The cactuses store water in their stems and thus are called stem succulents. In the course of its evolution the cactus has eliminated leaves, and their function has been taken over by the green outer covering of the stems. Thus the amount of transpiration (moisture loss through plant breathing pores) has been greatly reduced. [Illustration: Saguaro buds.] The cactuses are thought to have evolved from relatives of the rose family in the West Indies, beginning some 18,000 to 20,000 years ago. From there they spread to most parts of the Western Hemisphere, but particularly to the drier regions, changing their forms to meet new conditions. One of the youngest of plant families, the cactuses are still evolving rapidly. This doesn’t make the task of classifying them easier for the taxonomists. In varied forms, cactuses enliven the paloverde-saguaro community. In size they range from tiny button and pincushion types, some of which weigh only a few ounces, to the giant saguaro, the bulkiest of which may weigh several tons. Cactuses as a group are easily recognized, although many people mistakenly believe that any desert plant with spines or thorns is a cactus. Shreve describes their main characteristics thus: Several structural features have served to give the cacti their outstanding appearance, so unlike that of other plants. Most general have been the loss of the leaf as a permanent organ, the enlargement of the stem to accommodate water-storing tissue, and the development of local spinebearing structures known as “areoles.” In several genera, the stem is segmented into sections which are flat and somewhat leaflike; in others the stem is round, much branched, and the surface occupied by close-set tubercles. In a large group, including massive erect forms, as well as slender climbing ones, the stem is grooved or fluted and thus able readily to accommodate its surface to great fluctuations in the water content of the tissues. If you are trying to identify species, however, cactuses can be annoying, since they often hybridize. You must expect to find some individuals that don’t fit the book descriptions. _The Saguaro—Monarch of the Monument_ Of all the species of cactus recorded in Saguaro National Monument, the giant SAGUARO (pronounced sah-WAH-roe) holds the center of interest. From the visitor’s standpoint, all other plants, no matter how bizarre in appearance or peculiar in living habits, are merely stage scenery for setting off the star of the desert drama. For size, this vegetable mammoth tops all other succulents of this country; heights of more than 50 feet and weights of more than 5 tons have been reported. There have been specimens with more than 50 arms, or branches. Although no accurate method of determining saguaro age has been devised, it is estimated that an occasional veteran may reach the two-century mark.