58 SAP. 



route through the alburnum or newly formed 

 woody tissue. 



When it reaches the vicinity of the leaves it 

 is attracted into them, and there, having been 

 exposed to light and air, is converted into the 

 secretions peculiar to the species. 



It finally, in its altered state, sinks down the 

 bark, whence it is given off laterally by the 

 medullary rays, and is distributed through the 

 system. 



The cause of the motion of the sap is the at- 

 traction of the leaf-buds and leaves. 



The leaf-buds, called into growth by the 

 combined action of the increasing temperature 

 and light of spring, decompose their carbonic 

 acid, and attract fluid from the tissue imme- 

 diately below them ; the space so caused is 

 filled up by fluid again attracted from below, 

 and thus a motion gradually takes place in the 

 sap from one extremity to the other. 



Consequently the motion of the sap takes 

 place first in the branches and last in the 

 roots. 



For this reason a branch of a plant sub- 

 jected to a high temperature in winter will 

 grow while its stem is exposed to a very low 

 temperature. 



