Anatomy and Development of Leaves in Chamaecrista mimosoides and C. nomame (Leguminosae-Caesalpinioideae)

Abstract

Chamaecrista mimosoides (L.) Greene has a unique even-pinnate leaf with crenate-crested protuberances on the adaxial side of the rachis between leaflet pairs, which is one of the important characteristics for distinguishing it from other species with leaf rachises canaliculate. The present study clarified anatomical and developmental features of the leaves with the crenate-crested rachis in C. mimosoides in comparison with those with a canaliculate rachis in C. nomame (Makino) H. Ohashi. Transverse sections showed that the crenate-crested rachis has a protuberance constructed by continuous parenchymatous tissues composed of cells rich in chloroplasts, whereas the canaliculate one has two ridges constructed by two independent parenchymatous tissues. Developmental observations showed that the crenate-crested rachis is initiated as a single swelling of tissues, whereas the canaliculate one initiated as two parallel swelling of tissues. Both rachises have commonly two ridge bundles divided from the main vascular bundles in the petiole, but the two ridge bundles are fused into one bundle in the proximal half of the crenate-crested protuberance between leaflet pairs, and then divided into two again in the distal half. In contrast, the two ridge bundles remain separated throughout the length in the canaliculate rachis. The fused ridge bundles in the crenate-crested protuberance, moreover, branch a small bundle into the protuberance. At the distal part of the petiole there is an extrafloral nectary (EFN) in both species. The vascular bundles are supplied to the EFN from each of two ridge bundles in the petiole in C. mimosoides, whereas from the main vascular bundles in C. nomame. The repetition of fusion and separation of two ridge bundles in the crenate-crested leaf rachis seems to support the idea that the rachis is produced by fusion of two ridges of the canaliculate rachis. The chloroplast-rich cells observed in the crenate-crested protuberance as well as many stomata in the epidermis suggest that the protuberance may improve the ability of photosynthesis.