SECT. VIII. DESCENT OF PROPER JUICE. 185 



he calls central tubes through which the coloured 

 infusion ascended, together with their appendages, 

 the spiral tubes ; but also another set of vessels sur- 

 rounding the central tubes, which he distinguishes 

 by the appellation of external tubes, and which ap- 

 peared to be conveying in one direction or other a 

 fluid that was not coloured, but that proved upon 

 further investigation to be the descending proper 

 juice. In tracing them upwards they were found to 

 extend to the summit of the leaf; and in tracing 

 them downwards they were found to extend to the 

 base of the leaf-stalk, and to penetrate even into the 

 inner bark. According to Mr. Knight, then, there 

 are three sets of vessels in leaves, the central tubes, 

 the spiral tubes, and the external tubes. And yet Sir 

 J. E. Smith* represents him as meaning to speak of 

 two sets of vessels only, admitting that his language 

 seems to imply three, but cautioning the reader 

 against falling into the mistake. 



But whatever Mr. Knight's meaning may have 

 been, he positively speaks of and specifies three 

 distinct sets of vessels, the central, the spiral, and 

 the external, as is plain from the circumstance of 

 his trying to ascertain the respective functions of 

 each. The first, he says, conducts the ascending 

 sap from the tubes of the alburnum to the leaf-stalk 

 and leaf; the second does not seem to conduct any 

 fluid ; the third contains the proper juice and con- 



* Introduction, p. 51. 



