108 THE PLANT 



The order of the signs, , denotes that the water increases and decreases 

 with an increase and decrease of the i actor, or the reverse, h=. 



154. Response. The normal functional responses to water stimuli are 

 absorption, diffusion, transport, and transpiration. Of these, absorption and 

 transpiration alone are the immediate response to soil water and humidity, 

 respectively. Consequently they are the critical points of attack in study- 

 ing the fundamental relation of the plant to the water of its habitat. In 

 determining the pathway of the response, it is necessary to trace the steps 

 in diffusion and transport, but, as these are essentially alike for all vascular 

 plants, this task lies outside the scope of the work in hand. As previously 

 suggested, the relation between absorption and transpiration is strictly com- 

 pensatory, though, for obvious reasons, the amount of water transpired is 

 usually somewhat less than the amount absorbed. Absorption falls below 

 transpiration when extreme conditions cause temporary or permanent wilt- 

 ing ; the two activities are essentially equal after a growing plant reaches 

 maturity. In all cases, however, the rule is that an increase or decrease in 

 water loss produces a corresponding change in the amount of water 

 absorbed, and, conversely, variation in absorption produces a consequent 

 change in transpiration. This is strictly true only when the stimuli are 

 normal. For example, a decrease in humidity causes increased water loss, 

 which, through diffusion and transport, is compensated by increased activity 

 of the root surface. Frequently the water supply is insufficient to compen- 

 sate for a greater stimulus, and the proper balance can be attained only by 

 the closing of the stomata. In the case of excessive stimuli, neither com- 

 pensation suffices, and the plant dies. Many mesophytes and all xerophytes 

 have probably resulted from stimuli which regularly approached the limit 

 of compensation for each, and often overstepped, but never permanently 

 exceeded it. For hydrophytes, the danger arises from excessive water 

 supply, not water loss. There is a limit to the compensation afforded by 

 transpiration, which is naturally dependent upon the amount of plant sur- 



