Deserts and shrublands

Some deserts are so absolutely dry that they are almost completely devoid of plant life. The most extreme deserts, such as the Sahara in north Africa, occupy a band on about 20° north latitude. These deserts usually receive an annual precipitation of less than 4 inches (10 centimeters), and in some areas, no rain falls for years. Other deserts, such as those in the southwestern United States, northwestern Mexico, and west-central Australia, are less severe. These deserts receive an average annual rainfall of up to 10 inches (25 centimeters) all falling in one short season, which is sufficient to permit some specially adapted plants to live there.

Not only do these plants have to deal with severe drought conditions, but also the high temperatures that occur during the day—for example, up to 134° F. (57° C) in Death Valley, California—and often near-freezing temperatures at night. Temperatures are also seasonal in some deserts; those that are cold in winter are called “cold” deserts, such as the Gobi, where winter temperatures frequently drop as low as —40° F. (—40° C). Most desert plants are xerophytes (they conserve water); most also share the same means of water storage and heat endurance.

C4 photosynthesis involves two fixings of carbon. The first, indicated in red, occurs in the middle layers (mesophyll) of the leaf, when carbon dioxide (CO2) enters them and combines with the 3-carbon substrate phosphoenol pyruvate to form the 4-carbon compound oxaloacetic acid. This acid is converted to 4-car-bon malic acid, which is then carried to the specialized vascular bundle sheath cells. Here, CO2 is split off, and a second fixing, marked in blue, takes place. The CO2 combines with ribulose bisphosphate, then continues into the sugar-forming reactions.

Adapted photosynthesis

Most plants have C3 photosynthesis, but this method loses a great deal of water through transpiration. Some have adopted strategies such as C4 photosynthesis in which carbon dioxide is fixed temporarily, released, and then refixed. The second fixing takes place in specialized vascular bundle sheath cells, and the process reduces water loss and respiration. These plants, which are mainly grasses, are found in the lower tropical latitudes. Despite their adaptation they still need some water for photosynthesis to take place. The stomata in the leaves have to open, to allow carbon dioxide to enter the plant for use in photosynthesis, which inevitably results in some water being lost. Succulents avoid this problem by keeping their stomata closed during the day, but open at night, admitting the carbon dioxide and losing less water than if they were open during the day. The carbon dioxide is then stored until daylight, when it is metabolized in a process known as crassulacean acid metabolism (CAM).

Alternatively, some plants do not photosyn-thesize during the hot, dry season, or at the hottest time of the day, but survive in a considerably desiccated state until the air is more humid.

Annuals

Desert plants are able to deal with the problems posed by intense heat and an irregular water supply in two ways—they endure them with the aid of morphological and physiological adaptations, or they avoid them, for example by remaining dormant.

The “avoiders” are known as ephemerals. They are mostly annuals, which survive drought in their seed form. Few of them have morphological adaptations to their environmental conditions, and most rely on their dormant seeds for survival. The seeds germinate quickly after rain; the seedlings grow rapidly, and flowering may begin very soon so that the plants can pass through their whole life cycle from seed to seed in a matter of weeks.

Virtually all annuals have mechanisms that allow germination only after large amounts of rain. These plants also tend to vary the time of germination within a single seed crop; a phenomenon known as seed polymorphism. The seeds on the parent plant of some annuals are retained even after they are ripe. When the plant receives drops of water, the bracts that hold the seeds in place open and allow only some of the seeds to fall away at each wetting. This means that their germination can be staggered, and the chances of success are correspondingly increased. In addition to varied times of germination, the seeds contain a substance called an inhibitor that stops them from germinating. This inhibitor can be washed out by water. The seeds, therefore, need at least two phases of wetting—one to cause their release, and another to remove the inhibitor and allow germination.

The seeds of other annuals germinate in the dark only, after a series of cycles of wetting and drying that alter the seed coat and allow free passage of oxygen to the embryo. This combination of requirements tends to cause the seeds to germinate only when buried and after several rain showers.

The strict requirements of seeds of different plants for germination cause different species of ephemerals to appear in response to rains and low or high temperatures in winter and in summer.

