Adaptation of Plants to a Desert Environment (continued)

NON-SUCCULENT PLANTS
(continued)

The Perennials

Chief among the requirements for year-round survival in the desert is a plant's ability to control transpiration (loss of water through surface areas) and thus maintain a balance between water loss and water supply. In this struggle, the hours of darkness are a great aid because in the cool of the night the air is unable to take up as much moisture as it does under the influence of the evaporating heat of the sun. Therefore, less exhaling and evaporating of water occurs from plants, thus reducing both the rate and the amount of water loss. This reduction in transpiration at night allows the plants to recover from the severe drying effects of the day. As one biologist stated, "If the celestial machinery should break down so that just one night were omitted in the midst of a dry season, it would spell the doom of half the non-succulent plants in the desert."

One of the common trees in the desert part of the monument is the MESQUITE (Prosopis juliflora). In general appearance it resembles a small but spiny apple or peach tree with divided leaves, but its roots sometimes penetrate to a depth of 40 feet. Thus the roots secure moisture at the deeper, cooler soil levels, from a supply which remains nearly constant throughout the year. This enables the tree to expose a rather large expanse of leaf surface without losing more water than it can replace. A number of mechanical devices help the tree to reduce its water loss during the driest part of the day (10 a. m. to 4 p. m.). Among these are its ability to fold its leaves and close the stomata (breathing pores) thereby greatly reducing the surface exposed to exhaling and evaporating influences. In April and May, mesquite trees are covered with pale-yellow, catkinlike flowers which attract swarms of insects. These flowers develop to stringbean-like pods rich in sugar and important as food for deer and other animals. In early days, the mesquite was also a valuable source of food and firewood for Indians and pioneers.

Another desert tree abundant in the monument is the YELLOW PALO VERDE (Cercidium microphyllum). It is somewhat similar in size and general shape to the mesquite. Without the deeply penetrating root system of the mesquite, the palo verde (Spanish meaning "green stick") cannot count on a dependable moisture source so has taken even greater precautions to retain as much as possible of the water which its roots gather. In early spring, while the soil still contains moisture from the winter rains, the tree leafs out with dense foliage, followed closely by a blanket of yellow blossoms. At this season, the palo verdes provide one of the most spectacular displays of the desert, particularly along washes where they grow especially well. With the coming of the hot drying weather of summer, the trees need to reduce their moisture losses. They gradually drop their leaves until, by midsummer, each tree has become practically bare. However, the trees do not enter a period of dormancy but are able to remain active because their green bark contains chlorophyl. Thus the bark takes over some of the food manufacturing function that the leaves normally perform but without the high rate of water loss.

A good specimen of palo verde.

Carrying the drought evasion habits of the palo verde a step further, the OCOTILLO (Fouquieria splendens) comes into full leaf following each rainy spell. During the intervening dry periods it sheds its foliage. The ocotillo, a common and conspicuous desert dweller, is a shrub of striking appearance with slender, thorny, whiplike, unbranching stems 8 to 12 feet long growing upward and outward from a common base in the shape of a funnel. In late spring, brilliant scarlet flower clusters appear at the tips of the stems making each plant a glowing splash of color.

The ocotillo.

A number of desert shrubs fail to display as much ingenuity as the palo verde. Some of these, such as the DESERT-THORN (Lycium sp.), evade the dry season simply by going into a state of dormancy. It bursts into full leaf soon after the coming of the first winter rains and blossoms as early as January. The small tomato-colored, juicy fruits are sought by birds which also find protective cover for their nests and for overnight perches in the stiff, thorny shrubs. In the past, the berrylike fruits were important to the Indians who ate them raw or made them into a sauce.

Commonest and most widespread of the conspicuous desert non-succulent shrubs, is the CREOSOTEBUSH (Larrea tridentata), found principally on the extensive fills of rock debris forming desert flats between the mountain ranges. It is also well represented in the monument. A new crop of wax-coated musty-smelling leaves, giving the plant the local name "greasewood," appears as early as January. The leaves are followed by a profuse bloom of small yellow flowers and cottony seed balls. During abnormally moist summers or in damp locations, the leaves and flowers persist the year round, but usually the coming of dry weather brings an end to the blossoming period. If the dry spell is exceptionally long, the leaves turn brown, and the plants remain dormant until awakened by next winter's rainfall. Pima Indians formerly gathered a resinous material known as lac, which accumulates on the bark of its branches, to mend pottery and fasten arrow points. They also steeped the leaves to obtain an antiseptic medicine. Ground squirrels commonly feed on the seeds. Dr. Forrest Shreve, well-known authority on desert plant life has written of the success of creosotebush in flourishing under desert conditions, "It exhibits an unusually high degree of what may be designated as 'physiological elasticity' by virtue of which its size, rate of growth, density of stand, amount of foliage, size and structure of leaves, and size of seed crop vary within wide limits ac cording to habitat and season."

