Desert species

In many ecosystems, plants tend to pattern themselves as pure stands or as individuals spaced in rather specific densities or configurations. Many desert species show obvious zones of inhibition around which few, if any, alien species are able to invade. These patterns often cannot be adequately explained by competition alone, and are probably caused by a combination of factors including allelopathy. The phenomenon happens with herbaceous plants as well as woody shrubs and trees. 

The rings in trees are not always annual. In many desert species a ring forms when large xylem cells are produced after a suitable rainy period followed by smaller cells, and this can occur more than once or sometimes not at all in a particular year. Moreover, trees from the wet tropics can have no annual rings. 

As we have seen, the presence of partially oxidized carbons lowers the energy output of a fuel. Fats are commonly used as an energy store, to be used only when the more readily accessible carbohydrates have fallen into short supply. In Arctic species, the stored fat also acts as a layer of insulation in desert species (such as the camel), the fat is also a source of water, one of its oxidation products. 

Several foliose desert species and sometimes even Ramalina maciformis (Galun, 1970) have developed a specialized life form. Thethalli lie loosely on the ground and in the dry state they curl up their lobes and can be blown... 

The attachment of terriculous lichens to the soil has not been studied in any detail. Except for a few species such as Lecidea uliginosa (Brodo, 1961) and some desert species (Shields et al., 1957), few lichens have the ability to invade and consolidate sand. Most either are merely buried to a slight extent at the base (McLean, 1915), or, more frequently, become attached to dead vegetation (see p. 404). 

Modern plants have also been found to contain beryllium. Hickory is the best beryllium accumulator found so far, containing as much as 1 ppm dry weight. Most studies have shown that leaves contain more beryllium than either twigs or fruits, although some desert species contain more beryllium in the twigs. Romney and Childress [22] have shown that beryllium inhibits the growth of several different crop plants at levels above 2 ppm in nutrient solution and 4% of the cation exchange capacity in soil. 

Jojoba is a desert crop that gives a small bean containing about 50% of a wax, a fatty acid ester with a fatty alcohol. The only other large source of such a wax is sperm whale oil, traditionally used in fine lubricants (see Lubrication and lubricants). Because the sperm whale is an endangered species, relatively Httle sperm whale oil is available and there is a large market for a substitute. Jojoba oil has been found to be usable for most of these appHcations. The jojoba oil is obtained by simply pressing the nut followed by conventional refining. Some jojoba oil is used in cosmetics (qv). 

Figure 6.5. 5 0 of tooth enamel phosphate versus body size (log scale) for Kenyan fauna analyzed in this study. With the exception of the dikdik, there is a general association between the two variables. In contrast to tbe body-size model (Bryant and Froelicb 1995) wbicb predicts a range of values close to l%o, however, the measured range in 8 0 values for species averages is 5%a. This and the anomalous values for the dikdik reflect physiological and behavioral adaptations by these desert adapted species. 

These data show that both models identify important variables that affect 5 Obody w.ier and 8 Ophospha in mammals. Both serve to identify the dikdik as an outlier which may be explained by their sedentary daytime pattern. On the other hand, the body-size model (Bryant and Froelich 1995), which may reliably predict animal 5 0 in temperate, well-watered regions, does not predict 8 Opho,phaw in these desert-adapted species. The second model (Kohn 1996), by emphasizing animal physiology independent of body size, serves to identify species with different sensitivities to climatic parameters. This, in conjunction with considerations of behavior, indicate that certain species are probably not useful for monitoring paleotemperature because their 5 Obodyw er is not tied, in a consistent way, to The oryx, for example, can... 

Water is, of course, essential for plant growth, but one of the themes of this chapter is that it may not be necessary for plant survival. Although most agronomically important plants are very sensitive to internal water deficits, the majority of plants at some stage of their life cycle are tolerant of desiccation. Few of these have vegetative parts which are desiccation tolerant, but the survival of even so-called drought-evading species, such as the ephemeral desert annuals, rests on the tolerance of their seeds to desiccation. 

Ephedra is a genus of 40-50 species widespread in Old World and New World desert areas. Preparations of many different species have been used in folk remedies for a very long time. The physiological effects of these preparations are very likely... 

Madder, also known as Turkey red, is a scarlet dye extracted from perennial herbaceous plants of the order Rubiacea, of which there are about 35 species (Chenciner 2001 Farnsworth 1951). A well-known plant from this order is Rubia tinctorum, found naturally in Palestine and Egypt, abundant in Asia and Europe, and extensively cultivated in the ancient world, was widely used for production of the dye since remote antiquity. The use of madder for dyeing seems to have originated in the Middle East it was identified in many textiles found in Egyptian tombs and in woolen fabrics from the Judean Desert in Palestine. It was also used by the ancient Persians, Greeks, and Romans. Madder from other varieties of Rubiacea plants were used by the Incas in ancient Peru (Schaefer 1941 Fieser 1930). 

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