347 
the Ascent of the Sap in Trees. 
only that the plant possesses an intrinsic power of moving, but 
that it also possesses some vehicle of irritation ; and, without this, 
it will I think be difficult to explain how the heat applied to the 
branch of the vine, within the stove, can put the sap in the roots 
and external stem into motion. It may be objected, that these 
are always ready, when the branch calls for nourishment ; and 
that they are no way affected by the internal heat. But this I 
cannot admit to be the case ; because I have found that the stem 
suddenly becomes extremely susceptible of injury from cold, as 
soon as the branch begins to vegetate; and that its whole powers 
will be paralyzed for some days, by exposure for a few hours to 
a freezing temperature. 
I have had very frequent opportunities of observing a remark- 
able power in trees, of transferring their sap from one tube to 
another ; for I have often intersected, in the trunk, every tube 
which led to a lateral branch, and still this branch has derived 
a considerable portion of nourishment from the trunk. And, if 
the tubes of an annual shoot of the oak be traced downwards 
in the autumn, they will be found to pass along the layer of 
wood of the preceding summer, without any apparent commu- 
nication between them and the tubes of any former year’s 
growth. Yet the sap rises through the whole of the white 
wood ; and it must be transferred from the internal tubes to 
those near the surface, which alone appear to communicate 
with the central tubes of the young shoots and leaves. Indeed 
we have frequent evidence that trees possess this power; for 
we see that the whole sap of the stock is carried into an inserted 
bud or graft. 
I at one time suspected, that a small portion of sap, in its 
Yy 2 
