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Plant Adaptations


Many animal adaptations for desert dwelling are behavioral; some will be considered when we look at various groups. Plants have little ability to avoid the more serious dangers of the desert by behavior, so most adaptations are physical (including chemical). Only a few of the more notable adaptations will be covered in this section.

The most serious problem most plants have in the desert environment is associated with water: how to get it and, particularly important, how to keep it. Roots have the dual functions of bringing water and dissolved minerals from the soil into the plant body to be distributed as needed. There are two major root types: tap and fibrous. Tap roots consist of a main root with branch roots coming off of it; tap roots tend to grow deep into the soil. A carrot is an example of a tap root, though most of the side branches are removed before sale and it's relatively short and fat compared to most tap roots. Fibrous roots consist of a number of roots of which the main ones are more or less the same size; fibrous roots tend to spread out relatively near the surface of the soil. Grasses generally have fibrous root systems. Some desert plants have a double system, one set of roots near the surface and another that delves deeper; creosotebush is an example.

Plant roots normally take up water by osmosis—by having more dissolved material within the cells of the roots than outside, causing the water to diffuse into the roots. In situations where there is too much dissolved material (such as salts) in the soil, a plant can die from lack of water even when in moist soil. The beds of playa lakes often present this situation, leaving them entirely free of plant life or with small numbers of plants especially adapted for such conditions.

Plant leaves tend to be potential areas of water loss. When surfaces are unprotected or underprotected, molecules of water diffuse out into the surrounding air. Although at a cost, this is solved by many desert plants by applying a layer of waterproofing to the outside of the leaf. This substance, usually waxy in nature, does a reasonable job if fairly thick, but is metabolically expensive to manufacture. Decreasing the size of leaves both decreases the surface area through which water loss can occur and requires less waterproofing material; of course, it also decreases the leaf area for photosynthesis, one reason that many desert plants are slow growers.

Equally or more important is that in order to carry out photosynthesis, the plant needs to get carbon dioxide into the leaf, and the source is the air outside of the leaf. Small openings in the leaf (each one a stoma; plural, stomata) allow outside gases into the interior of the leaf. A stoma, however, is a two-way door: water inside of the leaf is free to diffuse to the outside, a process known as transpiration. The stomata, like doors, can be opened and closed; but closing the stomata to conserve water means that the carbon dioxide necessary for photosynthesis cannot get in. Thus some plants can access carbon dioxide only during the cooler parts of the day, when transpiration is much slower than at hotter temperatures. When the stomata close, making carbon dioxide unavailable, photosynthesis ceases, yet another reason for slow growth.

Many desert plants have solved the problem to a degree. A number of them use physiological pathways that quickly capture carbon dioxide, allowing the stomata to be closed more often than in plants not adapted for this. These so-called C4 plants (as opposed to C3 plants without the adaptation) also do much better in strong light and hot temperatures. C3 plants under such conditions tend to burn more carbohydrates than they manufacture. Many warm-climate grasses and such things as four-wing saltbush are C4 plants. Cacti and many succulents (such as the century plants, or agaves) utilize a different physiological strategy call CAM photosynthesis. The stomata generally are opened at night, when temperatures tend to be cooler and relative humidity higher, and the carbon dioxide stored in the form of an acid; during the day, the stomata are closed and photosynthesis uses the stored carbon dioxide. CAM plants also can go into a slowed mode for extended periods of time during particularly hot, dry periods by closing the stomata and utilizing carbon dioxide from cellular respiration to manufacture food and burning food to supply energy for respiration. This makes some sense if you know that plants, like animals, produce carbon dioxide as a waste product when breaking down food stuffs to produce usable energy. Of course, energy is lost at each such cycle and eventually the process runs down.

Other strategies for water conservation include dropping the leaves during dry periods, thus greatly decreasing transpiration. Ocotillo is noted for this.

A different strategy for surviving in a water-challenged habitat is undertake the life cycle only during times when enough water is available. Annuals are plants that complete their life cycle within 1 year (and usually far less than that). A number of desert annuals germinate during the spring, taking advantage of winter precipitation and finish flowering and setting seed before the hottest, driest time of the summer. Mexican poppies are an example. In common with many similar desert plants, few or none germinate if the winter precipitation fails; the seeds lie dormant until conditions are suitable. Several years may pass between conditions suitable for germination, and for many such species, only a portion of the seeds germinate during the good times—a hedge against times when things start out well and then turn ugly.

A somewhat different twist is waiting until precipitation sufficient to support the plant through maturity and reproduction falls.


Last Update: 25 Jun 2006