LRCT. VII.] ANATOMY OF STEMS. 329 



supposed that the liber annually changes, by hard- 

 ening, into the alburnum or young wood, an 

 opinion which is still maintained by some of the 

 ablest phytologists * ; but which I shall afterwards 

 prove to you is founded upon mistaken principles. 

 It is through the liber, however, that the matter 

 in which the new wood is formed, which annually 

 augments the diameter of the trunk and branches, 

 is secreted; and hence the importance of this 

 portion of the bark. 



Such is the structure of the bark of the stems 

 of woody dicotyledons ; and that of the root does 

 not materially differ from it; any difference de- 

 pending, perhaps, altogether on the medium in 

 which these two parts are situated. In the bark 

 the secreted juices of plants, and consequently 

 their medicinal qualities, are chiefly deposited; 

 but the consideration of the functions of this part 

 and its properties must be deferred, until the 

 whole of the structure of the stem has been de- 

 scribed. 



b. The WOOD. Pursuing our investigation in 

 the young stem of the Horse Chesnut ; when the 

 whole of the bark is removed, we find, imme- 

 diately under and slightly adhering to it, a 

 firmer and more compact substance, which, both 

 in a longitudinal and a transverse section, ap- 



* " Le liber endurcie, de verdatre qu'il etait, devient blanch- 

 " atre, et prend le nom d'aubier." MIRBEL, Element de Phys. 

 vcg. t. i. p. 106. 



