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Plant Life

[Introduction | Kinds | Climate | Geography | Plant Life | Animal Life | Human Life]

One of the most important characteristics of any biome is its plant life. Not only do plants provide food and shelter for animals, they recycle gases in the atmosphere and add beauty and color to the landscape. Deserts support many types of plants, although not in large numbers.

Algae, fungi, and lichens

Although it is generally agreed that algae (AL-jee), fungi (FUHN-jee), and lichens Algae: Most algae are single-celled organisms; a few are multicellular. Certain types of algae make their own food by means of photosynthesis, while others absorb nutrients from their surroundings. (Photosynthesis is the process by which plants use the energy from sunlight to change water and the air's carbon dioxide into the sugars and starches they use for food.) Although most algae are water plants, blue-green algae do appear in the desert. They survive as spores during the long dry periods and return to life as soon as it rains. (Spores are single cells that have the ability to grow into a new organism.)

Fungi: Fungi are commonly found in desert regions wherever other living organisms are found. They, too, reproduce by means of spores. Fungi cannot make their own food by means of photosynthesis. Some, like mold and mushrooms, obtain nutrients from dead or decaying organic matter. They assist in the decomposition (breaking down) of this matter, releasing nutrients needed by other desert plants. Other fungi are parasites and attach themselves to other living plants. Parasites can be found wherever green plants live, and some often weaken the host plant so that it eventually dies. Others actually help their host absorb nutrients more effectively from the soil.

Lichens: Lichens are actually combinations of algae and fungi living in cooperation. The fungi surround the algal cells. The algae obtain food for themselves and the fungi by means of photosynthesis. It is not known if the fungi aid the algal organisms, although they may provide them with protection and moisture.

Lichens are among the plants that live the longest. Some living in polar deserts are believed to have survived at least 4,000 years. Although lichens grow slowly, they are very hardy and can live in barren places under extreme conditions, such as on bare desert rock or arctic ice. Crusty types, colored grey, green, or orange, often cover desert rocks and soil. During very dry periods, they rest. When it rains, they grow and make food.

Green plants

Most green plants need several basic things to grow: light, air, water, warmth, and nutrients. In the desert, light, air, and warmth are abundant, although water is always scarce. The remaining nutrients—primarily nitrogen, phosphorus, and potassium—which are obtained from the soil, may be in short supply.

Because water is limited, desert plants must protect against water loss and wilting, which can damage their cells. Large plants require strong fibers or thick, woody cell walls to help hold them upright. Even smaller plants have a great number of these, which makes them fibrous and tough. Their leaves tend to be small and thick, with less surface exposed to the air. Outer leaf surfaces are often waxy, which helps prevent water loss. Pores in the surface of green leaves allow the plant to "breathe," taking in carbon dioxide and releasing oxygen. The leaves of some desert plants may have grooves to protect their pores against the movement of hot, dry air. Other leaves curl up or develop a thick covering of tiny hairs for protection. Still others have adjusted to the dry environment by adapting the shape of their leaves. For, example cactus leaves are actually spines (needles). These spines provide less surface area from which water can evaporate. As a result, more water is stored within the plant.

Growing season

The growing season in deserts is limited to the brief periods of rain, which, in some cases, do not occur for several years. In some coastal deserts, certain plants absorb mist from the nearby ocean through their leaves. Where the soil is rich and rain more regular and dependable, desert plants may flourish.

In cold deserts, where real winter occurs, plants behave much like those in temperate climates. The portion above ground dies off, but the root system goes deep and is protected from freezing by layers of snow.

Deserts are home to both annuals and perennials. Annuals live only one year or one season and they require at least a brief rainy period that occurs regularly. Their seeds seem to sprout and grow overnight into a sea of colorful, blossoming plants. This period of rapid growth may last only a few weeks. When the rains disappear, the plants die and the species withdraws into seed form, remaining dormant (asleep) until the next period of rain.

Unlike annuals, perennials live at least two years or two growing seasons, appearing to die in between but returning to "life" when conditions improve. Those that live many years must be strong and use several methods to survive. During their youth, these plants devote most of their energy to developing a large root system to collect any available moisture. Plants are often many yards apart, because their roots require a large area of ground in order to find enough water. The above-ground portion of young perennials is small in comparison to the root system because their leaves do not have to compete for sunlight or air as they do for water. Some perennials are succulents (SUHK-yoo-lents), which are able to store water during long dry periods. Some, like the century plant, store water in their leaves, while others store it in their stems or in large roots.

