Water Problem in Terrestrial Environment 107 



tions, the water factor thus exerts a major controlling influence on 

 the minute organisms as well as on the more conspicuous species. 



WATER PROBLEM IN THE TERRESTRIAL ENVIRONMENT 



When we turn to dry land as an environment, the water problem 

 becomes extremely acute. Everyone is familiar with the quick death 

 which awaits aquatic organisms brought out on land. Many terres- 

 trial forms can endure the land environment only if they remain in 

 damp places. This is tiue for a great many species of plants and 

 also for animals without special protection. A salamander that has 

 escaped from its terrarium and run across the dry floor will soon be 

 dead if it is not rescued and put back in a humid atmosphere. 

 Earthworms frequently crawl out on a concrete sidewalk during a 

 spring rain. If the sun comes out and dries the sidewalk, many of 

 the animals will be killed before they can find their way back to the 

 moist ground. 



The successful invasion of the dry land is dependent upon the 

 possession of methods for securing and retaining sufficient water and 

 at the same time allowing adequate exchange with the environment. 

 Terrestrial plants and animals as well as aquatic forms must maintain 

 a proper ivater balance between water income and loss. Great varia- 

 tion exists in the moisture content of land organisms and also in the 

 amount of water intake and output that is necessary to provide for 

 metabolic processes. Insects subsisting on dry food probably hold 

 the record for the minimum water exchange. At the other end of the 

 scale are many types of higher plants that are extravagant water 

 users. Some species of plants absorb and transpire more than 2000 

 grams of water for every gram of dry matter produced by assimila- 

 tion. An oak tree may transpire as much as 570 liters of water in a 

 single day. 



How many taxonomic groups have succeeded in colonizing dry 

 land? Actually, only a very few of the prominent kinds of plants and 

 animals have accomplished this. In the plant kingdom it is chiefly 

 the vascular forms that are able to live in really dry places, but some 

 low-growing bryophytes, algae, and fungi— particularly the rock lich- 

 ens—form noteworthy additions to the list. Among the vertebrates, 

 only the reptiles, birds, and mammals can live freely exposed to dry 

 air. Although several phyla of invertebrates are reported in the land 

 fauna, only the insects, spiders, and snails occur in important numbers 

 in dry habitats. By contrast, there are a great many plant and ani- 

 mal groups that are exclusively aquatic. 



