126 THE STEM. 



older wood-cells are more solidified than the new, the annexed 

 figures are given from corresponding parts of the same trunk of 

 White Oak; Fig. 161, 162, from sap-wood a year old; Fig. 163, 

 164, from a layer of heart- wood twenty-four years old. The walls 

 in both are greatly thickened with lignine ; but in the latter the 

 calibre of a large part of the cells is almost obliterated. In many 

 of the softer woods, there is little solidification in this way, and 

 scarcely any change in color of the heart-wood, except from 

 incipient decay, as in the White Pine, Poplar, Tulip-tree, &c. 



211. Each layer of wood, once formed, remains unaltered in 

 dimensions and position, and unchangeable except from internal 

 deposition and from decay. The heart- wood is no longer in any 

 sense a living part of the tree ; it may perish, as it frequently 

 does, without affecting the life of the tree. 



212. The Bark is much more various in structure and growth 

 than the wood : it is also subject to grave alterations with advan- 

 cing age, on account of its external position, to distention from the 

 constantly increasing diameter of the stem within, and to abrasion 

 and decay from the influence of the elements without. It is never 

 entire, therefore, on the trunks of large trees ; but the dead exte- 

 rior parts, no longer distending with the enlarging wood, are grad- 

 ually fissured and torn, and crack off in layers, or fall away by 

 slow decay. So that the bark of old trunks bears but a small 

 proportion in thickness to the wood, even when it makes an equal 

 annual growth. 



213. The three constituent strata (197-200), for the most part 



readily distinguishable in the 

 bark of young shoots, grow in- 

 dependently ; each by the addi- 

 tion of new cells to its inner 

 face, so long as it grows at all. 

 The green layer does not in- 

 crease at all after the first year 

 or two ; the thickening of the 

 opaque corky layer soon ex- 

 cludes it from the light ; and it 

 gradually perishes, never to be renewed again. The corky layer 

 commonly increases for a few years only, by the formation of new 



FIG. 165. Transverse section of a minute portion of Birch-bark, the corky layer, highly 

 magnified : a, the firm tabular cells : b, the delicate thin- walled cells in alternate layers. 



