Anatomical adaptations of four Crassula species to water availability

The genus Crassula contains a number of highly adaptable species, which can inhabit a wide range of environments. This investigation aimed to examine whether there are any differences in the anatomical adaptations in relation to water availability of four species of Crassula: the New Zealand pygmy weed, Crassula helmsii (T Kirk) Cockayne (from an aquatic habitat); the fairy crassula: Crassula multicava Lemaire ssp. multicava (from a subtropical habitat); the jade plant, Crassula ovata (Miller) Druce; and the anteelplakkie, Crassula socialis Schonland (both from semi-arid habitats). Plants were grown in a greenhouse and the anatomical features of stems and leaves were examined using light microscopy. Plant material was sectioned by hand and sections were stained with Toluidine blue O. Cuticle thick- nesses were measured by treating sections with Sudan black B. Stomatal and hydathode densities on leaves and stems were measured using epidermal peels. Two measures of leaf succulence were used: degree of succulence and succulence quotient. The aquatic species C. helmsii had significantly fewer features associated with conserving water, including the thinnest cuticles on the adaxial leaf (P , 0.001) and abaxial leaf (P , 0.001). In contrast, the semi-arid species C. ovata had significantly the highest hydathodes on adaxial leaf surfaces (P , 0.001). Crassula ovata also had significantly the highest degree of succulence (P , 0.001), while C. socialis had the highest succu- lence quotient. The subtropical species, C. multicava, had significantly the thickest cuticles on adaxial leaf (P , 0.001) and stem (P , 0.001). Crassula species from arid environments had significantly more water conserving anatomical features, such as reduced stomatal densities, than those from less arid environments. However, all species studied possessed varying degrees of similar anatomical features. These features make Crassula a highly adaptable genus able to inhabit a wide range of environments.