Spine anatomy and its systematic application in Astragalus sect. Rhacophorus s. L. (Fabaceae) in Iran

Cross sections of petioles in 35 species of Astragalus mainly belonging to sect. Rhacophorus were studied for circumscription of natural groups within the section and reliable characters in delimiting polymorphic species. The following characters were determined to be informative: number of parenchymatous cell layers in pith, diameter of median sclerenchymatous vascular bundle sheath and diameter of the cross section as whole. Astragalus amblolepis, A. andalanicus, A. floccosus, A. meyeri, A. paralipomenus, A. pycnocephalus and A. stenolepis are characterized by large pith and relatively large sclerenchymatous bundles. Most of these species belong to sect. Rhacophorus s. str., comprising large-flowered species as circumscribed by some taxonomists, but few species of sects. Stenonychium and Platonychium also show these features. Our results show that spine anatomy is not useful for circumscribing sections in spiny Astragalus, but applicable in delimiting small natural groups and circumscribing species.