335 
the Ascent of the Sap in Trees. 
below the incisions scarcely grew at all, till a new communi- 
cation with the leaves through the bark was obtained, by means 
of a lateral shoot, below the incisions. It now appeared to me 
to be probable, that the current of sap which adds the annual 
layer of wood to the stem, must descend through the bark, from 
the young branches and leaves ; and to these my attention was 
in consequence directed. 
Towards the end of the summer, when some young luxuriant 
shoots of my apple-trees had attained a proper degree of firm- 
ness,. I made four circular incisions through the bark of each, as 
in the preceding instances ; and I removed the bark in two 
places, leaving a leaf between the places where the bark was 
taken off. Examining them frequently during the autumn, I 
found that the insulated leaf acted just as the lateral branch had 
done ; the part of the bark and stem between it and the lower 
incision being apparently as well fed as any other part of the 
tree ; and it grew as much. Making similar incisions on other 
branches of the same age, I left similar portions of insulated 
bark, without a leaf between the incisions ; but in those no 
apparent increase in the size of the wood was discoverable. 
I was still unacquainted with the channel through which the 
sap was conveyed into the leaf ; and therefore, having obtained 
a deeply tinged infusion, by macerating the skins of a very black 
grape in water, I prepared some annual shoots of the apple, 
and of the horse-chesnut, in the manner above mentioned; then, 
cutting them off a few inches below the incisions of the bark, I 
placed them for some hours in the coloured infusion. Making 
transverse sections of them afterwards, I found that the in- 
fusion had passed up the pores of the wood, beyond both my 
incisions, and into the insulated leaves ; but it had neither 
coloured the bark, nor the sap between it and the wood ; and 