Annuals, like many other desert plants, depend mainly on insects for pollination. (Although the grasses and sedges are wind-pollinated.) The flowers are often large and showy and are attractive to insects, of which there may be few in these uncertain environments. The rains that stimulate plant growth, however, also encourage the emergence of insects from their dormant stages, so insects may be abundant when the plants are in flower.

Stoneplants (Lithops spp.) are heavily camouflaged through their likeness to the surrounding pebbles. These succulents have no stems— the visible fleshy parts are the tips of leaves that are partly buried in the soil. Two leaves grow during each rainy season when a single flower appears between them. The old leaves then wither away.

Drought-deciduous plants

Another group of desert plants, which lies between the annuals and those that tolerate the desert conditions, is the drought-deciduous plants. These plants are perennials, unlike annuals, and are woody, but they avoid drought by shedding their small leaves as soon as the water availability is reduced. They remain in a state of drought-dormancy until the rain falls again, when they quickly grow a new set of leaves and flower.

Some grasses are also drought-deciduous, but they are not dormant because they rely on water and nutrients stored in rhizomes underground, until the water supply is improved.

Succulents

Succulents are also perennials. They are not woody but have a distinctive fleshy appearance because of their water-storing facility. Water is conserved in large, thin-walled cells (parenchyma tissue) in the stems or in the leaves. They usually store water in their stems, because most have reduced leaves, or none at all, such as most cacti. The thick, outer cuticle of these plants reduces dehydration, but in addition, the stomata open only at night to allow carbon dioxide entry into the plant.

A number of succulents are spherical, which is the most efficient form for water storage. The sphere is, however, a form of limited potential in a plant, and very many more desert plants have a cylindrical form, which allows taller growth.

Neither spherical nor cylindrical cacti are smooth-surfaced; almost all have longitudinal ridges or a large number of conical projections on their surface. In combination with the internal, flexible network of woody strands, the ridged surface of these plants allows expansion and contraction according to the amount of water stored in them.

Moreover, succulents possess other structural features that tend to reduce water loss. Some are pale and shiny, and reflect much of the radiation that falls on them; others are covered with white hairs that perform the same function. Some grow partially buried in the soil or sand with only the tips of the leaves showing on the surface. The leaf tips are transparent and are lined inside with green photosynthetic cells, which are illuminated by the light entering through the “window” in the leaf.

Desert plants are particularly vulnerable to damage by herbivorous animals, because they grow very slowly and do not recover well from damage. Many of them survive, therefore, with the help of spines, such as those found on some cacti, or detachable barbs, which discourage feeding animals; others survive by being well-camouflaged. An additional advantage for these plants is the conservation of water when they nestle among the stones.

Xeromorphs

Most xeromorphs are shrubs and trees, which tolerate drought because of the various specializations of their tough leaves. Some have small leaves (or none at all), which may be needlelike, or curled up; their small size means that less surface area is presented to the sun’s heat. Xeromorphs have the stomata on one side of the leaf only (usually the upper side), which are covered when the leaf curls up; this protection has the effect of reducing transpiration, particularly in windy conditions. Others have stomata on both sides and absorb moisture from the atmosphere through them, especially when there is dew or fog around them. Those trees that do not have leaves or which lose their leaves in the dry season photosyn-thesize through their stems in addition to their small leaves.

Deserts support some trees, particularly . around oases or watercourses where brief heavy floods recharge deep soil moisture reservoirs at long intervals. These trees have very deep roots, often reaching a depth of 100 feet (31 meters), that exploit the deep, moist soil layers.

Shrubs with xeromorphic leaves are more typical of Mediterranean-type vegetation such as California chaparral. These shrubs keep their leaves throughout the year. In some plants, such as manzanita (Arctostaphy/os spp.), the leaves are supported vertically to reduce the area exposed to the sun, and others hold their leaves parallel to the sun’s rays for the same reason. The tough structure of the leaves and their internal sclerenchymous tissue prevent the plants from wilting under water stress or from being damaged by strong winds.