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Popular Science Monthly/Volume 66/February 1905/A Botanical Laboratory in the Desert

< Popular Science Monthly‎ | Volume 66‎ | February 1905

By Professor FRANCIS E. LLOYD,


TWO years ago the Carnegie Institution determined to establish a laboratory to be devoted to the special study of desert vegetation. The plan originated with Mr. Frederick V. Coville, who, under the auspices of the Smithsonian Institution, had for some dozen years previously been a close student of the plants of the southwestern American desert. His interest was fixed by his experiences as a member of the memorable Death Valley Expedition of 1891. After Mr. Coville's plan had been adopted by the Carnegie Institution, an advisory board, consisting of Mr. Coville and Dr. D. T. MacDougal, was appointed. The first work of this board was the choice of a proper site—a task which will be conceded to be neither easy nor unimportant when the great extent and variety of the North American Desert is appreciated. Both of these gentlemen were, however, possessed of wide personal knowledge and experience of this region and brought to the solution of the problem ripe judgment. After a further personal examination of all of the most promising areas, including the deserts of Texas, northern Chihuahua and Sonora in Mexico, New Mexico, Arizona and of California, the choice rested upon Tucson, in southern Arizona. The results of this investigation are embodied in an extensive report[1] which is full of valuable data and most instructive and beautiful illustrations. The wisdom of the choice of the advisory board may very naturally be questioned, and I confess to have entertained some doubt in this regard. After a personal examination, however, of nearly all the above mentioned regions, and after spending the major portion of the past summer at the Desert Botanical Laboratory, I am now of the opinion that the action was well-advised and is fully justified. I am therefore taking occasion at this time to give an account of the laboratory and its surroundings from my own point of view.

The city of Tucson, with a population of 10,000, is situated in the valley of the Santa Cruz. Its position is central with respect to the deserts of California, Mexico, Texas, New Mexico and northern Arizona. With an elevation of 2,390 feet above sea level, it has a hot, though dry and bracing, climate. The soil is a fine clay or adobe, underlaid by a white hard pan, locally known as caliche. Two miles

PSM V66 D334 Desert botanical laboratory of the carnegie institution.png
The Desert Laboratory, looking North.

to the westward are to be seen the outposts of the Tucson Mountains, rugged hills of volcanic origin. On the more gradual northerly face of one of these, on a shoulder of gentle slope, stands the laboratory, a building appropriately constructed of the volcanic rock. The style is simple and well adapted to the climate. The thick stone walls heat slowly, particularly as they are for the most part protected from the direct rays of the sunlight by an overhanging roof. This latter is so constructed as to form a large ventilated air chamber, itself a protection from the effect of intense insolation as well as affording comfort to the occupants by modifying the strong light.

PSM V66 D335 Floorplan of the desert botanical laboratory.png

The form of the building is in the shape of the letter L, with the longer outer elevation facing the north. The dimension and disposition of the space within are indicated upon the accompanying plan. The windows are abundant, and when open to the brisk winds allow a current of air throughout the building which makes one forget the heat. About this factor much misapprehension is abroad. To be sure, the mercury stands high, and a registration of 100-105° F. is not at all unusual; occasionally it is even higher, and doubtless if one did not forget to look at the instrument, still higher temperature could be discovered. Fortunately, this is just what one does. There is so little discomfort attendant upon the heat that it is usually quite disregarded, a condition happily due to the low relative humidity, which on many occasions during six weeks in July and August of this year ran down as low as 7 per cent. The only feature of discomfort to some persons is the intense illumination, and one may require dark glasses, although I personally found no discomfort even upon the mesa. On
PSM V66 D336 The laboratory with palo verde on the right.png
A Corner of the Laboratory. A Palo Verde on the right.

the volcanic hills, the dark color of the ground affords very appreciable relief.

The general Laboratory is furnished with individual tables, a general work table and two sources of water supply. It should be noted that, although two miles from town and upon an elevation of 300 feet above it. there is abundant water supply. It is to the enlightened interest of the citizens of Tucson that this convenience, as well as the electrical connections, acreage and roadways, are to be credited. The immediate source of water is a 300-gallon tank, supplied from a pumping station at the foot of Sentinel Hill, which stands immediately to the southeast of the laboratory.

PSM V66 D337 The general laboratory.png

The General Laboratory.

