349 
the Ascent of the Sap in Trees. 
young leaves are excessively tender, and they perspire much, 
and cannot, like animals, fly to the shade and the brook. In 
the mature annual branches, and in those of more than one year 
old, the medulla is dry, and, I think, it is evidently lifeless; but 
the space it occupies is never filled with wood, as some natu- 
ralists have imagined. 
The heart or coloured wood, distinguished from the alburnum, 
seems to execute an office somewhat similar to the bone in the 
animal economy. The rigid texture of the vegetable fibre, has 
rendered this substance unnecessary in the young subject ; but, 
as the powers of destruction, 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 vege- 
tation appeared in the succeeding 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 without 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 
