_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 studying 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 compensatory, 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 wilting; 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 compensate 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 compensation 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 surface exposed to the air. No compensation occurs in the case of submerged plants; floating hydrophytes possess a single transpiring leaf surface, while the leaves of amphibious plants behave as do those of mesophytes. The whole question of response to water stimuli thus turns upon the compensation for water loss afforded by water supply where the latter is moderate or