ever, comparable to that of palo verde. Mesquite timber is seldom used, though it makes a very good cabinet wood, much like walnut. In summer and sometimes in the fall also the mesquite bears a crop of beans, which are relished by all herbivorous animals. These beans were a staple food of the Papago Indians, and their mortars, called "béchete holes," can be found in many localities. (See Pl. V, B.) The screw bean (Prosopis pubescens) is a closely related species, although the tree is usually of smaller size. The pods are twisted and not flat, hence the common name. The screw bean is common near Yuma but is rarely found in the rest of the area.

The palo verde is the most widely distributed tree of the region. (See Pl. VII, A.) Its green branches and trunk are particularly striking in appearance and give rise to the name (Spanish for green tree"). There are three species-Parkinsonia aculeata, Parkinsonia microphylla, and Parkinsonia torreyana. They are not easily distinguished and have in common the green trunk and branches, finely divided locust-like leaves, which are shed in the dry season, and yellow flowers, which make a great display in the spring. Palo verde grows on the rocky slopes of the mountains as a small shrub. On the plains it is larger, and particularly along stream channels it attains a diameter of 12 to 18 inches and a height of 30 to 40 feet. According to Mearns, the common form on rocky slopes is Parkinsonia microphylla, and the very large individuals along stream channels are Parkinsonia torreyana. Parkinsonia aculeata has been found in the wild state in Arizona only at La Osa, in the Altar Valley, and La Ventana, in Baboquivari Valley, and in Sonora only along Sonoita River.3

34

Some conception of the age of desert plants and the stability of any particular association of plants can be obtained from the investigations of Shreve.35 He found that of 146 trees of Parkinsonia microphylla growing in a tract on Tumamoc Hill, near Tucson, 16 were more than 300 years old, of which one was 420 years old. Only 10 were less than 25 years old. Of 542 seedlings that sprouted after the summer rains in 1910, all but 62 were dead in 16 months, the deaths being distributed as follows:

34 Mearns, E. A., Mammals of the Mexican boundary of the United States: U. S. Nat. Mus. Bull. 56, pt. 1, pp. 60, 61, 1907.

35 Shreve, F., Establishment and behavior of the palo verde: Plant World, vol. 14, p. 293, 1911.

The dead seedlings on the same ground in 1910 numbered 160 and represented the deaths of the preceding 12 or 18 months. Of these only three were more than 3 years old. It seems, then, that palo verde and probably most other desert trees have a very high mortality as seedlings but once past the third year are likely to reach maturity and often live to great age.

The leaves and branches of palo verde are eaten by stock, and in dry seasons the trees are sometimes cut down by cattlemen to furnish forage. The trunks are used by the Papagos in constructing shelters from the sun and as roof timbers for their houses. The wood is a poor fuel, burning without coals and having a very disagreeable odor.

Palo fierro (also written palo hierro), or Sonoran ironwood (Olneya tesota), is a handsome tree with an erect bushy crown and blue-green leaves. Like palo verde and mesquite it is a legume and has numerous thorns from a quarter to half an inch in length. Palo fierro is usually found along stream channels on the higher parts of the alluvial slopes and is especially noticeable around the base of the mountains, where it grows in open groves like an orchard. Here individual trees may have a trunk 1 to 2 feet in diameter and are about 30 feet high. Palo fierro is more common west of Ajo than in the region east of that place. The leaves are eagerly eaten by stock, and it is a common practice for prospectors to cut down the trees near their camp, so as to provide fodder for their burros and horses. The wood is very hard and durable, and although it has a somewhat more pronounced odor than mesquite it is better fuel. The seeds when roasted have a fine flavor and were a staple food of the Papagos.

Under the name catsclaw are included a number of species of Acacia, of which the common one in the neighborhood of Tucson is Acacia greggii. This is a shrubby tree rarely over 18 feet high, in habit somewhat like the locust. It has many curved thorns about half an inch long, from which the popular name, a translation of the Spanish uña de gato, is derived. The catsclaw grows along arroyos on the alluvial slopes and in the foothills, and in places forms impenetrable thickets.

