Eight species in the Namib Desert, South Africa were assessed for their leaf area ( A), thickness ( z), saturated ( Q) and dry mass, relative volume of air ( F _a), water and dry mass, intrinsic water-use efficiency (based on δ ¹³C), and N, P and cation (Na+K) contents. As water-storage capacity is a function of Q _v and z, this means Q/ A (= Q _v • z) is an ideal index of succulence compared with specific-leaf-area and other indices that highlight mass rather than volume. Specific gravity ( ρ _l) has a different relationship with the F _a of sclero-mesophylls: rising ρ _l infers decreasing air content is replaced by water rather than dry matter. The trend among succulent species, including Argentinian/Spanish added to our study, was Q/ A exceeding 1 mg water/mm ² whose overall slope was ten times that for co-occurring sclerophyll-mesophyll species, and shows the futility of seeking a universal relationship among plants regarding their water-storing properties. (Na+K), N and P concentrations varied on a dry-matter, but not water-volume, basis. W _i relationships were essentially functions of variations in z and increased metabolic efficiency. We conclude that z and Q _v are keys to the special physiological properties of succulent leaves. Adding succulents would force many current monotonic relationships to dichotomize.