128 



THE STEM. 



the bark. The thin-walled cells are those of ordinary parenchyma, 



mingled, at the inner jiart of each stratum, with larger and longer 



ones, marked (on some sides at least) with the thin and reticulated 



spots or punctuations already described (215). These last may be 



- termed the proper cells of the liber, as they are peculiar to this part 



^of the bark, are seldom if ever absent, they contain an abundance of 



mucilage and proteine, and in all probability they take the principal 



part in the descending circulation of the plant, if it may so be called, 



, i. e. in conveying downwards and distributing the rich sap which has 



been elaborated in the fohage. It is evident that the bast-cells, 



which in Linden (Fig. 53) are seen to be almost solid, are not 



adapted to this purpose. 



228. That bast-cells are not an essential part, is further evident 

 -tfrom the fact, that they are altogether wanting in the bark of some 

 ' plants, and are not produced after the first year in many others. The 



latter is the case in Negundo, where abundant bast-cells, like those 

 of Bass-wood, compose the exterior portion of the first year's liber 

 (Fig. 194, 195, J), but none whatever is formed in the subsequent 

 layers. In Beeches and Birches, also, a few bast-cells are produced 

 the first year, but none afterwards. In Maples a few are formed in 

 succeeding years. In the Pear bast-ceUs 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 agree- 

 ing with the proper liber-ceUs in their 

 structure and markings. Although 

 the liber of Birch produces no bast- 

 cells after the first year, it abounds 

 in short cells equally solidified by in- 

 ternal deposition, and of a gritty tex- 

 ture, which might be mistaken for 

 ''"' bast-ceUs on the cross-section (Fig. 



201). A longitudinal section discloses the difference. 



229. The bark on old stems is constantly decaying or falhng away 

 from the surface, without any injury to the tree ; just as the heart- 

 wood may equally decay within without harm, except by mechani- 



FIG. 201. Cross-section of a cluster of solidified and indurated cells from the liber of the 

 White Birch. (After Link.) 