The ocotillo (Fouquieria splendens) is one of the most striking of desert plants. It consists of several wandlike stems about 1 inch in diameter, which rise from a common base to a height of 5 to 10 feet. Each stem has a graceful curve away from the center, so that the whole plant has a vaselike shape. The stems are coated with wax and bear numerous thorns. At each thorn are leaves, which turn yellow and drop off in the dry season. The ocotillo bears masses of bright-red flowers in the spring. Its habitat is the upper

and dryer parts of the alluvial fans and pediments, where it is commonly associated with the sahuaro and other cacti. Ocotillo branches are used by the Papagos to make temporary shelters. The butts of the branches are stuck in the ground along the two long sides of a rectangle 8 to 10 feet long and 4 to 5 feet wide. The ends are brought together and tied, and the ribs thus formed are fastened to longitudinal strips. The result is a framework in shape like the cover of a prairie schooner. Brush, skins, or blankets are used to keep out rain and sunlight. Ocotillo branches are also used for fences and for the sides and tops of arbors. The ends, when buried a few inches and given a little water, will grow and make an arbor which keeps green throughout the greater part of the year and is adorned with the beautiful red flowers in the spring.

The indigo thorn (Parosela spinosa) is a spiny tree between 20 and 30 feet in height and from 8 to 15 inches in diameter. It has violet-colored blossoms with a delicious fragrance. The tree grows in the sandy bottoms of arroyos rather than on their banks. It is found in the Sonoita Valley, around Tule Well, and more abundantly in southeastern California.

The wild china tree (Sapindus marginatus), called cherioni by the Mexicans, is found sparingly along arroyos in the Papago country. The specimens seen were about 4 inches in diameter and 20 feet high. Cherioni is popularly thought to be an unfailing indicator of water, a tradition derived from the Papagos. The Cherioni Well, in the valley of the Ajo, was dug on this assumption but failed to find any large amount (p. 181). The wood is tough and is occasionally used for handles of tools and the repair of wagons.

The desert willow (Chilopsis linearis) grows to a height of about 20 feet. (See Pl. VI, B.) It has willowlike leaves and on the ends of the branches bears bunches of purplish flowers and later seed pods 15 inches long. The flowers and pods resemble those of the catalpa, a nearly related tree. This tree grows along sandy arroyos and has a reputation as an indicator of underground water hardly borne out by the facts. Its wood, however, resists decay and it makes very durable fence posts.

The crucifixion thorn (Holocantha emoryi) is another impressive desert plant. (See Pl. VIII, B.) It is usually little more than a shrub, but sometimes grows to a height of more than 20 feet. The main stalk and branches have a green bark, and there are no leaves. The smaller branches are fleshy, and each twig is a thorn, so that the plant appears to be a mass of thorns, most of which are. about 11⁄2 inches long and a quarter of an inch in diameter at the base. The seeds are reddish berries, which grow in dense clusters and remain on the plants for many months. The crucifixion thorn

grows in the most rocky and barren soil; most of the specimens seen were in the region west of Ajo.

Trees of the high mountains of southeastern Arizona are found on Tumacacori Mountains, the westernmost wooded range in southern Arizona. The dominant trees are oaks of three species-Quercus oblongifolia, Quercus arizonica, and Quercus emoryi. These species have small evergreen leaves. The first is commonest on the lower slopes and drier situations of the mountains, where the individuals have trunks from 6 to 12 inches in diameter and 20 to 30 feet high. The second is similar in form and habit, and the rough whitish bark, dense foliage, and equal spacing give a stand of these trees an orchard-like appearance. The third species is usually called the bellota oak by the Mexicans. The largest and best specimens grow in valleys, where they attain 3 or 4 feet in diameter and nearly 100 feet in height. The acorns are sweet, and flour made from them was once a staple article of food with the Papagos and early Mexican settlers.

