ASCENT OF THE SAP IN TREES. 95 



both from winds and gravity, increase in a compound ratio with the 

 growth of the tree, some stronger substance than the alburnum may 

 be supposed to be wanting to support the additional weight of fruit 

 and seeds. In the root this substance cannot be wanted, and there 

 it is not found ; but if the mould be taken away from the roots round 

 the trunk, so that they are exposed to the air, and made to support the 

 weight of the tree, they become as full of coloured wood as the trunk 

 and large branches. Having cut through the alburnum of an oak 

 all round, not the slightest mark of vegetation appeared in the succeed- 

 ing spring ; and, having been unable to impel either air or water through 

 its tubes, I conclude that the coloured wood of the oak is without 

 circulation: I see very little reason, however, to admit that it is with- 

 out life in a young or middle-aged tree. The new matter which enters 

 into the internal part of the alburnum, on its conversion into heart 

 or coloured wood, seems to be of a nature different from the alburnum 

 itself; for it not only changes its colour, which is nearly white, to a dark 

 brown, but it renders it at least ten times more durable. Some portion 

 of this increased durability may, perhaps, be attributable to the superior 

 solidity of the coloured wood; but a little attention to the common 

 kinds of English timber, (omitting the resinous tribe,) will convince 

 us that these qualities, though frequently found together, have very little 

 connexion with each other. If a number of oaks of the same age be 

 examined, it will be found that, in some individuals, the alburnum 

 consists of a greater number of annual layers than in others, and that 

 the coloured wood will have approached nearer the bark on one side 

 than on the other, in the same tree ; the termination also of the coloured 

 wood, and the commencement of the alburnum, are often found in the 

 middle of an annual layer of wood ; and each substance, at the points 

 of contact, possesses all its characteristic properties. The alburnum, 

 I think, evidently extends itself laterally, without any radicles descend- 

 ing from the leaves or buds above. I have often procured a union, 

 by grafting, between trees of different kinds, and have sometimes found 

 mere varieties of the same species of tree, whose wood was sufficiently 

 distinguishable, in every stage of future growth, to allow me readily 

 to trace their line of union. The wood of the graft does not at all 

 descend below its original place of junction with that of the stock ; 

 which, immediately below, wholly retains its native character; and, 

 in the part where both are spliced together, each constantly extends 

 itself in the direction of the divergent laminae of its silver grain. The 

 heart wood also appears to increase by lateral extension ; but I am 



