128 THE STEM. 



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

 mingled, at the inner part 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 foliage. 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 

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

 plante, 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, h), 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 INIaples a few are formed in 

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

 in very small quantity, compared Avith the parenchymatous part of 

 the liber. In Pines, at least in Wliite 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-cells in tlieir 

 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-cells on the cross-section (Fig. 



201). A longitudinal section discloses the difference. 



229. The bark on old stems is constantly decaying or falling 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 ) 



