114 The Plant Wori.d. 



nomena lies elsewhere. It is apparent that the plant roots 

 can be actually in contact with only a very small part of the 

 soil at one time, and, as water absorption goes on, this soil 

 which is touched by the roots would be very soon dried out 

 were it not for one thing. More water is immediately supplied 

 from other parts of the soil by capillary movement. * If the 

 rate of this movement be low, the rate of water absorption must 

 necessarily be low also even though there be plenty of water in 

 the soil as a whole. The dearth of water is in the contiguous 

 soil only. It is not that there is too little water but that it is 

 too slowly supplied. This is often the case when plants wilt 

 and die from sudden hot winds. The rate of transpiration sud- 

 denly jumps up and the plant is dried out and killed before the 

 saving water can move through the soil and reach the roots. 



It will be noticed that factors depending upon the concen- 

 tration of the cell solution appear in both sides of the equation — 

 as controllers both of transpiration and of absorption. This is 

 why it was necessary above to say that the rates of absorption 

 and of transpiration could vary almost independently. The 

 concentration relations are the reason for the "almost." It 

 will be remembered that transpiration decreases with increase 

 in sap concentration, while under the same conditions absorp- 

 tion increases. It is apparent, therefore, that if some means 

 exist by which the sap concentration at leaves and roots is kept 

 substantially the same, there really is some relation between 

 transpiration and absorption. To be more precise, there is an 

 influence of the water content of the plant on both transpira- 

 tion and absorption, as well as of them on it. If the plant be- 

 comes too dry the concentration of the cell solution increases, 

 and, in turn, transpiration decreases and absorption increases; 

 both of which changes tend to remove the condition which gave 

 them rise. But this process of natural regulation is efficacious 

 between narrow limits only. It maintains the balance of 

 transpiiation and absorption under normal conditions — ^pre- 

 vents a dessication or a drowning due to any possible " mistake" 

 of the transpiring or absorbing mechanism. It is competent 

 also to care for the constantly occurring slight variations in 

 external conditions, which are the evaporating power of the air 

 and the supply of moisture through the soil. . 



*As discussed in the previous paper. _^ 



