Nature excels in nichemanship. Wherever there is the slightest chance for survival, animal and plant life contrive to fill the niche—but only by some very ingenious and specialized adaptations. Thus, all plants that grow in desert-lands are cleverly equipped to withstand the heat and make full use of what little water exists in arid areas.
Deserts confront plants with many problems. It is always very hot during the day—soil temperatures may rise to 150 degrees but can drop swiftly to the freezing point or below at night; the air as well as the soil is often bone dry. When rain does fall, it often comes in torrents, and much of it runs off swiftly instead of sinking into the ground.
Cacti and other succulents, such as the agaves and the aloes, brilliantly solve the problem of growing where the rainfall is low but where there are occasional heavy showers. Cacti, which are native to the southwestern United States and Mexico, have no leaves, except for a few which have tiny leaves for a short time during the year. Their stems do the job which leaves perform for other plants. Thick and pulpy, the stems are first-class water reservoirs—a cactus is about 90 per cent water. But large reservoirs are not much use unless the plants can fill them during a quick run-off of water. Most cacti have very extensive root systems. A tiny barrel cactus, only five inches high may have lateral roots four or five feet long. This far-ranging root system is typical of most of the hardy trees that live in lands where water is scarce. The desert eucalypti of Australia, standing only as tall as a man, may have root systems extending a quarter mile. These great networks of roots, mostly close to the surface, enable the cactus and other desert plants to soak up quickly any water that reaches them.
Because the cactus has no leaves, it can better conserve water it has stored. With their enormous leaf surface, oak and beech trees lose hundreds of gallons of water through evaporation on a hot clay. But a barrel cactus weighing 200 pounds may only expose a total surface of 10 square feet. Cacti, too, have thickened, tough skins with a waxy covering to guard against excessive evaporation. Moreover, they have fewer breathing pores than other plants, and these are sunken into the skin to form a cushion of still air that retards loss of moisture.
Hardy because they can sit out the longest of droughts, cacti are among the slowest-growing and longest-living forms of life. One of the barrel cacti of Central America grows from six to nine feet tall, three to four feet in diameter, weighs over two tons and lives for a thousand years. A cactus of this family, a bisnaga, at the University of Arizona is just over two feet tall although it was planted more than 40 years ago.
The largest and most spectacular of the cactus family is the saguaro—the "sage of the desert." This giant cactus, whose fluted columns may soar 40 feet or more, is a spiny fortress of the desert for hawks that build their nests in the forks and for woodpeckers and owls that make nests in holes in the fluted stems.
Cacti and other succulents do not grow in deserts from choice. Many American and African deserts were at one time great lakes surrounded by jungles of water-loving plants. When the climate changed and the lakes dried up, most plants were unable to survive, but others like the adaptable cactus dispensed with leaves and developed special talents to meet the changed conditions. Many cacti still flourish in wet conditions; some prow even in cold climates and survive winter snow.
The most famous of the cacti are the prickly pears or opuntias. They have been exported to many parts of the world including Australia, North Africa and the northern Mediterranean. In Australia the prickly pear quickly overran millions of acres of grazing land and was only finally controlled when the cactoblastis insect was imported from North America.
Among the cacti is one that has no spines and relies on protective coloring, much as a stick-insect does; this is the living rock cactus of Mexico and southern Texas, with its grey warty and triangular projections.
Many cacti have very large and beautiful flowers. Some, such as the famous Mexican Queen of the Night, open at sunset, but by morning are faded and dead-appearing. The plant which looks like a mass of dead twigs all the year bursts into colorful display with great white flowers six inches and sometimes a foot long. Near Tucson the night-blooming selenicerens explodes into fragrant glory about June 15 each year. It is still the custom in some old Spanish-American homes to invite friends for a fiesta when La Reina de la Noche blooms.
Among the more bizarre members of the lily family are the thirty kinds of yuccas, native to arid regions of Mexico, the West Indies and the United States. Some of the yuccas are Spanish daggers, desert candles and the Joshua trees. Desert candles in bloom are a magnificent sight; from the porcupine-like rosettes of basal narrow leaves rise the strong flowering stalks crowned with an immense cluster of creamy-white flowers. No plant is more dramatic than the Joshua tree which grows to 40 feet tall with an irregular head of bent and twisted branches, covered with short stiff narrow leaves. The Mormons gave the trees this name when crossing the California deserts on their way to Utah because its outflung branches looked like the arms of Joshua pointing the way out of the wilderness.
