128 Water 



other instances animals are able to respond directionally to a humidity 

 gradient. However, the whole problem of control of locomotion by 

 moisture in land animals calls for further investigation. 



The discussion of water relations in this chapter emphasizes the 

 universality of the water problem. Plants and animals in aquatic, 

 amphibious, and terrestrial situations are faced with the difficulty of 

 providing for water exchange and at the same time maintaining the 

 proper water balance. The amount of water present in the environ- 

 ment and its transfer into and out of the organism are results of an 

 equilibrium of forces. In the terrestrial environment the organisms 

 themselves, especially the plants, can affect the amount of moisture in 

 their surroundings. The total amount of water is not of as great 

 significance as its availability. In some situations the environment 

 tends to extract water from the plant or animal at an alarming rate. 

 If the organisms cannot obtain water fast enough and retain it in suf- 

 ficient quantity, the lack of this essential material becomes a critical 

 limiting factor. 



