Cactus flowers are distinguished by the presence of an inferior ovary - that is, the ovary is located beneath the other parts of the cactus flower. Cactus flowers come in all colors except for blue.

The presence of an Areole separates the cacti from other succulents. Spines, hairs, flowers, new growth, and fruits originate from this pad-like structure in the majority of cactus species.

Photo shows a longitudinal section of a tuberculate cactus similar to plants in the Mammillaria and Coryphantha genera. The tubercle, and the rib in columnar cacti, are expandable to allow the cactus to store water during times of plentiful rainfall. In times of drought, these organs contract. The vascular tissue illustrated consists primarily of two tissue types: xylem vessels and phloem. The xylem vessels transport water from the roots. Phloem facilitates the transfer of sugars throughout the plant. As plants age, the vascular tissue produces wood.

Drawing shows four common cacti of the Sonoran desert: Cholla, Saguaro, Prickly Pear, Barrel Cactus. Even though their growth habits are different, the cholla and prickly pear are both members of the Opuntia genus. The Saguaro, whose scientific name was named for Andrew Carnegie, is the typical monarch of the desert. Barrel cacti of the desert are predominately Ferocactus and Echinocactus species.