}2Q PROCESS OF NUTRITION. CHAP. III. 



through which the sap ascends.* That this was 

 the conclusion deduced from Dr. Darwin's expe- 

 riment, there is no doubt. But it does not appear 

 that Mr. Knight has deduced any such conclusion 

 from any experiments of his own ; but certainly 

 not inasmuch as regards the ascent of the sap 

 through the alburnum, in which he denies the ex- 

 istence of the spiral tubes altogether, except as 

 already stated. And although his central tubes, 

 which conduct the sap through the leaf and leaf- 

 stalk of woody plants, as well as through the stem 

 of herbaceous plants, are accompanied with spiral 

 tubes as appendages ; yet these spiral appendages 

 are represented as conveying no fluid. How then 

 Dr. Smith came to regard it as Mr. Knight's opi- 

 nion that the sap ascends through the channel of 

 the spiral tubes, it is not easy to say, except from 

 Mr. Knight's occasional obscurity and perplexity 

 of expression, if not of thought, that so often 

 embarrass the reader, even in his most luminous 

 papers. 



Which the But what is the office of the spiral tubes where 



bable opt they are certainly known to exist ? The opinion of 



Grew and Malpighi, as it is the most ancient, is 



perhaps also tne most correct : at least we may 



fairly regard Crew's reformed opinion in this light; 



namely, that they transmit not only air but sap. 



It is indeed the opinion of Knight that they are 



altogether incapable of transmitting moisture : but 



* Introduction, p. 4-9. 



