378 CONSERVATIVE ORGANS. [_LECT. VI t. 



If we examine a tree in the first stage of its 

 growth, or a fresh shoot, we shall be satisfied 

 that the alburnum and bark form so very small 

 a portion of their diameter, that, were there not 

 some means of extending the surface of these 

 parts, there would be neither sufficient space for 

 the formation and arrangement of the vessels 

 essential to their future existence ; nor a suf- 

 ficient support to maintain the plant in its erect 

 position, or the shoot in its projection. To supply 

 these purposes the pith is given to young plants 

 and shoots ; for, its cells being filled with an 

 aqueous incompressible fluid, both extension and 

 support are thus afforded by it, as long as these 

 are required, or until the first ligneous zone is fit 

 to allow the next zone of alburnum to be formed 

 on it, and consistent enough to afford sufficient 

 stability to the plant. When this takes place, the 

 moisture passes from one cell to another until it is 

 altogether carried off, leaving the pith in the dry 

 spongy state in which it is found in the older stems 

 and branches. The comparative diameter of these 

 stems and branches depends equally on the evo- 

 lution of the pith ; for, in moist seasons, when these 

 push out thicker and more vigorous than usual, 

 we find their mass consist chiefly of pith ; and the 

 ligneous circle, although much widened, yet, not 

 containing a proportionate increase of substance. 

 In such shoots also we have another proof that 

 it is the turgidity of the medullary cells with 



