An Interpretation of the Flower of Petalostemon (Leguminosae)

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Abstract
There is no clearly defined disjunction betweenPetalostemon andDalea, and evidence from vegetative anatomy, floral morphology, and chromosome number indicates a common ancestry. Yet it is apparent thatPetalostemon represents a highly specialized branch within the heterogeneous daleoid complex. Such specialization has resulted in the loss of a whorl of stamens and migration of 4 petals to the apex of the staminal tube. The species ofDalea possessing petalostemoid characteristics appear to arise from diverse sections of the genus and probably represent specialized and independent, albeit somewhat convergent evolutionary trends. A complete reassessment of the placement ofPetalostemon must await revision of the remainder of the daleoid complex but it is desirable at present to redefine the generic delimitations ofPetalostemon to exclude all taxa possessing more than 5 stamens as well as those with petal insertion substantially below the apex of the staminal tube. The reticulate, often convergent pathways taken by the evolutionary processes in many groups often makes generic lines somewhat indistinct. This is certainly true in many groups of legumes and emphatically true in other plant families (e.g. Cruciferae, Graminae). Therefore, at least more than one viewpoint of what constitutes generic delimitation in a particular taxon is admissible. The alternative to the present disposition ofPetalostemon, as advocated by Shinners (1949) and more recently by Barneby (1965), would be to reduce it to some subgeneric ranking withinDalea; but again, the same problems would be encountered only at a different taxonomic level. The acceptance ofPetalostemon as herein defined seems to result in a reasonably natural genus of perennials, easily definable from the diverse genusDalea.