108 THE PLANT 
The order of the signs, +, denotes that the water increases and decreases 
with an increase and decrease of the factor, or the reverse, +. 
Humidity + Water-content + 
Temperature = Temperature = 
Wind = Wind + 
Precipitation + Precipitation + 
Pressure + Pressure = 
Soil texture 0 Soil texture 
Altitude = Porosity += 
Capillarity + 
Slope = Slope += 
Exposure += Exposure = 
Cover + Cover + 
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- 
