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 anesthetics as well as 
with other poisons, may temporarily or per- 
manently abolish root pressure. 
No very satisfactory explanation has been 
given of ‘root pressure, nor indeed of any other 
form of excretion. Weare 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. 