Adjoining the general laboratory is the stock room, liberally supplied with the necessities for work. This leads into a photographic dark room, with water supply, ventilation and a cement floor, and which may be used as a physiological dark room, with constant temperature.

The library, which adjoins the stock room, though at present small, contains a carefully selected lot of periodicals and books, the latter chosen with special reference to their bearing upon desert exploration and vegetation. From the library one enters the office of the resident officer, Dr. W. A. Cannon, whose generous treatment and constant sympathy, coupled with the material opportunities afforded, leave little to be desired.

The view commanded from the laboratory site is a panorama of rare beauty. To the north lies the range of the Santa Catalina Mountains, extending through 40° of the horizon. Its rugged topography scarcely noticeable in the glare of the high sun, is thrown into bold relief when the shadows begin to lengthen. Then the dazzling purples and yellows of midday give way to the deep blues and purples of the valleys contrasted with the reddening tones of the higher slopes and ridges. The well-wooded and watered regions of this fine country are within two days' travel of Tucson, and excursions may be made thither with comparative ease.

To the east stands the rounded mass of the Rincon Mountains, the illumination of which by the afternoon sun is most remarkable. The vividness of details, the shimmer of heat, the blaze of reflected light modified by the merest veil of purple—these are faint expressions of what one quite fails to describe. To the west the desolate defiles of the Tucson Mountain, seen at short range, gives us, with the sun in the same position, a contrast picture of hard and rugged profile, dark browns and black shadows. To complete the panorama, one needs but to climb to the top of the hill, from which may be seen in

PSM V66 D338 Prickly pear.png

Prickly Pear.

the far distance the deserts of Sonora and the malpais—the rendezvous of the few 'bad men,' now so hard to find.

Aside from the conditions for study offered by the Desert Laboratory as such, the matter with which the student is especially concerned is the plant life. In seeking for the right place to plant a laboratory for the study of desert vegetation, it is obvious that some practical conception of what such a vegetation is had to be formulated by the advisory board. It was necessary for this board to find a locality with a desert climate and possessed of as rich and varied a flora as possible, while still of a distinctly desert character. Since it is the chief object of the laboratory to study 'drought-resistant vegetation' it would have been absurd to put the laboratory in an out-and-out desert, and but little better to have selected a semi-arid region with a rich flora. Nor would it have been foresighted to have chosen a locality which might sooner or later be threatened by irrigation. The conditions above stated may, of course, be met in many places, but scarcely better than on the hills west of Tucson, and on the adjacent slope and mesa. The general character of the vegetation here is in the main similar to that of the mesa and rocky ridge of the whole territory between Texas and western Arizona, but is, also, within the limits of distribution of the saguaro or giant cactus (Cereus giganteus). It is, therefore, representative in this important respect, of a very wide stretch of country which is of an undoubtedly arid character, the plants of which are, with the supply of water derived from a meager rainfall and a little snow, able through long periods of drought to sustain their powers of growth unimpaired. A more immediate view of a few of the more striking of these desert plants will here be of interest.

PSM V66 D339 Ephedra.png


The vegetation of the country about Tucson is very naturally and obviously divided into two formations which occupy, the one the level mesa, the other the rocky hills and slopes. We may consider first the mesa, where the most prominent element is the greasewood, or creosote bush (Covillea tridentata), a plant of rather singular aspect. The average plant in point of size stands two and a half to three feet in height. The numerous branches, which arise together from the soil, spread radially at an angle with its surface, and bear in their upper parts a meager foliage which, in times of drought, is olive-brown in color. The leaves, the form of which is peculiar to the family (Zygophyllaceæ) of which the plant is a representative, are coated with appressed hairs and varnished with a resinous secretion. After the advent of a rain, or if watered, the plant quickly responds, the leaves becoming bright green and its delicate yellow flowers coming out in great numbers. The shrub is flat topped,PSM V66 D340 Yellow flowered opuntia.pngYellow-flowered Opuntia. and in large areas where few other so large specimens grow, the foliage of many plants lies in a uniform layer parallel to the ground. The hard twigs and branches make excellent fuel particularly for the camp cook's fire.

