Mojave National Preserve - Cacti / Desert Succulents

Cacti are xerophytes, plants specially adapted to arid climates. There are 93 genera and approximately 2000 species of cacti worldwide. The North American deserts contain a wide variety of cacti, and 20 species are known to occur within Mojave National Preserve. Of these over half (11 species) belong to the genus Opuntia , which includes the chollas and prickly-pear cacti. Opuntia cacti have jointed stems which are cylindrical in the case of chollas and flat in the case of prickly-pear. All Opuntia also have glochids, sharp, barbed bristles that are different from spines. Some Opuntia cacti, such as the beavertail cactus ( O. basilaria ) even generally lack spines altogether. Prickly-pear and cholla are quite common and easily seen throughout the preserve.

Cacti employ several evolutionary adaptations which allow them to survive in the harsh climate of the desert. These modifications generally serve to reduce transpiration and consequently conserve water. Some species, such as barrel cacti ( Ferocactus spp.), subsist on water stored in fleshy stems during the rainy season. Cacti also have small leaves which are modified into spines, thereby reducing the surface area to volume ratio of the leaf and providing protection from herbivory to the plant. The stomata are located on the lower leaf surface, shaded from the sun, and are often recessed to provide some shelter from the wind.

Perhaps the most interesting adaptation that cacti employ is CAM photosynthesis. Photosynthesis is the process by which plants produce food using energy from the sun. CAM stands for crassulacean acid metabolism, for the family (Crassulaceae) in which it was first discovered. CAM photosynthesis is a process by which plants assimilate carbon dioxide by an alternative photosynthetic pathway than the typical pathway. In most plants, the stomata must remain open during the day in order to allow for the uptake of carbon dioxide, which is a vital step in photosynthesis. In plants which utilize CAM photosynthesis the stomata stay closed during the day, when transpiration (and consequently water loss) would be the most severe. Instead, mesophyll cells in the leaves incorporate carbon dioxide into organic acids by way of special enzymes. During the day these organic acids are broken down to release the carbon dioxide within the leaf, and photosynthesis continues via the conventional photosynthetic pathway. The behavior exhibited by the stomata of CAM plants is an example of a short-term physiological adjustment as well as a long-term evolutionary adaptation.