Morphological Variations in Hoya siamica Craib (Asclepiadaceae) in Thailand

Abstract

Hoya siamica Craib is a climbing epiphytic plant belonging to the family Asclepiadaceae. Morphological variations were observed in 33 H. siamica samples collected from five floristic regions in Thailand and from 8 specimens kept in the BKF, BK, K and BCU herbaria representing a further 4 locations. This plant was found to have extremely variable vegetative and reproductive structures. Based on the qualitative morphological characters, H. siamica can be divided into five Forms that can be distinguished by their leaf shape and venation and the shape of the corona scale, pollinium and corpusculum. Form I has an obscure lateral vein, a long petiole, a broadly ovate and oblong-obovate shaped corpusculum and pollinium, respectively, whilst Forms II - V have a conspicuous lateral vein, short petiole, and an arrow head and narrowly oblong-obovate shaped corpusculum and pollinium, respectively. It is clear that “Form I” is distinguishable from the four others and the holotype and so may possibly be a cryptic species or variety of the Hoya siamica complex in Thailand. Although Forms II - V have some degree of variation in their leaf shape, corona shape and outer angle of corona between them, they nevertheless still share some important vegetative and reproductive characters. Thus, they may possibly simply be variable forms of the H. siamica complex in Thailand.