ASCENT OF THE SAP IN TREES. 87 



mentioned vessels, appear other vessels, which, I conclude, return the sap 

 to the tree : for when a leaf was cut off which had imbibed a coloured 

 infusion, I found that the native juices of the plant flowed from these 

 vessels, apparently unaltered, as has been remarked by Dr. Darwin. 

 These vessels descend through the inner bark, (as delineated in plates 

 i., ii., and in.) and appear to extend from the extremities of the leaves to 

 the points of the roots. 



The whole of the fluid, which passed from the wood to the leaf, seems 

 to me evidently to be conveyed through a single kind of vessel ; for the 

 spiral tubes will neither carry coloured infusions, nor in the smallest 

 degree retard the withering of the leaf, when the central vessels are 

 divided. But the annexed figures appear to point out at least two kinds 

 of returning vessels. And I think it by no means improbable that two 

 kinds exist with distinct offices ; for there is a new layer of alburnum 

 and a new internal bark to be formed. I have, however, seen it asserted 

 somewhere, in the writings of Linnaeus and other naturalists, that the 

 internal bark is annually converted into alburnum. But this is totally 

 erroneous ; and a vigorous shoot of the apple-tree often presents in its 

 transverse sections, when three or four years old, as many layers in its 

 bark, each of which once formed its internal vascular lining. 



As the bark appeared to me now to receive its nutrition through the 

 leaf, I wished to see what effect would be produced by gradually reducing 

 the quantity of the leaves. I had a luxuriant shoot of the vine in my 

 vinery, exactly in the stage of growth I wanted ; and this branch there- 

 fore was towards its point every day deprived of a small portion of its 

 leaf. The bark, in consequence, became shrivelled and dry; and at 

 length the buds below vegetated, and the point of the shoot died, appa- 

 rently from the want of nourishment. I here observed, as I had frequently 

 done before, that almost the whole action of each leaf lies between itself 

 and the root ; for the branch, in this case, was perfectly well fed below 

 the uppermost unmutiiated leaf, but failed immediately above it. 



Every branch in which I had yet attempted to trace the progress of 

 the sap having contained its medulla uninjured, the action of that sub- 

 stance next engaged my attention, and I made the following experiments 

 on the vine : Having made a passage about half an inch long, and a line 

 wide, into a strong succulent shoot of this plant, I totally extracted its 

 medulla, as far as the orifice I had made would permit me. But the 

 shoot grew nearly as well as the others whose medulla had remained 

 uninjured, and the wound soon healed. Making a similar passage, but 

 of greater length, so that part extended above, and part below, a leaf and 



