THE LIVING PARTS OF A TREE. 129 



cally impairing the strength of the trunk. Great differences are 

 observ'able as to the time and manner in which the older bark 

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

 ture in each species. Some trees and shrubs have their trunks in- 

 vested \\'ith tlie hber of many years' growth, aUhough only the in- 

 nermost layers are alive ; in others it scales off much earlier. On 

 the stems of the common Honeysuckle, of the Nine-Bark (Spirtea 

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

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

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

 shreds in the latter. 



230. While the newer layers of the Avood 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 bai'k by 

 the medullary rays, many pi-oducts which jirobably 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, Avhich, 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. Tliese are all that is concerned in the 

 life and growth of the tree ; and these are annually reneAved. The 

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

 nication, by means of the newer layers of Avood, Avith the fresh 

 rootlets, Avhich 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, haA'ing been elaborated and converted 

 into organic nourishing matter, is partly expended in the upAvard 

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



