=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 constitutes 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 humidity 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 precipitation, 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. The order of the signs, ±, denotes that the water increases and decreases with an increase and decrease of the factor, or the reverse, ∓.