Four succulent families and 40 million years of evolution and adaptation to xeric environments: What can stem and leaf anatomical characters tell us about their phylogeny?

Four families of the eleven or so families of Caryophyllales have been particularly successful at adapting to xeric environments: (1) ice plants (Aizoaceae), (2) cacti (Cactaceae), (3) Old World cacti (Didiereaceae); and (4) portulacs (Portulacaceae). Ice plants adapted to the harsh conditions of the deserts of southern Africa; many evolved leaf succulence to an extreme degree, some went underground, others lost the ability to make normal wood, and all had to adapt their reproductive strategies in pollination or seed dispersal. Cacti adapted to deserts of South, Central, and North America by losing their leaves over time and evolving various types of succulent stems. Cacti also adapted by greatly reducing the presence of vessels in their wood, evolving a novel tracheid type termed wide-band tracheids, and evolving CAM and C-4 metabolisms. Plants of Didiereaceae adapted to the harsh Madagascan environment by evolving both stem and leaf succulence and the columnar stem form, but these plants maintained their normal, non-anomalous woody growth. Members of Portulacaceae evolved in many different xeric areas of southern Africa, but all evolved leaf succulence and limited stem succulence, much the same as found in Aizoaceae. Each family evolved varying strategies for living in water-stressed environments. Common themes among these distantly related plants are leaf and/or stem succulence, and anatomical and physiological adaptations.