The morphology and evolution of the genus Conophytum N.E. Br. (Aizoaceae)

Date of Completion

January 2004


Biology, Botany|Biology, Genetics




Plants of the genus Conophytum are dwarf xerophytes from the arid, winter-rainfall regions of southern Africa, remarkable for the extreme succulence of their fused leaf pairs, and for the frequently cryptic appearance of their vegetative bodies, which has earned them the common name “living stones.” About 10 other genera in the family Aizoaceae contain sphaeroid succulents, with analogous shoot morphology, but none appear to be closely related to Conophytum. With about 100 species, exhibiting a range of growth forms and ecological specializations, Conophytum is by far the largest and most diverse genus of living stones. ^ My investigations of the microscopic characteristics of Conophytum leaves, using SEM and light microscopy, has revealed hitherto unknown variability between species in the expression of characteristics such as sunken stomata, bladder cells, the hypodermis, and epidermal layers of calcium oxalate crystal sand. The patterns of distribution of these anatomical and micromorphological features provide new insights into the taxonomy, systematics, and ecology (many of the characters are relevant to survival in dry climates) of the genus. The chromosome counts that I have obtained for Conophytum enable comparative study of the cytology of the genus, which consists predominantly of diploid populations, but with several complexes of tetraploid species. ^ Using evidence from my SEM, anatomical, and cytological work, along with characters from gross vegetative and floral morphology, I have undertaken the first morphological cladistic analysis of Conophytum. The resulting phylogeny resolves, with bootstrap support, a number of the sections of the genus, while indicating that other traditionally recognized groupings are actually paraphyletic or polyphyletic. Several biologically significant characters, such as nocturnal flowers and the windowed-geophyte growth form, seem to have evolved more than once, in parallel. A striking trend in the character evolution of Conophytum is the reduction of plesiomorphic anatomical adaptations to drought, like sunken stomata, with a compensatory rise of novel strategies for surviving in xeric environments, like protective tunics formed from old leaves, and the retreat of the plant body underground. ^