Comparative anatomy of two forms of Sri Lankan Calotropis gigantea (L.) R. Br. (Family Apocynaceae s.l. – Subfamily Asclepiadoideae) -Taxonomic implications

Calotropis gigantea (L.) R. Br. is considered a versatile plant that has been described in Ayurvedic medicine. Two forms of C. gigantea, differing in flower colour (purple and white) occur in Sri Lanka. In the absence of flowers, it is difficult to differentiate the two forms due to the similarities in their vegetative morphology. Accurate botanical identification of the two forms of C. gigantea is essential in developing conservation strategies and establishing a proper management system. Anatomical studies of the vegetative structures of the two forms of C. gigantea were carried out using Light Microscopy and Scanning Electron Microscopy to study the anatomical features between the two colour forms. The adaxial and abaxial leaf surfaces of the same form, as well as the two forms, differ in terms of cuticular material deposition and cuticular material organization on the leaf surfaces. The amount of leaf cuticular material was higher on the abaxial surface than on the adaxial surface of the purple form, compared to the white form where both surfaces have more or less similar amounts of cuticular material based on the observation. Irregularly scattered and sunken amphistomatic, anomocytic, elliptical-shaped stomatal complexes, unbranched, and uni/ multicellular trichomes in the leaves and stems were observed in both forms. Simple, branched, “Y”, “H” shaped laticifers were observed throughout the vegetative parts. Stem and stomatal anatomy and the arrangement of cuticular materials are useful to differentiate the two-colour forms of C. gigantea which is important taxonomically as well as in conservation.