DESERT PLANTS AS A SOURCE OF DRINKING WATER. 



Bv Frederick Y. Coville. 



A stranger left alone in a desert would die of thirst, and yet there 

 is water in all deserts, and both the native animals and the native 

 races know how to find it. This water is gathered and stored by 

 plants, which have built and tilled their reservoirs for their own pur- 

 poses, but which yield it up. when required, for the use of the animal 

 world. 



The extent of the root sj^stem in desert plants, b}- means of which 

 the}^ absorb their water from the soil, is often astonishingly great. 

 In the Mohave Desert of (California a branching cactus {Opuntia 

 echlnoearpa) 48 centimetei's (19 inches) in height was found to have a 

 network of roots extending over an area of ground about 5.5 meters 

 (18 feet) in diameter." The roots lay near the surface, at a depth of 5 

 to 10 centimeters (2 to i inches), a situation which enabled them to 

 take advantage of a single substantial downpour and, before the pre- 

 cipitation had been again absorbed into the parched air, to suck up a 

 supply of water sufficient, if need be, for a whole year's use. Other 

 desert plants send their roots deep into the ground for water, and a 

 certain shrubby species of acacia found about Tucson, Arizona, pos- 

 sesses, according to Professor R. H. Forbes, a double-root system, in 

 which one series of roots spreads out horizontall}', close beneath the 

 surface, and a second series, sharply defined, goes directly downward 

 into the soil. Such an arrangement enables the plant to seize upon 

 water either from light precipitation or when deeplv percolating under 

 dry stream beds. 



While the devices for absorption in desert ]:)lants are unusual, the 

 mechanical contrivances bv means of which these plants are enabled 

 to retain the moisture they have absorbed are still more remarkable. 

 Other factors l)eing eciual, the amount of water transpired, or evapo- 

 rated, from a plant is proportional to the area of its green surface, 

 which, in ordinary plants, is a foliage surface. A specimen of coflfee 

 plant {Coffea at'ahlca) weighing 2<».5 grams is found to have a leaf 

 surface computed at KU.-lTB square millimeters, which gives a ratio of 



« Coville, Contributions from the United States National Herbarium, Vol. IV 

 (Botany of the Death Valley Expedition), pp. 46-7, 1893. 



499 



