GENERAL RELATIONS 107 
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- 
tutes 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 water 
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. 
