescence and splitting-up tendencies. Splitting up of staminal tubes may not necessarily take place along the borders of phylogenetically original filaments and pseudostaminodia. The Amaranthus-type of pollen is found in the majority of genera of the subfamily Amaranthoideae, but also in the Chenopodiaceae. A group of genera within the subfamily Gomphrenoideae also has pollen very similar to, or identical with, this type. Most genera of the subfamily Gomphrenoideae have pollen of the Gomphrena-type. Pseudoplantago has unilocular (at anthesis) anthers, a characteristic of the subfamily Gomphrenoideae, but floral structure as well as pollen morphology connect the genus to a group of genera within the Amaranthoideae, subtribe Achyranthinae. The combination of subcuboidal shape and opercula with radially arranged hooked protuberances, makes the pollen of Pseudoplantago unique among the angiosperms studied so far. Floral morphology and palynological characteristics indicate a close relationship between Pfaffia and Alternanthera. Both genera, as currently accepted, are relatively homogeneous from pollen morphological points of view. There are no correlations between pollen morphology and the variation in the androecium in Pfaffia, nor would pollen structure support recognition of Hebanthe as a distinct genus. Woehleria and Irenella may be derived from, or be of the same origin as, Dicraurus and Iresine. All four genera are placed in the subfamily Gomphrenoideae because of the bisporangiate anthers, but their pollen structure is very close to, or identical with, that of the Amaranthus-type. Pseudogomphrena combines characteristics of Gomphrena and Pfaffia.