1 66 PLANT LIFE. 



quantity of leafy shoots. Even the root and stool of a 

 cut down tree contain much food material ; and the 

 method of regenerating forests known as " coppicifigl' 

 consists in allowing these shoots from the stump to 

 replace the original trees. 



It is clear, therefore, that living timber consists of a 

 whole series of live and dead cells, each of which has its 

 own special part to play, a part which is subordinate to 

 the necessities of the whole community. 



The pressure of the sap within the wood vessels is 

 often very considerable. In some cases it reaches an 

 internal pressure of eight atmospheres, or i 20 lbs. to the 

 square inch. This, however, is by no means excessive 

 compared with that existing in the cambium and 

 medullary ray cells, which is, according to some authori- 

 ties, 10, 15, or even 20 atmospheres, that is from 150- 

 300 lbs. to the square inch.^ This tension exceeds 

 that of the steam in the boilers of an ordinary loco- 

 motive engine. That such a pressure is necessary 

 can be understood if one remembers the height of 

 trees and the amount of energy that must be ex- 

 pended in producing all the annual shoots on the 

 outside of all the branches. Yet these cells in the 

 medullary rays are by no means unhealthy It has been 

 found that medullary rays, knovv^n to be 50 years old 

 from their position, were still alive, so that existence at 

 high pressure is not fatal to them. These tensions can 

 be easily shown on a small scale by taking a fresh 

 Dandelion, or the flower stalk of an Arum Lily, and slitting 

 it lengthwise into four equal parts ; the pieces at once 

 curl backwards, and when put in water the rolling is 

 very marked indeed. The inside part of each piece was 



^ D'Arsonval even states that an internal pressure of 1 60 atmospheres 

 may occur in the cells of some mould fungi. 



