50 MR. KNIGHTS EXPERIMENTS. 



lower ends of some cuttings of the Apple-tre<f 

 and Horse-chesnut into an infusion of the skins 

 of a very black grape in water, an excellent 

 liquor for the purpose. The result was simi- 

 lar. But Mr. Knight pursued his observations 

 much further than Dr. Darwin had done ; for 

 he traced the coloured liquid even into the 

 leaves, " but it had neither coloured the bark 

 nor the sap between it and the wood ; and the 

 medulla was jnrot affected, or at most was very 

 slightly tinged at its edges." Phil. Trans, 

 for 1801, p. 335. 



The result of all Mr. Knight's experiments 

 and remarks seems to be, that the fluids des- 

 tined to nourish a plant, being absorbed by 

 the root and become sap, are carried up into 

 the leaves by these vessels, called by him cen- 

 tral vessels, from their situation near the pith. 

 A particular set of them, appropriated to each 

 leaf, branches off, a few inches below the leaf 

 to which they belong, from the main channels 

 that pass along the alburnum, and extend 

 from the fibres of the root to the extremity 

 of each annual shoot of the plant. As they 

 approach the leaf to which they are destined, 

 the central vessels become more numerous, 



