With the exception of some mesquite, iron-wood, and palo-verde trees, scattered here and there along the Gila and its bottoms, the whole country from Yuma to Tucson is practically treeless, and must continue so from want of rains. Sage-brush and grease-wood abound, as in Utah and Idaho, and throughout the great internal basin of the continent generally; and on the uplands, you find the great columnar cactus in full vigor and maturity. Indeed, from the time we struck the Colorado Desert, we were fairly into the cactus region of the continent, but the varieties were few, and the size moderate, till we got well into Arizona. Here they increased in height and bulk, until we reached the Maricopa Desert, where we found them thirty and forty feet high, by two or three feet in diameter, with perpendicular branches halfway up, nearly half as large as the main stem. This variety is a green fluted column, with its edges armed with semi-circular thorns, and bears a cluster of apples on top, from which the Indians extract a rude molasses or sugar. Inside, it is a frame-work of reedy poles, that serve many useful purposes in that woodless region. These immense cacti dot the country over to Tucson, and beyond--indeed, down to Mexico, and largely through it--and are a leading feature of southern Arizona. Sometimes you miss them altogether; but, as a rule, they occur more or less on the _mesas_ or plateaus nearly everywhere, and seem in the distance like monumental columns. Their clustering groups and varying heights, when seen from afar, have all the effect of a rural cemetery; only here the shafts are emerald green, instead of marble white. In fights with the Indians, they often prove of value as a defence, and their huge trunks secrete a fluid much akin to water, that has saved the life of many a thirsty traveller, when lost amid these arid wastes. How such a gigantic vegetable or immense plant can thus nourish here, where nothing else comparatively will grow, is a continuing mystery and perpetual astonishment. It would seem more fit for a luxuriant soil and a tropical climate. Yet here it is, _magnum opus_, mocking the naturalist apparently to scorn.