44 HORTICULTURAL SOCIETY OF NEW YORK. 



be transmitted through the conducting system to the leaves, 

 etc., and this rate will probably decrease as the plant develops, 

 since the path traversed becomes longer. We know practically 

 nothing of the fluctuations of this latter rate, but it is probably 

 safe to assume that it does not vary rapidly and is practically 

 constant as long as it is not altered by growth. 



Evidently, a resultant of the conditions just described is 

 this, that the plant is able to maintain in its tissues a water con- 

 tent adequate to life and growth only so long as the "transpira- 

 tion rate does not exceed the maximum rate of water supply 

 from the soil. It is thus possible for a plant to grow normally 

 in a soil with a low water content, if only the evaporating power 

 of the air about its leaves and branches (and hence its tran- 

 spiration rate) is riot excessive. The same plant may be ob- 

 . served to wilt and die at a later stage, for either, or both, of two 

 very different causes : the water content of the soil may be de- 

 creased till the rate of supply to the transpiring tissues becomes 

 less than that of loss, or the evaporating power of the air may 

 be increased till the rate of loss becomes greater than that of sup- 

 ply. It will be seen that both causes bring about the same con- 

 dition within the plant, namely, a shortage of water in the tran- 

 spiring tissues. 



From my own observations it appears to be true also, that 

 a quiescent existence can be maintained with the rate of tran- 

 spiration approaching or equalling the maximum rate of supply, 

 but that growth cannot occur unless there is still a considerable 

 margin of possible supply over and above the transpiration loss 

 which is being experienced. 



It is realized by every worker with plants that the water 

 supply is the most important and fundamental of the conditions 

 which are effective in determining growth. But it is not nearly 

 so clearly appreciated that this is not alone dependent upon the 

 amount of water in the soil but is to a great extent determined 

 by the evaporating power of the air. At first thought it might 

 appear that with high evaporation rate the soil would rapidly 

 dry out and that at the same time the rainfall would be scanty, 

 so that the air condition would be directly transmitted to the 

 soil. But such is not the case, at least in many instances; for 



