Most species grow in hot, dry regions, especially in Mexico and the Southwestern United States. But cactuses are also found in rain forests and on mountains. Some even grow in Alaska and near Antarctica. Cactuses vary widely in size and shape. For example, the giant saguaro looks like a bare tree with a thick trunk and long, upturned branches.
Some small cactuses look like pincushions, starfish, or even blades of grass.
Scientists believe cactuses may have grown on earth for millions of years. The first cactuses had leaves, branches, and woody stems like those of a tree. These features are still found in some primitive species. Most cactus species, however, changed in appearance over the years. The branches became shorter, and the leaves developed into spines.