THE LIVING PARTS OP A TREE. 129 



eally impairing the strengtiT^ the tmpk. Great differences are 

 observable as to the tiijfie and manner in which the older bai-k 

 of different shrubs and irees is thrown off, according to the struc- 

 ture in each species. S|me trees and shrubs have their trunks in- 

 vested with the liber of j many yeara' growth, although only the in- 

 nermost layers are ahve j in others it scales off much earlier. On 

 the stems of the commc n Honeysuckle, of the Nine-Bark (Spirsea 

 opulifolia), and of Graps-vines (except of our Muscadine Grape), 

 the liber lives only one season, and ig detached the following year, 

 hanging loose in papery layers in the forqier species, and in fibrous 

 shreds in the latter. | 



230. Wliile the new«- la^^ers of llie wAod abound in crude sap, 

 which they convey to the leaves (224), those of the inner bark 

 abound in elaborated sap (79, 227), which they receive from the 

 leaves and convey to the cambium-layer or zone of growth. The/ 

 proper juices and peculiar products of plants (88) are accordingly, 

 found in the foliage and the bark, especially in the latter. In the 

 bark, therefore, (either of the stem or of the root,) medicinal and 

 other principles are usually to be sought, rather than in the wood. 

 Nevertheless, as the wood is kept in connection with the bark by 

 the medullary rays, many products which probably originate in the 

 former are deposited in the wood. 



231. The Living Parts of a Tree or Shrub, of the Exogenous kind, are 



obviously only these : — 1st. The summit of the stem and branches, 

 with- the buds which continue them upwards and annually develop 

 the foliage. 2d. The fresh tips of the roots and rootlets annually 

 developed at the opposite extremity. 3d. The newest strata of wood 

 and bark, and especially the interposed cambium-layer, which, annu- 

 ally renewed, maintain a living communication between the rootlets 

 on the one hand and the buds and foliage on the other, however dis- 

 tant they at length may be. These are all that is concerned in the 

 life and gi-owth of the tree ; and these are annually renewed. The 

 branches of each year's growth are, therefore, kept in fresh commu- 

 nication, by means of the newer layers of wood, with the fresh 

 rootlets, which are alone active in absorbing the crude food of the 

 plant from the soil. The fluid they absorb is thus conveyed directly 

 to the branches of the season, which alone develop leaves to digest 

 it. And the sap they receive, having been elaborated and converted 

 into organic nourishing matter, is partly expended in the upward 

 growth of new branches, and partly in the formation of a new layer 



