Main sources: Kearney, Thomas & Robert Peebles, et al, 1960, Arizona Flora, pp. 397-99, Berkeley: University of California Press; Petrides, George & Olivia Petrides, 1992, A Field Guide to Western Trees, pp. 119, pl. 20; Hodgson, Wendy, Food Plants of the Sonoran Desert, p. 163; Hultine, Kevin, "The Taxonomy of North and South American Mesquites", Aridus 13:2:May 2001.
Acacias include over a hundred species of trees and shrubs, many of which grow in extremely harsh desert environments. In our area they run in size and texture from small, almost non-woody herbaceous plants to woody shrubs and small trees. As legumes, they have two-valved seedpods, but otherwise they differ considerably in appearance.
The ancestors of Acacias as well as most other legumes must have arisen during the late Jurassic Era, after the breakup of the great global-continent Pangea some 165 MYA, in the ancient continent of Gondwanaland -- the supercontinent composed of ancestral continents of South America, Africa, India, Antarctica, and Australia -- because Acacias are native to Australia as well as these other (now widely separated) southern continents (but not to North America, which had separated from all of them with the breakup of Pangea). This indicator of great age distinguishes them from mesquite, whose origins are later, tracing only to South America.
Below, the essential feature of leguminous plants: simple, dry, bean-bearing pods, in this case slowly ripening Cat's Claw Acacia pods, photographed on a terrace above the floodplain of the San Pedro River July 6, 2007. (Click on the image to enlarge it.)
Click on image below to enlarge:
Incidence of thorns is highly variable -- compare the plant above with this one below (Click on the image to enlarge it):
Whitethorn is winter-deciduous, but renews its leaves only after adequate summer rains. It is eaten by a variety of birds and small mammals. Like Catclaw, it is long-lived and may develop into a small tree. Like Catclaw, it is an invader of desert grasslands.