Associated with the creosote bush, in numbers varying with the locality, are several species of cacti of the genus Opuntia, but of the type of Opuntia arborescens. and not possessed of the flattened branches of the common prickly pear. Of these, which are more dense tree-like in form, the one known locally as the cholla[2] attracts one's attention most quickly. Its smaller branches, clothed with numerous white spines, are curiously massed into formless bunches many of the smaller joints hanging down and sooner or later breaking off. Thus collect beneath the plant piles of dejecta membra, many dead, some dying. These broken off parts may serve to propagate the plant; when seed-pods are so cast off they more likely serve this end. In the agglomerations of branches just described birds cunningly build their nests; and these are hardly to be noticed but for the knowledge that they are PSM V66 D340 Twig of palo verde.pngTwig of Palo verde. frequently found in such places. The few stout ungainly stems of these plants, and, as one may say, the weird disposition of the lesser branches, glistening in their coating of gray shining spines, produce a habit which, in the language of the advertisement, is peculiar to itself. When, as often happens, the cholla has the monopoly of the situation, the desert aspect becomes very pronounced. Two other species of this type are common, the so-called Opuntia spinosior, with pink flowers, and another (O. versicolor), with yellow ones. These, however, do not show the peculiar massing of branches, nor are their ultimate articles so readily detached. Perhaps more interesting than all these is the little half-viny Opuntia leptocaulis, with stems, of scarcely the thickness of a lead pencil, interweaving among the branches of some companion plant, usually the creosote bush. It is never to my knowledge found growing except in the immediate proximity of another plant, and thus often escapes detection. It is not very well supplied with spines, and might easily be destroyed by trampling if away from the company of a plant of sturdier growth. Perhaps this is the reason why it usually is so found—it is stamped out elsewhere. Or possibly it needs the partial shade afforded it. This is a question worth an answer.

If you travel for a distance in any direction, other plants will be found. Among these are sweet-smelling acacias, ungraciously, but, I fear, not undeservedly, called cats-claws, with their finely divided leaves and small yellow pompons of flowers. The low leafless shrub Ephedra, with its vertical green stems which look like the scouring rush and is as rough and hard to the touch, is another—a typical desert plant if ever there was one. This plant is a relative of our yew, but is possessed of very unique characters, the description of which would take us too far into details.

Here and there a 'salt' or 'alkali' spot is to be found. Here grow few enough plants, and these such as can endure the hardships of a most unfavorable soil, as for example Atriplex and Dondia. Near the water courses, from which for the greater part of the year water is conspicuously absent, one finds, on the other hand, larger shrubs and very small trees of Acacia, mesquite (Prosopis velutina) and a species of palo verde (Parkinsonia). Ten miles south of Tucson, near to the mission of San Xavier del Bae, the river bottom is occupied by a veritable forest composed wholly of large mesquite, and this the Papago Indians of the region draw upon chiefly for wood. I should mention the presence in parts of the mesa of the many low shrubs which are noticeable chiefly for their inhospitable thorniness. The palo christi (Koeberlinia) is an extreme type. For a crown of thorns no better material could be imagined than this.

On approaching the rocky slopes leading to the higher elevations, a different vegetation is met—different in species, but not in general character. At closer range a distinctively green note in the coloring is appreciated, which at a greater distance was a uniform brown. It is the palo verde (Parkinsonia microphylla), so called in the Spanish on account of the uniform green color of all its members, which gives the impression of verdure. This is a small, somewhat gnarled tree, related to our locust, of the general appearance of a trimmed orchard tree. It is usually leafless, although the younger shoots are sometimes supplied with a meager foliage of compound leaves. The very small leaflets, which are foliar members of the third order, are sensitive, the pairs folding together at night PSM V66 D342 Ocotillo.pngOcotillo and opening at dawn. Closure again occurs at the period of excessive insolation. All the ultimate branchlets taper into long stiff thorns, an example of direct metamorphosis, since they originate as normal shoots. In the absence of leaves it is evident that the chlorophyll tissues of the stems are chiefly concerned in the food making process, which would appear true also from the fact that the smaller branches and twigs are used as green forage for horses in winter. Although a particular branch removed from the tree and regarded alone is a rather graceless object, the whole tree with its smooth green bark and gradually tapering limbs and twigs is singularly beautiful, its outline most delicate. PSM V66 D342 Twig of ocotillo.pngTwig of Ocotillo

