118 PHYSIOLOGY OF VEGETABLES. 



the wood, and the liber, or inner layer of bark, is on 

 the other, annexed to the layers formed in the preced- 

 ing seasons ; and neither have any share in the pro- 

 cess of vegetation for the year ensuing. Still, as they 

 continue for a long time to be living bodies, and help 

 to perfect, if not to form, secretions, they must receive 

 some portion of nourishment from those more active 

 parts, which have taken up their late functions." 



The vital functions of these systems diminish with 

 their age. Thus, on one hand, the layers of bark, 

 with their systems of vessels, are pushed outward by 

 the new layers within, gradually diminishing in vitali- 

 ty, until they finally become quite dead, split and scale 

 off, as we see them in old trees. On the other hand, the 

 layers of wood are successively covered by the new 

 layers of alburnum, gradually losing their vital powers, 

 until at last they become quite destitute of vitality, and 

 then constitute the heart-wood. The heart-wood some- 

 times goes a step farther, decomposes and leaves the 

 tree hollow. 



7. Facts which tend to prove the preceding theory of 

 vegetation. 



1. The old physiologists had an idea, that the ves- 

 sels of the alburnum contained nothing but air. Dr. 

 Darwin and Mr. Knight, by placing the cut ends of the 

 twigs of various plants in coloured fluids, succeeded in 

 making these vessels absorb them in such a degree, 

 that they were enabled to trace them in the vessels 

 quite into the leaves 3 thus proving that they carried 

 fluids. 



2. When a tree is wounded, it heals from the upper 

 edge of the wound downward, while below the wound, 

 the tree dies in a sort of triangular space, until by the 

 anastomosis or lateral communication of the vessels of 

 the bark with each other, the parts are supplied with 

 what is necessary to their vitality. 



