ROOTS AND THEIR FUNCTIONS 85 



out from the surface of the wood. If vines 

 are pruned too late the water thus pressed 

 out by the root cells through the xylem will 

 flow for many days, and it is squeezed out 

 at a pressure often amounting to several 

 atmospheres. The maintenance of the pres- 

 sure depends on the living cells of the root, 

 hence it is called " root pressure." Anything 

 which interferes with the life of the root cells 

 causes the pressure to diminish. Thus chil- 

 ling the roots, depriving them of oxygen, or 

 treating them with anaesthetics as well as 

 with other poisons, may temporarily or per- 

 manently abolish root pressure. 



No very satisfactory explanation has been 

 given of Toot pressure, nor indeed of any other 

 form of excretion. We are sure, however, that 

 as our knowledge of the physical and chemical 

 processes of protoplasm increases the diffi- 

 culties will one day vanish. In the mean- 

 time the problems connected with water 

 absorption and its movements within the 

 plant are still in the interesting condition of 

 incomplete solution. We know more or less 

 what happens, but we do not as yet fully 

 understand the how of the happening. 



