SECT. II. ASCENT OF THE SAP. 117 



of the leaf-stalk, and one on each side. In the 

 latter they are attached by means of several such 

 bundles. Now the coloured fluid was found in each 

 case to have passed through the centre of the se- 

 veral bundles, and through the centre only, tinging 

 the tubes throughout almost the whole length of 

 the leaf-stalk. In tracing their direction from the 

 leaf-stalk upwards, they were found to extend to 

 the extremity of the leaves ; and in tracing their 

 direction from the leaf-stalk downwards, they were 

 found to penetrate the bark and alburnum, the 

 tubes of which they join, descending obliquely 

 till they reach the pith which they surround.* 

 From their position Mr. Knight calls them central 

 tubes, thus distinguishing them from the common 

 tubes of the wood and alburnum, and from the 

 spiral tubes with which they were every were ac- 

 companied as appendages ; as well as from a set 

 of other tubes which surrounded them, but were 

 not coloured, and which he designates by the ap- 

 pellation of external tubes. 



The experiment was now transferred to the Flower 

 flower-stalk and fruit-stalk, which was done by 

 placing branches of the Apple, Pear, and Vine, 

 furnished with flowers not yet expanded, in a de- 

 coction of Logwood. The central vessels were 

 rendered apparent as in the leaf-stalk. When the 

 fruit of the two former was fully formed, the ex- 

 periment was then made upon the fruit-stalk, in 

 * Phil. Trans. 1805. 