Agaves, whose basal rosettes twist into giant spine-tipped and serpentine curves, are natives of the deserts of Mexico and California but have migrated with man's help to many parts of the world. They grow so profusely all along the Riviera coast that they're considered natives there. Agaves are popularly called century plants because they flower infrequently. They produce a flowering stalk 20 feet tall once every 20 or 30 years and sometimes die afterwards.
In Africa and Asia, too, other plants that had been water-lovers adapted themselves to change and developed into the euphorbias (or spurges) and mesembryanthemums. (This last Greek-derived name means "mid-day flower" because of the habit of opening only in sunshine.)
Euphorbias are cactus-like succulents of the arid regions of Africa and the Arabian Peninsula. One of the most famous and most prickly is popularly known as the Crown of Thorns. Unfortunately for the legend, it does not grow in Jordan but is a native of Madagascar, where it is known by the impressive name of Soongo Soongo. Euphorbias are perfect examples of convergent evolution—two quite unrelated groups of plants gradually came to look alike after long exposure to the same environment. Many of the euphorbias resemble cactus so closely that laymen confuse them.
Mesembryanthemums are tough, sturdy African plants which are very popular in gardens because of a brilliant range of daisy-like flowers. Among the family are some very unusual spineless ones which mimic pebbles and stones. These plants, called split rock, stone faces and flowering quartz blend so well with their backgrounds that it is almost impossible to see them unless they are in bloom. Some have mottled patterns resembling lichen. Other unusual members of this family are the window plants which grow embedded in rocks with only their flat tops exposed to the sun; these surfaces have translucent membranes that filter the hot sun just as windows do. In some of the African deserts these plants may get no water for years other than the night dew.
Aloes, those other king-sized members of the lily family, are mainly African plants and are most abundant in the Cape area. Some grow on the Arabian Peninsula where their white, yellow and red clusters of tubular flowers on tall stalks growing out of rosettes of thick succulent leaves bring a sudden beauty to dry areas.
In all the arid and semi-arid regions of the world are lovely wild flowers whose seeds or bulbs lie dormant, often for years when no rain falls. Within a few days or weeks of rain, the dry lands explode into masses of vivid color.
Trees, too, have developed special talents to survive in desertlands. Like the cactus and other succulents, they, too, have water reservoirs in their trunks or in their vast boles below the ground. The most stoical desert trees have sparse, leathery or spiky foliage or even dispense with leaves completely, as the cacti do. Many are protected by thorns. In the most inhospitable areas of the American Southwest, the spiny mesquite or screw bean survives—even in Death Valley. In the waterless places of Arabia the nibq (also called the 'ilb, sidr, or ber), struggles against adversity where the dates cannot prow "with their feet in water and their heads in the fires of heaven," and produces a fruit, the dom, so profusely that passers-by are allowed to shake the small berries from the branches. Even in Saudi Arabia's Empty Quarter, a Texas-sized desert, H. St. John B. Philby in 1932 observed almost fifty species of trees and shrubs, including four species of acacia, which the Bedouin called salam, and the sturdy tamarisk.
In the production of desert trees, nature seems sometimes to have been experimenting with the truly strange. No plant is weirder than the tree that isn't a tree of the Mossamedes Desert of Angola (South West Africa). This tree with the awesome scientific name of Welwitschia Bainesii grows sideways, instead of up, and often goes 10 years without a single drop of rain. Looking like a foot-high, round table, the tree stops growing upward when it produces its first pair of leaves! It then grows sideways and increases its diameter until its circumference may be as much as 14 ft. The sideways-growing tree has no immediate relatives, living or fossil, and lives for a thousand years!
The desert plants are indeed nature's triumph of stoical endurance and bold opportunism. The plants may have to wait somnolent during years of drought; when rain comes they leap into frenzied life, flower, propagate and store up water in a great burst of energy. They combine the talents of the marathon runner and the sprinter!