262 BIOLOGICAL PHYSICS 



channels, the medullary rays, and it may be the permanent 

 ligneous structure, the earthy salts held in solution, or 

 suspension, by the indispensable element of water. On 

 the passage of these two fluids through, the cambium, the 

 branch terminals and leaves, they become intermixed, 

 when the phenomena of lignification begin, and result, in 

 temporary, or permanent, additions, to the structure of 

 the plant, in the form of leaves, and increases in the 

 length, and girth, of the plant. 



Towards the accomplishment of the process, the pro- 

 vision of pith channels and medullary rays, to a greater, 

 or lesser, extent, is universal, throughout the whole range 

 of, stem, and branches, while the whole external, or 

 peripheral, aspect of the plant, is the scene of one 

 continuous circulation, of sap proper, which, meeting with 

 earthy material, through the outer layer of the permanent 

 ligneous tissue, is the means of laying down, another, and 

 new, layer, and, so, of adding, year after year, to the 

 lateral extension of the plant, or tree, and, also, on the 

 admixture at the extremities of the branches, a more, or 

 less, large addition is made to their permanent length, 

 while a very large amount of highly developed, functional, 

 material, is shed, in the form of leaves. 



