THE STRUCTURE OF TIMBER. 275 



durable wood. It seems to me that these rival theories both 

 assist in explaining the state of things that we find in a wood 

 ring, that of Sachs' being necessary to account for the variations 

 in the shape of the cells, while Hartig's more satisfactorily ex- 

 plains the different degrees of thickness of their walls. 



This cambium mantle, which produces practically all the wood 

 and bark of a tree, could never display so much energy were it 

 not well provided with nourishment, and to a certain extent the 

 amount of wood annually produced — in other words the breadth 

 of the ring — stands in intimate relationship to the nourishment 

 at the disposal of the cambium. Trees growing in good soil and 

 in a good climate grow faster, and their wood possesses broader 

 rings than trees reared under opposite conditions. But it is not 

 the nutritive substances which move up a tree from the roots, but 

 those which come down from the leaves that enable the cambium 

 to do its work, as a simple experiment will easily convince us. 

 If we remove a ring of bark from the bole of a tree some inches 

 in diameter, we shall in many cases find that the tree continues 

 to grow for years apparently as if nothing had happened. If, 

 after a few years, we fell the tree and examine a longitudinal 

 section we shall find that, after ringing, growth was entirely 

 confined to the stem above the point where the band of bark 

 was removed. Now, this simple experiment throws light on a 

 variety of points. It shows, in the first place, that the ascent 

 of the water and mineral matter must be through the wood and 

 not through any of the tissues of the bark, and it demonstrates 

 too that this ascending stream is incapable of supplying the 

 energy to the cambium that results in the formation of wood. 

 And, again, the production of new wood above the ringed por- 

 tion proves that it is something that comes down from the leaves 

 which feeds the cambium, and that this nourishing stream must 

 descend through the bark and is incapable of utilizing the wood 

 for its passage. And such, in point of fact, is the case. No 

 elaborated plant food could be sent down from the leaves if crude 

 food did not flow up from the roots, but it is well to bear in mind 

 that it is only the descending stream that has any power directly 

 to influence the production of wood. This holds true as well of 



