In 1846, Lieutenant Emory, as he rode southward with the Army of the West, first encountered the creosote as he approached Socorro. The northward extension of the plant is at the same point today, and the range of mesquite extends only a few miles farther north. These two remarkable plants are the main markers of the Lower Sonoran Zone. In the same arid, stony, broken country, the traveler will find an abundance of cacti, and these plants are usually supposed to be the most tenacious of all tenacious growing things. Actually, in southwestern Arizona and neighboring parts of California there are whole areas where the aridity is too severe for cacti, but for our state—let it stand. Everybody, of course, has noted the grotesque shape of the plants, but not everybody has noted that the shape was chosen with a canny intelligence. A globe exposes less surface (that is, the vulnerable area for evaporation) than any other solid of equal volume. And a cylinder is second best. Another thing—cacti being succulents are filled with a watery sap or juice which quickly coagulates after a wound and stops the waste of moisture. Furthermore, the plant’s aggressive root system is arranged so as to capture the grounds scanty moisture most quickly and completely after rains. Cacti of more than sixty species occur within our state, but more than nine tenths of the plants seen along the highway—unless they are brought together for a garden collection—can be grouped into one genus, the _Opuntia_. The family likeness is shown not by the shape or general appearance but by the fact that all are jointed. Both the cane cactus (also called cholla and elkhorn) and the pancaked prickly pear, unlike the big barrel cactus and the numerous small species which resemble it in shape, have joints and may therefore be rightly called opuntias. The “pancakes,” it may be remarked, are not leaves but divisions of the stem. A cactus gets along without leaves. So a good rough-and-ready test is this: if it has leaves, it is not a cactus—which eliminates ocotillo, yucca, agave, sotol, and so on. The barrel cactus is sure to be seen growing beside service stations in the southern part of the state. Its name as well as its size identifies it. The flowers and the large lemon-shaped fruits are worth a glance. If they do not occur on the south side of the plant, it has been transplanted and turned. The evidence is as trustworthy as the presence of moss on the north side of a tree in the forest. The barrel cacti, the opuntias, acacias, the ocotillos, along with the creosote and mesquite already mentioned, have a way of growing along together in what is called a plant society. These all belong in the bottom layer of the cake called the Lower Sonoran Zone, which is best seen on the Rio Grande mesas near El Paso.