Gray desert plants, notably cactuses and other thorny plants, partly replace in the south the bushes of the north.
MELocACTus, the genus of melon-thistle or Turk's-cap cactuses, contains, according to a recent estimate, about 90 species, which inhabit chiefly the West Indies, Mexico and Brazil, a few extending into New Granada.
The Lower Sonoran zone is noted for its cactuses, of which there is a great variety, and some of them grow to the height of trees; the mesquite is also very large, and the creosote bush, acacias, yuccas and agaves are common.
In the Lower Sonoran belt, soapweed, acacias (Palo Verde or Parkinsonia torreyana), agaves, yuccas and dasylirions, the creosote bush and mesquite tree, candle wood, and about seventy-five species of cactuses - among them omnipresent opuntiae and great columnar " Chayas " - make up a striking vegetation, which in its colours of dull grey and olive harmonizes well with the rigidity and forbidding barrenness of the plains.
Almost all of the United States east of the 98th meridian is naturally a forest region, and forests cover the greater part of the Rocky Mountains, the Cascades, the Sierra Nevadas and the Coast Range, but throughout the belt of plains, basins and deserts west of the Rocky Mountains and on the Great Plains east of the Rocky Mountains there are few trees except along the watercourses, and the prevailing type of vegetation ranges from bunch grass to sage brush and cactuses according to the degree of aridity and the temperature.