The systematic significance of pod morphological and anatomical variations in Indian Crotalaria L. (Leguminosae)

Abstract

Pod morphological and anatomical variations of Indian Crotalaria were evaluated to understand the systematic significance and dispersal mechanisms. Pods show variation between species with respect to size, colour, persistence of calyx, presence of trichomes and number of seeds per pod. Pods with length less than 1.5 cm have thicker endocarp when compared to the combined thickness of exocarp+mesocarp (exception being C. berteroana). These pods are likely indehiscent and are dispersed as a unit (enclosing the seeds). On the other hand, pods 1.5 cm or longer have thicker exocarp+mesocarp with respect to the endocarp. These pods dehisce and disperse seeds by explosive mechanism. Seed dispersal is assisted by the sclerenchymatous zone. The angular arrangement of sclereids in opposite directions in the two halves of the narrow fruit enables spiralization soon after explosion of the pods. The orientation of the fibres in the endocarp relative to the longitudinal axis of the fruit is the major factor determining dehiscence. Pericarp anatomy has been categorised into two major types, Type I: with the non-sclerenchymatous zone (exocarp+mesocarp) thicker than the sclerenchymatous zone (endocarp); and Type II: with the sclerenchymatous zone (endocarp) thicker than the non-sclerenchymatous zone (exocarp+mesocarp). The seemingly identical dispersal mechanism in the form of diaspore shows convergent evolution which occurs shows convergent evolution which occurs multiple times in different clades independently (viz., Calycinae and Crotalaria). C. pusilla and C. medicaginea pods possess air cavities, which may be an additional adaptation for diaspore dispersal.