GENERAL RELATIONS I07 



HYDROHARMOSE 

 ADJUSTMENT 



152. Water as a stimulus. Plants are continually subjected to the 

 action of the water of the soil and of the air ; exception must naturally 

 be made of submerged plants. The stimulus of soil water acts upon the 

 absorbing organ, the root, while that of humidity affects the part most 

 exposed to the air, viz., the assimilative organ, which is normally the leaf. 

 But since both are simultaneous water stimuli, a clearer conception is gained 

 of this operation if they are viewed as two phases of the same stimulus. 

 This point of view receives further warrant from the essential and intimate 

 relation of humidity and water-content as determined by the plant. They 

 are in fact largely compensatory, as is shown at some length later. In 

 determining the intensity of the two, a significant difference between them 

 must be recognized. The total humidity of the air at any one time consti- 

 tvites a stimulus to the leaf which it touches. This is not true of the total 

 soil water. Part of the latter is not available under any circumstances, 

 and can not affect the plant, at least directly. The chresard alone can act 

 as a stimulus, but even this is potential in the great majority of cases, since 

 the actual stimulus is not the water available but the water absorbed. The 

 latter, moreover, contains many nutrient salts which are in themselves 

 stimuli, but as they normally have little bearing upon the action of wafer 

 as a stimulus they are to be considered only when present in excessive 

 amounts. 



153. The influence of other factors upon water. The amount of hu- 

 midity is modified directly by temperature, wind, precipitation, and pressure, 

 and, through these, it is affected by altitude, slope, exposure, and cover. 

 Naturally, also, the evaporation of soil water has a marked influence. In 

 determining water-content, atmospheric factors, with the exception of pre- 

 cipitation, are usually subordinate to edaphic ones. Soil texture, slope, and 

 precipitation act directly in determining soil water, while temperature, wind, 

 and pressure can operate only through humidity. This is likewise true of 

 altitude, exposure, and cover, though the latter has in addition a profound 

 effect upon run-off. Biotic factors can affect humidity or water-content 

 only through the medium of another factor. Light in itself has no action 

 upon either, but through its conversion into heat within the chloroplast, it has 

 a profound effect upon transpiration. The following table indicates the 

 general relation between water and the other physical factors of 'the habitat. 