Water supply

Except for occasional rivers and oases, water in the desert comes from the brief rains. Plants may grow in greater numbers in arroyos or wadis where some moisture may remain beneath the surface. In coastal deserts, some plants absorb moisture from fog and mists that condense on their leaves, while in cold deserts, spring thaws provide water from melting snow.

Reproduction

Pollination (the transfer of pollen from the male reproductive organs to the female reproductive organs of plants) is also often a problem in deserts. Although the wind may carry pollen from one plant to another, this method is not efficient because plants usually grow far apart. Also, insect pollination is rare because there are fewer insects than in other biomes. As a result, most plants have both male and female reproductive organs and pollinate themselves.

Common desert plants

Several species of plants grow in the desert, including cacti, shrubs, trees, palms, annuals.

Cacti: Cactus plants originated in southern North America, Central America, and northern South America. Instead of leaves, a cactus has spines, which come in many forms from long, sharp spikes to soft hairs. Photosynthesis takes place in the stems and trunk of the plant. Nectar, a sweet liquid that appeals to insects, birds, and bats, is produced in the often spectacular flowers.

The largest plant in the desert is the giant saguaro (sah-GWAH-roh) cactus. Its large central trunk can grow as tall as 50 feet (15 meters) and weigh 11 tons (10 metric tons). Ninety percent of its weight is in water, which it stores in its soft, spongy interior. During very dry conditions, as the plant uses up this stored water, the trunk shrinks in size.

Like many desert plants, the saguaro has a wide, shallow root system designed to cover a large area. After a long dry period, its roots can take up as much as 1.1 tons (1 metric ton) of water in 24 hours. The trunk then expands as it absorbs and stores the new supply.

Pores run in deep grooves along the saguaro's stems. These pores open during the cooler nighttime hours to take in carbon dioxide and release oxygen. For protection from wind and animals, long, sharp spines run along the grooves. These spines reduce air movement which conserves water and keeps grazing animals away.

Birds and bats love the nectar produced in saguaro flowers, and bats help to pollinate the saguaro. A bat's head fits the shape of the flowers almost perfectly and, as the bat drinks the nectar, its head gets heavily dusted with pollen grains, which it then carries to the next plant.

Woody shrubs and trees: Small, woody perennials that flourish in the desert climate include sagebrush, salt-brush, creosote, and mesquite. They have small leaves and wide-ranging root systems. Most shrubs have spines or thorns that protect them against grazing animals.

The mesquite (meh-SKEET) is a tree with roots that may grow 100 feet (30 meters) deep. Because of its long roots, it manages to find a constant supply of water and remains green all year. It produces long seed pods with hard, waterproof coverings. When the pods are eaten by desert animals, the partially digested seeds pass out of the animal's body and begin to grow.

Joshua trees, a form of yucca plant, live for hundreds of years in the Mojave Desert of California. Yuccas and treelike aloes store water in their leaves, not their stems. One type of aloe, the kokerboom tree of southwest Africa, can survive several years without rain.

Palms: Date palms, found at many oases in the Sahara and Arabian Deserts, can grow in soil with a high salt content. Only female trees produce dates, and only a few male trees are necessary for cross-pollination. The dates can be eaten raw, dried, or cooked and are an important food source for desert dwellers.

The Washingtonian fan palm is the largest palm in North America. Native to California, it requires a dependable supply of water to survive and sends out a thick web of tiny roots at its base. When its fronds (branches) die, they droop down around the trunk and form a "skirt," where animals like to live. The fan palm produces a datelike fruit, which is eaten by many desert inhabitants.

Annuals: Long grasses, such as alpha or esparto grass, often flourish in the desert after seasonal rains. Their stems can be used to make ropes, baskets, mats, and paper. Tufts of grasses sometimes become so entwined that they form balls which, when blown by the wind, drop their seeds as they spin across the landscape.

Almost every desert has its share of blooming annuals that add masses of color after a rain. Primroses and daisies adorn the California desert, while daisies, blue bindweed, dandelions, and red vetch beautify the Sahara.

Endangered species

Many desert plants, such as the candy cactus, saguaro cactus, and the silver dollar cactus, are nearing extinction because of their popularity as house plants and for landscaping. Even though they are available for sale in nurseries, some people steal them from the wild. Because desert plants are sparse to begin with, removal from their native home upsets the delicate balance of their reproduction.

[Introduction | Kinds | Climate | Geography | Plant Life | Animal Life | Human Life]

Source: "Desert." U·X·L Encyclopedia of Biomes, Vol. 1. U·X·L, Detroit: 2000.

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