Conspicuous as a member of the hillside vegetation is the ocotillo (Fouquieria splendens), a plant with a hypothetical relationship with the willows, but not in the least suggesting them by its habit and more obvious structures. The general disposition of its branches, which are highly suggestive of coach whips, is similar to that in the creosote bush. It is, however, a much taller shrub, with lithe, bespurred stems, bearing in spring each a brilliant mass of scarlet flowers. On the advent of the rains, the stems are quickly and completely clothed with rosettes of light green ovate leaves, each rosette in the axil of a thorn. Most interesting is the manner in which the thorns arise. The new shoots produce first the primary leaves, in the stalks of which a hard tissue is developed. Their leaf-blades rather soon wither away, and split away from the harder part of the stalks, which in this way are left as spines, in the axils of which, as above stated, the secondary

PSM V66 D343 Giant cacti and the view from laboratory hill.png

View on the Laboratory Hill, Giant Cacti.

leaves develop. The bark of the ocotillo is waxy and burns like a candle.

But whatever of unusual beauty or peculiarity the palo verde or ocotillo possess, the plant which of all claims and holds the attention the most constantly is the great sentinel-like saguaro or giant cactus (Cereus giganteus). Until middle age it is a single green fluted column, with a nature-wrought entasis, a form of severe simplicity scarcely less emphasized when, in its later growth, a few arms grow out. These are usually found at some distance below the middle point of the main shaft, and, though for the most part erect, sometimes, partly by accident, take most grotesque positions. The woody skeleton is not extensive, consisting of slender ribs which occasionally anastomose. These are used by the Papagos for palings and to form the frame of their burden frame or quijo.

The saguaro flowers in June, producing at the top of the stem a large cluster of tubular white petaled flowers in which are found hundreds of small beetles and wasps, apparently the chief agents of pollination. While the fruit is developing, the shriveled flowers, which remain

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Saguaro Flower and Fruit cut Longitudinally.

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Looking North from the Desert Laboratory.

PSM V66 D345 Barrel cactus cut across.pngBarrel Cactus Cut Across. for some time attached, Lend a shaggy top-knot appearance which for the while detracts from the dignity of the plant. When the fruits are ripe, the fleshy pericarps split, disclosing the crimson pulps studded with small. black seeds. The effect, at this time, if one stands at a modest range suggests a mass of crimson flowers. The fruits are sought for by Mexicanos and Papagos with an avidity which to my own taste is scarcely justified by their flavor, that of a ripening fig.

Another prominent member of the cacti is the barrel-cactus (Echinocactus Wislezeni), which is two feet in height PSM V66 D345 Barrel cactus.pngBarrel Cactus. and eighteen inches in diameter, and is armed with large hooked spines. In this plant, as will be seen in the illustration, the rind is remarkably developed, this tissue containing about eighty per cent, water. As the sap when extracted is potable, this species, as well as certain others, is used as a source of water to quench the thirst. PSM V66 D345 Mammillaria.pngMammillaria. The prickly pears are found in profusion, and represent at least two species, one a shrubby form, four feet in height, the other a sprawler. The fruits of these two plants are also collected and eaten.

Smallest among these curious plants is a species of Mamillaria, each protuberance of which is surmounted by a radiating group of

PSM V66 D346 Encelia.png


delicate spines. From the center of each group arises a black slender hooked spine. The flower is a relatively large one of a dainty rose color.

Space does not allow further description and the remaining shrubby vegetation, including the Lycia and Celtis, must be passed with mere reference. Two weeks after the advent of the rains the ground is clothed with many richly colored and often fragrant annuals and small perennials. Some of the latter, as for example an Encelia and a Cassia, persist through the drought, a hardiness explained in part, at least, by the felt-like protective layers on the leaves. Among the less resistant, but rapidly growing herbs are the fragrant flowered Martynia, with its large, double-hooked pods, and a Tribulus, bearing rich yellow poppy-like flowers.

With such surroundings, rich in material and opportunity, what may be accomplished at the desert laboratory? Much of the taxonomic work of this region has been done; but, in the light of modern research, much remains to do. The structure and development of scarcely one of these desert forms is properly understood, and a wide field awaits the student in these directions, while the peculiar physiology of these plants has scarcely been touched upon. Physiological and anatomical-physiological studies of wide extent may here be carried on. It is to the honor of American science that an opportunity such as this has been afforded by the Carnegie Institution.

  1. 'Desert Botanical Laboratory of the Carnegie Institution,' Publication No. 6, November, 1903.
  2. Pronounced Cho-ya (Opuntia fulgida).