Cactus Evolution – Part I

By Joe Merkelbach (February 2009)

As a group of plants, cacti are easy to recognize. They have green stems that can photosynthesize to produce food and therefore need no leaves. They also have an ability to store water in their stems and, generally, the presence of protective spines.

These characteristics are readily seen and recognized, so it easy to determine whether a plant is a cactus, but the origins of the Cactaceae family group are prickly to tease apart. Since cacti have developed and used these characteristics to thrive in dry, harsh habitats, they have left behind no fossilized evidence of their development. Water-distributed sediments are the mother lode formations of fossils, and cacti have had relatively little coexistence with these conditions – hence no known fossilized cacti.

More recently developed scientific techniques have begun to unravel cactus evolution. Biochemical comparisons of chromosomal similarities and DNA evaluations have indicated that the relationships are closest to other succulents, the family Portulacaceae. The common bedding plant rose moss and the elephant bush, a collected succulent, are examples. This family shows reduced leaf size, an amount of stem succulence and some hairiness at leaf axils. Development and honing of these traits in response to climatic conditions is what provides cacti their advantages in xeric habitats.

The stem succulence is actually a sophisticated adaptation; the surface cells are photosynthetic, but are long-lived instead of ephemeral. The waxy cuticle which covers them is a key part of their extended life cycle. The tan, barklike tissue that sometimes develops at the base of old plants in the wild is made of dead surface cells, their function assumed by cells higher up the stem. This is the reason plants which suffer pest attacks sometimes show tan coloration in damaged areas.

Another evolutionary adaptation is the development of crenellations and tubercles on the stem body, which allow the plant to take on water during the sparse periods of availability. These areas can swell and then shrink as water is absorbed and used without tearing the critical photosynthetic surface.

The taxonomy of cacti is notoriously changeable due to the absence of any fossil examples to indicate past relationships and the integration of new biochemical and DNA methods to reveal previously hidden relationships.

Next month, I will discuss some more of the evolutionary relationships of North American cacti in advance of a program about endangered cactus species in the United States.

Sources:
American Naturalist – “Pereskia and the Origin of the Cactus Life-Form” – 2006 – http://www.journals.uchicago.edu/doi/abs/10.1086/504605
University of Texas School of Biological Sciences – http://www.sbs.utexas.edu/mauseth/ResearchOnCacti/large%20photo%20Pereskia%20guamacho%20w%20bark.htm
Enchanted Learning – http://www.enchantedlearning.com/subjects/plants/types/cactus