MR. knight's experiments. 39^ 



round the pith, surrounded by an external circle of 

 vessels containing the white milky juice, or secreted 

 iiuid, so remarkable in the Fig-tree. Mr. Knight, in a 

 similar manner, inserted the lower ends of some cut- 

 tings of the Apple-tree and Horse-chesnut intoan in- 

 fusion of the skins of a very black grape in water, an 

 excellent liquor for tlie purpose. The result was simi- 

 lar. But I\Ir. Knight pursued his observations much 

 further than Dr. Darwin had done; for he traced the 

 coloured liquid even into the leaves, " but it had nei- 

 ther coloured the bark nor the sap between it and the 

 wood ; and the vicduUa was not affected, or at most 

 was very slightly tinged at its edges." Phil. Trails^ 

 for ]SQ\,p, 335. 



The result of all Mr. Knight's experiments and re- 

 marks seems to be, that the fluids destined to nourish 

 a plant, being absorbed by the root and become sap, 

 are carried up into the leaves by these vessels, called 

 by him ceiitral 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 approaii the leaf to which they are destined, 

 the central vessels become more numerous, or subdi- 

 vided. '' To these vessels," says Mr. Knight, " the 

 spiral tubes are every where appendages." p, 336. By 

 this expression, and by a passage in the following 



