Meeting Water Pwblcin on Land 119 



within its burrow, it breathes air two to five times more humid than 

 that outside. Measurements indicate that, if this animal remained 

 out of its burrow continuously, the rate of evaporation from its lungs 

 would exceed the rate at which it could obtain water, and the re- 

 sulting water loss would eventually cause death. Temperatures in 

 desert burrows have been found to be as low as 28°C at the same time 

 that values for the surface of the soil surpassed 71°C (cf Table 9). 

 Such critical differences in temperature and in other factors in micro- 

 climates will be considered further in subsequent chapters. 



Meeting Water Problem on Land 



Land plants can live only where sufficient water reaches them either 

 through the air or through the soil. Some species can get along with 

 relatively small amounts of water, and these plants, inhabiting deserts 

 and other arid regions, are termed xerophytes. Plants eke out an 

 existence on a reduced water supply in a variety of ways. Certain 

 annual plants are adapted to maintain themselves in desert areas by 

 remaining in the seed stage during the dry season and then passing 

 rapidly through their life cycles when a rainy period occurs. Other 

 desert plants, like the cacti, store water in succulent organs in suffi- 

 cient quantities to tide them over long intervals of drought. In addi- 

 tion, various species of non-succulent perennials, such as the creosote 

 bush, sage brush, and many grasses are included among the xerophytes 

 because they possess unusual ability to endure long periods of per- 

 manent wilting— in some instances running into years. Lichens and 

 some mosses similarly can survive a surprising degree of drying out. 

 Plants with water relations intermediate between those of the xero- 

 phytes and the hydrophytes are classified as mesophtjtes. 



Since land animals can move about, they can actively seek pools 

 of water from which to drink or can obtain water in the form of snow 

 or dew. Many are able to endure long periods between visits to the 

 water supply. Some terrestrial animals, such as frogs and toads, 

 have the ability to absorb water through their skins from damp sur- 

 roundings, whereas others find sufficient liquid water in the tissue 

 fluids of their food. Another method of solving the problem of 

 securing water in dry land habitats, and one employed widely by the 

 insects, is the use of metabolic water, that is, the water resulting from 

 the chemical breakdown of food materials. The clothes moth and 

 the meal worm have the ability to obtain metabolic water in sufficient 

 quantity to enable them to live their whole lives without any access 

 to free water in the environment. Experiments indicate that the 



