Ribbing, orientation of cladodes, and variations in stem height were examined theoretically and in the field to see whether the morphology of various cacti could be interpreted as adaptations for intercepting photosynthetically active radiation (PAR). The total daytime PAR incident on different parts of the stem was related to the nocturnal increase in tissue acidity of these Crassulacean acid metabolism plants. The acidity change was 90% saturated for a total daytime PAR of about 22 mol m-2, which indicates that the vertical stems of cacti are often light-limited in the desert.PAR interception by a cactus with ribs of various depths was simulated. Ribbing led to more surface area but a lower PAR per unit area, so net carbon gain was little influenced.Although the cladodes of Opuntia basilaris Engelm. and Bigel. var. basilaris were randomly oriented, those of another platyopuntia, O. chlorotica Engelm. and Bigel, preferentially faced northsouth. Compared to facing east-west, cladodes facing N-S would receive 52% more PAR at the winter solstice, a time when water is generally available and overheating is not a problem in the Mojave desert habitat of O. chlorotica.The maximum height of Stenocereus gummosus (Engelm.) Gibs. & Horak varied from 0.7 m in northern Baja California (31°52'N) to 4.1 m at 23°47'N. Stem height was positively correlated (r 2=0.93) with the height of the surrounding vegetation and negatively correlated (r 2=0.91) with the PAR 1 m above the ground. The great plasticity in stem height permits S. gummosus to obtain sufficient PAR in the presence of subtropical trees in the southern part of its range and yet expend less biomass to reach unobstructed PAR in the northern part of its range, where the surrounding vegetation is much shorter.