THE INNER ORGANIZATION OF TREES. 47 



which correspond to the medullary rays. In this manner 

 a new layer of wood and bark is annually formed. 



The parenchyma cells of the cambium layer retain their 

 vital activity longer than any other cells in the tree ; for, as 

 new cells are developed from this tissue every season, the 

 death and birth of its cells keep with one another equal 

 steps. These cells may continue to be vitally active to some 

 extent : even when the life-processes have ceased through 

 nearly the entire structure of the tree, here life may still 

 linger, these cells being the very last to yield up their 

 vitality. 



The vasiform or duct-cells in early spring, when the as- 

 cent of the sap is most powerful, at first convey it to the 

 leaves in conjunction with the fibre-cells of the wood. But, 

 as the flow of the sap becomes less vigorous, it gradually 

 disappears from the ducts, owing to their deficiency in the 

 requisite amount of capillarity, which thus become filled 

 with air ; the finer capillary tubes of the fibre-cells, however, 

 still induce a continuance of its flow through their cavities. 



As the sap speedily subsides in the ducts, earthy deposits 

 necessarily accumulate sparingly on their parieties or walls, 

 and their tabes remain permanently open. The interior 

 diameter of the duct-tubes is from the first much larger 

 than the bore of the fibre-tubes, hence the sap continues to 

 flow in the latter much longer than in the former : in fact, 

 throughout the season. As every additional deposit of 

 earthy matter on the walls of the fibre-cells necessarily gives 

 them a finer degree of capillarity, the sap continues to flow 

 in them through subsequent seasons, until their tubular 

 character is obliterated altogether. When this is the case, 

 the life of the fibre-cells is terminated, and they exercise a 

 purely mechanical function. 



This solidification of the fibre-cells is usually connected 

 with a change in the color of the wood, more or less marked. 

 If the transverse section of the stem of an oak or cherry- 

 tree be examined, a very perceptible difference will be seen 

 between the circular beds of wood toward the interior of 

 the stem, which are of a deeper color and more compact than 



