86 ACCOUNT OF SOME EXPERIMENTS ON THE 



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 pre- 

 pared some annual shoots of the apple and of the horse-chestnut 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 infu- 

 sion. Making transverse sections of them afterwards, I found that the 

 infusion 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 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-chestnut 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 

 discovered that they entered the inner bark, and had no immediate com- 

 munication with the tubes of the wood. I now endeavoured, 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 alburnum. They commence a few inches below the 

 leaf to which they belong, and they become more numerous as they 

 approach it ; everywhere surrounding the medulla in bundles, as repre- 

 sented in plate i. To these vessels the spiral tubes are everywhere 

 appendages. I do not know that any specific name has been given to 

 these vessels ; and, therefore, as they constitute 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 n. and in. 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 leaf, where I believe they 

 terminate. Plate iv. presents two sections of the leaf-stalk of the 

 horse-chestnut ; the first being taken from the middle of the stalk, and 

 the second from its base. Lying parallel with, and surrounding the above- 



