THE VARIABLE DESERT 49
But the subjective monotony of the traveler has no objective reality in the paucity of conspicuous forms. Among the cacti^ there are the generally procumbent flat-stemmed opuntias^ or prickly pears^ the shrub or tree-like round-stemmed opuntias, Cylindropuntias, or chollas, the barrel cacti, or bisnagas and above all the splendid fluted columns of the giant cactus or sahtuvro. The latter is represented by but a single species, but there are five or six quite distinct and highly interesting round-stemmed cacti that are indifferently called chollas and a score of species of prickly pears and other cacti that can not be distinguished from a distance. When leafless, two acacias will not be distinguished by the novice from the mesquite belonging to quite a different genus. Add to these the palo verde, the ocatiUo, the yuccas and agaves, Dasy- lirion, the onmipresent creosote bush and several other shrubs, which are less dominant in the facies of the vegetation: a respectable begin- ning, then, has been made upon a rather thick flora of the region.
In fact, the flora is not at all meager. The paucity of species is only apparent; it arises from the facts that only the more conspicuous ones are seen at all by the average tourist, that things which are quite distinct are liable to be confused, that those which do occur together in the same plant association are not all in a vegetative condition, and hence not easily distinguished by the novice, at the same time.
Standing as it does in a transition zone between the highlands of New Mexico and of northern and eastern Arizona and the great desert that stretches away southwest to the Colorado delta, with the valley of the Santa Cruz connecting it with the Sonoran Highlands on* the south, with the diversity in environmental conditions which accompany a range of elevation of several thousands of feet within a radius of but a relatively few miles, it is inevitable that this region would exhibit a marvelous conmiingling of t&xonomically and floristically diverse plant organisms.
The bare statement that the region contains a flora rich in genera and species and of diverse geographic origin or affinity is entirely inade- quate as a description of its real biological diversity. The plants which one sees are of the most highly contrasted structural types.
Some few species have roots extending far down to a permanent water supply, in the few places where this is possible, others have a spreading underground system lying immediately beneath the surface. Growing side by side, one may see large bisnagas or magnificent sahuaros, whose stems contain hundreds or thousands of liters of water and hard dry- stenmied shrubs. During the brief moist seasons, plants with leaves as tender as those of our eastern forests hasten through their development in the shade of tiny-leaved trees, many of which, notwithstanding their small size, were old before the Spanish came down through the valley of
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