With these oaks are associated-although they form only a small part of the forest-the Mexican piñon (Pinus cembroides) and the alligator juniper (Juniperus pachyphloea). These trees, together with the oaks, occur in small numbers and very much reduced in size in the higher parts of the Baboquivari Mountains and perhaps other ranges.

Along the flood plains of Gila River and the upper Santa Cruz the common tree is the cottonwood (Populus fremontii). Wherever there is permanent water, as on Arivaca and Sonoita creeks and around many springs, the cottonwood is found, but it is often difficult to determine whether or not it has been introduced by man. With it is commonly associated the black willow (Salix nigra).

In Santa Cruz and Arivaca valleys the leatherleaf ash (Fraxinus velutina), the western walnut (Juglans rupestris), and the Mexican elder (Sambucus mexicana) are large trees common along permanent water. With them is usually found the Arizona sycamore (Platanus wrightii), which also occurs as a lone tree or small clump at the springs and wells along the base of the Baboquivari Mountains.

The hackberry is called cumaru by the Mexicans and kom by the Papagos. It plays a large part in the legends of the Papagos, as may be seen in many place names, such as Comobabi and Comovo. The principal species is Celtis occidentalis, a large tree that may reach 3 feet in diameter and 50 feet in height. It occurs in the upper Santa Cruz Valley and along Arivaca Creek, and there are many small groves and single trees along stream channels at the foot of the Tumacacori and Baboquivari mountains. The tree seems to require considerable water for full development and probably deserves its

reputation as an indicator of underground water. A smaller species, Celtis reticulata, is included under the popular name hackberry or cumaru. It is more widely distributed and doubtless can grow on the moisture from flood waters alone.

THE CACTI

The cacti of southern Arizona are remarkable for their size and peculiarity of form. Of these the giant cactus, or sahuaro (Carnegiea gigantea), is the most conspicuous and in primitive times was the most important to man. It is shown in a number of photographs in this report. (See Pls. V, A; XV, C; and XXVII, B.) It has a straight trunk with vertical parallel ribs, from 12 to 18 inches in diameter, and may attain a height of 40 feet. One or more clublike branches are given off at irregular heights and grow up more or less parallel to the parent stem. They produce many fantastic shapes, which have a large part in the weird and unearthly appearance of the desert. The sahuaro occurs along the Mexican boundary from Nogales west to the Gila Mountains. Farther north it occupies suitable locations in the broad desert valleys adjacent to Gila River and its tributaries as far east as Graham County and as far north as the foot of the Bradshaw Mountains. Near the northern boundary of its range the trunk is commonly unbranched and often not more than 10 feet in height. In common with most of the other cacti, the sahuaro prefers a rocky soil and is usually found on the upper parts of alluvial fans, on pediments, and on the lower slopes of mountains. The white flowers are borne near the top of the column in May, and the fruit ripens in July. The fruit is prized by the Papagos, who use it for making sahuaro sirup and wine; the seed is used for making flour. The spines of sahuaro are dry and will burn, if lighted at the base, with startling rapidity and with a bright flare. This quality leads to the occasional use of the sahuaro for signaling at night. When the plant dies the flesh shrivels up, leaving the parallel woody ribs standing and bound together with only a network of fibers. Each rib is a pole about an inch in diameter, and such poles are frequently used by the Indians for forming the side walls of their rude huts and in constructing corrals. The pitahaya dulce or organ-pipe cactus (Cereus thurberi) consists of a clump of columns, each 3 to 4 inches in diameter and 3 to 8 feet high. (See Pl. XXIV, B.) The pitahaya is much more rare than the sahuaro and is more closely confined to rocky slopes. The fruit also is larger and more prized than that of sahuaro.

The bisnaga or barrel cactus (Echinocactus wislizeni) is from 18 inches to 2 feet in diameter and rarely grows over 4 feet high. The ribs twist in a spiral from the base to the top and bear clumps of large