82 MORPHOLOGY OF STEMS. 



Linden, and of other plants with a similar fibrous bark, may be 

 taken as best representing the liber. Here it consists of alter- 

 nate strata of fibrous bast, and of the peculiar liber-cells called 

 sieve-cells, in which nourishing matter is especially contained 

 and elaborated. While the latter, or their equivalents, occur 

 and play an important part in all inner bark, the bast-cells are 

 altogether wanting in the bark of some plants, and are not pro- 

 duced after the first 3'ear in many others. The latter is the case 

 in Negundo, where abundant bast-cells, like those of Basswood, 

 compose the exterior portion of the first year's liber, but none 

 whatever are formed in the subsequent layers. In Beeches and 

 Birches, also, a few bast-cells are produced the first }'ear, but 

 none afterwards. In Maples, a few are formed in succeeding 

 years. In the Pear, bast-cells are annually formed, but in very 

 small quantity, compared with the parenchymatous part of the 

 liber. In Pines, at least in White Pines, the bark is nearly as 

 homogeneous as the wood, the whole liber, except what answers 

 to the medullary rays, consisting of one kind of cells, resembling 

 those of bast or of wood in form, but agreeing with the proper 

 liber-cells in their structure and markings. 



151. The bark on old stems is constantly decaying or falling 

 away from the surface, without airy injui\y to the tree ; just as 

 the heart-wood within may equally decay without harm, except 

 by mechanically impairing the strength of the trunk. There are 

 great differences as to the time and manner in which the older 

 bark of different shrubs and trees is thrown off. Some have 

 their trunks invested with the liber of man}^ years' growth, 

 although only the innermost layers are alive ; in others, it scales 

 off much earlier. On the stems of the common Hone} T suckle, of 

 the Nine-Bark (Spiraea opulifolia) , and of Grape-vines (except 

 Vitis vulpina) , 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. 



152. While the newer laj'ers of the wood abound in crude sap. 

 which the}' convey to the leaves, those of the inner bark abound 

 in elaborated sap, which the}' receive from the leaves and convey 

 to the cambium-la3*er or zone of growth. The proper juices and 

 peculiar products of plants 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. 



