26 WATER-RELATION BETWEEN PLANT AND SOIL. 



Nevertheless, it is obvious enough that the water-extracting power 

 of the environment as a whole can not be considered as completely- 

 vanishing when only one of its component factors, as here expressed, 

 becomes zero; wilting and death of leaves may surely be produced by 

 high transpiration rates, even in plants rooted in water. In such a 

 case the resistance offered by the plant itself to water movement takes 

 the place of the soil resistance that has vanished. From this it is clear 

 that the resistance offered by the soil to root absorption can be a 

 limiting condition for plants only when its magnitude is greater than 

 the internal resistance to absorption and conduction. If this internal 

 resistance is such that, under a given set of other conditions, water can 

 move from roots to leaves no faster than with a certain rate, then it 

 is quite immaterial at what rate water may be supplied to the root 

 surfaces, so long as the latter rate is equal to or greater than the 

 possible internal movement. 



It thus appears that the statement D^ = EI may be expected to 

 express the approximate truth only when the internal resistance to 

 water movement is less than the value /. We may denote the internal 

 resistance here in question by R. Then, so far as we have gone, 

 Z)e = EI when R<I. But, even within the Umits set by this condition, 

 our general statement can not uniformly hold; for R may be less than 

 I and at the same time E may be nil, in which case D should again 

 vanish, which we know it does not. Under such conditions there is no 

 water-extracting power exerted by the aerial surroundings, and the 

 only desiccating power effective to tend to remove water from the 

 plant is /, the attraction of the soil for water. As long as R<I, R 

 does not play a controlling part in this relation, and our expression 

 becomes simply D^ = I. 



A consideration of all the logical possibilities in this connection 

 would lead farther afield than it is requisite to go at present. It 

 simply needs emphasis here that the statement D^ = EI is to be 

 regarded as approximately true only within the limits set by the 

 conditions, i2< Zand ^>0. As to the possible values which R may 

 assume, it is clear that there must always be some resistance to 

 water movement from absorbing to transpiring periphery. It there- 

 fore follows that R may be supposed to have any value greater than 

 zero. Hence, when R<I and R>0, then />0. 



Thus the conditions under which the environmental desiccating 

 power may be taken as proportional to the product EI are: (1) that 

 the internal resistance to water movement is less than the attraction 

 of the soil for water {R<I); (2) that the latter attraction is always 

 positive (/>0) ; and (3) that there is some tendency for evaporation to 

 occur from aerial surfaces {E>Q). We venture the conviction that 

 these conditions are fulfilled in the vast majority of cases involving 

 ordinary plants, with roots in soil and stems and leaves in air. This 



