LANDRUM, VIC. Biology Department, Washburn University, 1700 SW College Ave. Topeka, KS 66621. - Four families and 40 million years of evolution and adaptation to xeric environments.
Many of the eleven or so families of the Caryophyllales have been
particularly successful at living in xeric environments. Four families
have shown the greatest amount of adaptive changes: (1) the ice-plants
(Aizoaceae), (2) the cacti (Cactaceae), (3) the Old World cacti
(Didiereaceae), and the portulacs (Portulacaceae). The 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. The cacti adapted to deserts of South, Central, and
North America by losing their leaves over time and evolving two types
of succulent stems: columnar (ex. saguaros) and rotund (ex. peyote).
Cacti adapted by greatly reducing the presence of vessels in their
wood, evolving a novel tracheid type termed wide-band tracheids, and
evolving CAM and C4 metabolisms. Plants of the Didiereaceae adapted
to the harsh Madagascaran environment by evolving both stem and leaf
succulence, and evolving the columnar stem form, but these plants
maintained their normal woody growth. Members of the Portulacaceae
evolved in many different xeric areas of southern Africa, but all
evolved leaf succulence rather than stem succulence, much the same as
was found in the 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, anatomical
and physiological adaptations, and varying reproductive strategies.
Key words: Aizoaceae, Cactaceae, desert, Didiereaceae, Portulacacese, succulent