128 THE STEM. 



terposed cambium-layer, it is finally thrown off at the surface, and 

 no more is ever formed. A singular anomaly occurs in a species 

 of Cocculus, where Decaisne has shown that the bast-cells remain 

 connected with the face of the wood, and are covered by its sec- 

 ond layer, so as eventually to be found in the interior of the 

 wood. Laticiferous vessels or canals (63) abound in the newer 

 parts of the liber. 



215. Sometimes thin plates of delicate cells, like those of cork, 

 are formed in the liber alternately with its proper tissue ; these 

 early give way in the external layers, so that the outer part of the 

 liber, as it grows older, scales off in plates year after year ; as is 

 strikingly the case in the Buttonwood or Plane-tree, in the Shell- 

 bark Hickory, in the Larch, Pine, &c. Even the liber of only 

 one or two years old is thus annually detached in membranous 

 layers or fibrous shreds from the stems of the Currant and Honey- 

 suckle, the Spiraea opulifolia or Nine-Bark, and most strikingly in 

 the Grape-vine. In the latter cases, the green and the corky lay- 

 ers are thrown off the first or second year ; in other cases, they 

 disappear at a later period. 



216. Obviously the recent liber and the newer layers of wood, 

 with the interposed cambium-layer, are alone concerned in the 

 life and growth of the tree. The old bark is constantly decaying 

 or falling away from the surface, without any injury to the tree ; 

 while the heart-wood may equally decay within without harm, ex- 

 cept by mechanically impairing the strength of the trunk. 



217. The crude sap rises to the leaves principally through the 

 newer wood (210). The elaborated sap (79) is returned into the 

 newest bark, thence sent to the cambium-layer, and horizontally 

 diffused through the medullary rays (which may be viewed as in- 

 ward extensions of the bark) into the sap-wood and all other liv- 

 ing parts. 



218. The proper juices and peculiar products of plants (80) are 

 accordingly elaborated in the foliage and the bark, especially in 

 the latter. In the bark, therefore, 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 through the medul- 

 lary rays, many products which probably originate in the former 

 are found in the wood. 



219. Exogenous plants almost always develope axillary buds, 

 and produce branches : hence their stems and branches gradually 

 taper upwards, or are conical. 



