The evolutionary significance of the legume genus Chamaecrista, as determined by nodule structure

Nodulated plants of 14 species of Chamaecrista (Leguminosae, sub-family Caesalpinioideae) were grown. The structure of cells from the active N₂ -fixing region of nodules varied greatly with species. At one extreme bacteroids were confined solely by peribacteroid membranes, typical of advanced papilionoid legumes such as soybean. At the other extreme, bacteroids were retained within sheath-bounded persistent infection threads, previously reported as typical of the Caesalpinioideae and also found in certain of the base genera of the Papilionoideae. Other species had an intermediate nodule structure. Nodules of the genus Chamaecrista thus appeared to be at an evolutionary crossroads between the primitive and more advanced forms.