336 Mr. Knight’s Experiments on 
the medulla was not affected, or at most was very slightly tinged 
at its edges. 
My attention was now turned to the leaves : these, in the apple- 
tree, are attached to the wood by three strong fibres or tubes, 
(or rather bundles of tubes,) one of which enters the middle 
of the leaf-stalk, and the others are on each side of it. In the 
horse-chesnut, there are seven or eight bundles of a similar kind 
of tubes in each leaf : through these the infusion had passed, and 
had communicated its colour to them, through almost the whole 
length of each leaf-stalk. Examining these tubes more minutely, 
I found that they were surrounded with others, which were free 
from colour, and appeared to be conveying, in one direction or 
the other, a different fluid. On tracing these downwards, I dis- 
covered that they entered the inner bark, and had no immediate 
communication with the tubes of the wood. I now endea- 
voured, in the same manner, to trace back those vessels which 
had carried the infusions into the leaves; and I readily found 
them to be perfectly distinct from the common tubes of the al- 
burnum. They commence a few inches below the leaf to which 
they belong ; and they become more numerous as they approach 
it ; every where surrounding the medulla in bundles, as repre- 
sented in Plate XXIV. To these vessels, the spiral tubes 
are every where appendages. I do not know that any specific 
name has been given to these vessels ; and therefore, as they con- 
stitute a centre, round which the future alburnum is formed in 
the succulent annual shoot, I will call them the central vessels, 
to distinguish them from the spiral tubes and the common tubes 
of the wood. In Plates XXV. and XXVI. the direction of 
these vessels, with the spiral tubes, in their passage from the 
sides of the medulla to the leaf-stalk, is delineated in a transverse 
and longitudinal section: they extend to the extremities of the 
