Stomatogenesis in the genus Portulaca L. (Portulacaceae)

Abstract

The stomata in the six Indian species of Portulaca L. are paracytic and predominantly dicyclic, completely enclosed at their polar ends by the subsidiaries, and mesogenous dolabrate and basipetal in differentiation except in P. oleracea, where they are diffuse in origin. The subsidiaries of the outer cycle develop directly from the stomatal meristemoids. The leaves are amphistomatic except in P. quadrifida, where they are epistomatic. The stomata are mostly orientated at c. 90o to the midrib of the leaf, but in P. oleracea they are irregularly orientated. The origin of the stomatal orientation appears to be genetically regulated rather than epigenetic. In Portulaca, as in some other angiosperms, the epidermal pattern of the costae is similar to that of the interstices, i.e. stomatiferous with nearly isodiametric cells. This is considered to be due to prevention by the thick chlorenchyma, characteristic of these plants, of the stomatal inhibitory stimuli originating in the vascular tissues from reaching the epidermis.