Petiolar Anatomy of North American Astragalus Species (Fabaceae) with Persistent Petioles

Abstract

The petiole and rachis anatomy of 11 North American Astragalus species that show a tendency towards persistent petioles is described and illustrated by line drawings of representative transections. The results are compared with those of a spine anatomical survey of 200 Old World Astragalus and Astracantha species. Attention was given to the anatomical characters previously determined to be taxon-specific such as the amount and position of sclerenchyma and distribution patterns of vascular bundles. Character evolution and the classification of species into sections are discussed. A fundamental difference between New World and Old World Astragalus s.1. species was found. Most of the sclerenchyma contributes to the stability of the persistent petiole by the inner vascular bundle sheath—primarily the median bundle—in North American Astragalus species. In the Old World species, however, the outer sclerenchymatous bundle sheaths form most of the sclerenchyma of the spine. In American as well as in Asian species, the persistence of petioles (and rachises) evolved several times in different sections by convergence. Species representing distinct stages of the evolution towards a spinelike organ occur in the three sections Jejuni, Humillimi and Neonix. Thus, unlike in the Old World species in which the petiolar anatomy of each section reached more or less the same evolutionary level, in the New World the sections investigated contain species with different levels of spine development.