The unique plants of the Sonoran Desert Region have developed specialized structures to help them survive in a harsh climate. Cacti are common desert plants with some easily recognizable anatomical features (flowers, fruit, branches, roots), but many of their structures require further study for accurate rendering.
Of these desert plants the Opuntia (the prickly pear cactus) are the most common cacti, and their relatively large size makes them a good plant for observation.
The largest feature in the above diagram is the cactus pad. This will be the most apparent structure in any member of the Opuntia.
Below is a drawing of two pads of Opuntia chlorotica by Daniela Siroky . Different specimens of the same species of cactus can have different combinations of structures. Here a pad from the specimen on the left is armed with both spines and glochids while the pad from the specimen on the right has only glochids.
Both spines and glochids grow from a structure called the areole. Also present in the areole are miniscule whitish hairs referred to as wool.
The first drawing below of Opuntia chlorotica by Daniela Siroky shows the long spines and the dense cluster of glochids present in the areole. The bottom drawing of Cylindropuntia acanthocarpa var. coloradensis by Marsha Bennett shows different structures in the areole: a long central spine that is covered in a sheath and several smaller and more flexible spines in a radial pattern.
The example below is a closeup of a portion of the painting of Opuntia engelmannii by Mary Emily Eaton. Areoles on the cactus pad are very distinct and have both spines and glochids. The whitish areas at the bottom right are not wool on the areoles but rather clusters of a cottony material that protects the cochineal insect that is feeding on the prickly pear.
The barrel cactus, or Ferocactus, has areoles with much stouter spines than those of the Opuntia. Below is another painting by Mary Emily Eaton, Ferocactus rectispinus. The areole with its whitish wool is easily seen as are the formidable central and radial spines.
Another formidable set of spines can be seen below in Paulus Roetter's drawing of Ferocactus wislizeni . For this scientific illustration, the entire areole is drawn separately from the plant in order to show as clearly as possible the individual spines and their relation to the areole.
