II 6 PROCESS OF NUTRITION. CHAP. III. 



stem* He then placed them in coloured infusions 

 obtained by macerating the skins of very black 

 grapes in water ; and on examining the transverse 

 section at the end of the experiment, it was found 

 that the infusion had ascended by the wood beyond 

 his incisions, and also into the insulated leaves, but 

 had not coloured the pith nor bark, nor the sap 

 between the bark and wood.* From the above 

 experiment Mr. Knight concludes that the sap as- 

 cends through what are called the common tubes 

 of the wood and alburnum, at least till it reaches 

 the leaves. This is no doubt the fact, though still 

 it is but vaguely stated. M. Mirbel is somewhat 

 more explicit. From a variety of experiments 

 made by immersing branches of the Elder in co- 

 loured infusions, he finds himself entitled to con- 

 clude that the sap ascends through the medium of 

 what he calls the small tubes ; because they only 

 were found to be coloured with the infusion, while 

 the large tubes were not ; and because in the bleed- 

 ing season they are found to contain a limpid juice, 

 which the large tubes do not. 



Who Thus the sap is conveyed to the summit of the 



mTinto alburnum. But Mr. Knight's next object was to 



the leaf, trace the vessels by which it is conveyed into the 



leaf. The Apple-tree and Horse-chesnut were still 



his subjects of experiment. In the former the 



leaves are attached to the plant by three strong 



fibres, or rather bundles of tubes, one in the middle 



* Phil. Trans. 1801. 



