National Geographic : 1945 Dec
The Saguaro, Cactus Camel of Arizona BY FORREST SHREVE OUTSTANDING features of the Ari zona desert are the mountains and the plants. Both come in all sizes, shapes, and colors, and together they give southern Arizona a landscape that seems to the trav eler to be "out of this world." In all this strange land the strangest sight, and most characteristic, is the giant cactus (Cereus gi ganteus), known as the saguaro (or sahuaro).* The principal characteristic of a desert is its little rain, but this desert is freakish in its rainfall. When it is dry, it is exceedingly dry. When a summer rainstorm breaks, the whole country is awash. The plants of arid regions solve the prob lem of an irregular water supply either by sending their roots deep toward moisture or by developing large masses of tissue in which water accumulates during the wet periods and forms a reserve to be drawn upon in the long dry spells. In the cactus family, of which the saguaro is monarch, the habit of main taining a reserve of water is most conspicu ously developed. Cacti Omit Leaves to Save Water Cacti found in the Southwest are composed entirely of stem, the leaf having been dis pensed with as an extravagance from the point of view of water economy. The stems have a green surface, which enables them to carry on the important leaf function of food mak ing. In the saguaro, the bisnaga, and other tall cacti, these thickened stems develop into massive columns. The endmost branches of the saguaro at their wettest are 90 percent water. After eight or nine months without rain, this per centage falls to a much lower figure. The problem for the cactus, as for the Arizona farmer, is how to equalize an irregular supply of water. By constructing dams such as the Roosevelt and the Coolidge, man spreads the water of wet years over intervening periods of drought (Plate VII). The saguaro and other giant cacti have a comparable technique. Over a large part of southern Arizona there are two rainy seasons, a gentle, pro longed one in the late winter and a shorter, more violent one in midsummer. The part of the country visited by rain twice a year is more favorable for plants than the region of winter rain to the west and that of summer rain to the east. Dr. Ellsworth Huntington dubbed southern Arizona "the greenest of deserts." The saguaro is found in varying abundance over almost all of Arizona where altitudes are below 4,000 feet. It constitutes a unique part of the landscape. It is infrequent or small along the Colorado River and on the nearly level plains of the lower Gila Valley. On higher ground, rocky slopes, and canyon walls it is common. In some of its best localities it forms veritable forests. For a plant that rarely exceeds 40 feet in height, the saguaro is one of the heaviest and most massive members of the vegetable king dom. Its erect trunk and gracefully curving branches may contain as much as six or seven tons of water, all accurately poised on a slender base. Yet this ponderous bulk is elastic enough to sway in the wind and strong enough to defy all save the most violent storms. A large portion of the tissue of the saguaro is much like an unripe watermelon, but there is a central skeleton of woody ribs, forming a cylinder that is remarkably stout at the base and tapers to slender, unconnected rods near the top. Strength is also added to the columns by the ridges and grooves which run from base to top. The Saguaro Wears Accordion Pleats Because of the intake of water in a rainy season and the gradual loss of much of it in dry periods, there are great variations in the diameter of the trunk. When the stems are gorged with water, the spiny ridges are far apart, and the grooves between them are shallow. When the water content is low, the diameter of the whole stem is less, the ridges are closer together, and the grooves are narrow and deep. It is obvi ous that the accordion-pleated surface is essential to these adjustments of volume. When the saguaro reaches a height some where between 6 and 20 feet, a small spher ical bud breaks through one of the ridges and a branch begins to appear. It grows outward and then upward in a sweeping curve and finally takes a position almost parallel to the main trunk. In favorable situations several branches may be formed, usually one at a time, and they are always so placed as to preserve the symmetry of the plant and to maintain a balance in its continually increasing weight. When there is a long dry period, the * See "Saguaro Forest," by H. L. Shantz, in the NA TIONAL GEOGRAPHIC MAGAZINE, April, 1937.