Ferns, notably plants of dank woods and other moist habitats, seem entirely out of place in the desert; nevertheless, members of the fern family have overcome drought conditions. The GOLDFERN (Pityrogramma triangularis) is not uncommon on rocky ledges where, by means of special drought-resistant cells, it persists in spite of trying conditions and hardships of the relentless desert climate.

Among the smaller perennials, there are many that add each year to the seasonal flower display while conditions of moisture and temperature are most favorable. Since the perennials do not need to mature their seeds before the coming of summer as do the ephemerals, a majority start blossoming somewhat later in the spring, and gaily flaunt their flowers long after the annuals have faded and died. When the heat and drought of summer begin to bear down, they gradually die back, surviving the "long dry" by their persistent roots and larger stems. One of the most noticeable and beautiful of this group of small perennials fairly common in the monument is PARRY'S PENSTEMON (Penstemon parryi). It occurs in scattered clumps on well-drained slopes along the base of the Tanque Verdes. The showy rose-magenta flowers and the glossy-green leaves arise from erect stems that may grow 4 feet tall in favorable seasons.

Although not limited to a desert habitat, one of the conspicuous perennials found here is the PRICKLEPOPPY (Argemone platyceras) bearing large, white-petaled, yellow-centered flowers on thistlelike stems with prickly, gray-green leaves. Of especial interest is the fact that a closely related species, the Mexican pricklepoppy, with yellow petals, is found in the United States only in the vicinity of Tucson and Saguaro National Monument.

The pricklepoppy blooms throughout the year.

Among the first of the shrubby perennials to cover the rocky hillsides with a blanket of winter and springtime bloom is the BRITTLE-BUSH (Encelia farinosa). Masses of yellow sunflower-like blossoms are borne on long stems that exude a gum which was chewed by the Indians and also burned as incense in early mission churches.

Two of the conspicuous perennials that survive the dry season as underground bulbs are the DESERT-MARIPOSA (Calochortus kennedyi), and the PAPAGO LILY (Dichelostemma pulchellum). Although neither occurs in massed bloom, they add spots of color to the desert scene. Usually appearing from February to May, the papago lily has violet flower clusters on long, slender, erect stems. The bulbs were dug and eaten by Papago and Puma Indians. The desert-mariposa is particularly noted for its deep, rich-orange, three-petaled blossoms appearing from March to May.

Although neither conspicuous nor attractive, the common WESTERN RAGWEED or BUR-SAGE (Franseria sp.) of the desert is so important a part of the palo verde, bur-sage, and cactus association (which covers much of the desert part of the monument) that it should not be omitted from this discussion.

One of the handsome shrubs abundant in the high desert along the base of the Tanque Verdes is the JOJOBA (Simmondsia chinensis) or deer-nut. Its thick, leathery, evergreen leaves are especially noticeable in winter, and furnish excellent browse for deer. The flowers are small and yellowish, but the nutlike fruits are large and edible, although bitter. They were eaten raw or parched by the Indians, and were pulverized by early-day settlers for use as a coffee substitute.

Jojoba nuts are attractive and edible.

Among the attractive flowering shrubs are the INDIGOBUSHES (Dalea sp.) of which there are several species adapted to the desert environment. Most publicized of these is the smoketree of the lower deserts not found in the monument. The local, low-growing indigobushes are especially ornamental when covered with masses of deep-blue flowers in spring.

Another small shrub, noticeable from February to May because of its large tassel-like pink blossoms and fernlike leaves is FAIRY-DUSTER (Calliandra eriophylla). Deer browse on its delicate foliage.

A larger shrub of open, sprawling growth sometimes reaching tree size and usually found along desert washes in company with mesquite is CATCLAW (Acacia greggii). In April and May, the small trees are covered with fragrant, pale-yellow, catkinlike flower clusters that attract swarms of insects. The seed pods were ground into meal by the Indians and eaten as mush and cakes.

Quick to attract attention because of their apparent lack of foliage, the JOINTFIRS (Ephedra sp.) of which there are several desert species, grow in clumps of harsh, stringy, yellow-green, erect stems. The skin or outer bark of the stems carries on the functions of leaves which have been reduced to scales. Small, fragrant, yellow blossom clusters at the stem joints are noticeable in spring and attract insects by their nectar. The device of lessening the water-transpiring surface by reducing or entirely eliminating their leaves is one which has been employed successfully by many plants that have adapted their way of life to a desert environment. Of these, the stem succulents are among the most numerous and conspicuous.