LKCT. VII.] ANATOMY OF STEMS. 377 



posed, by the remark of Sir J. E. Smith, that 

 " all the moisture in the medulla of a whole 

 " branch is, in some cases, too little to supply 

 " one hour's perspiration of a single leaf * ;" for 

 Mr. Knight could never mean to assert that the 

 fluid in the medulla is intended to supply the 

 entire loss of moisture from perspiration in the 

 leaf, although it might make up the excess. But 

 a more tenable objection to this hypothesis may 

 be deduced from the fact, that the succulent twig 

 perspires nearly as much as the leaves themselves ; 

 droops also, like them, from an excess of perspira- 

 tion in sultry weather ; and is again, in the same 

 manner, revived by the check given to that excess 

 in the night, or by a fresh supply of moisture to 

 the roots. What then, it may be asked, is the use 

 of the pith ? 



In answering the above question, I may re- 

 mark, that as it is more easy to discover the de- 

 fects of an hypothesis, than to suggest one ; and 

 as none is so unlikely to discover the errors of a 

 new opinion, as its proposer, it is very probable 

 that that which I am about to hazard on this sub- 

 ject, may be found as defective as any of those 

 we have criticised. 



* Introd. to phys. and syst. Bot. p. 41. Mr. Keith con- 

 curs in the objections of Sir J. E. Smith, regarding them as 

 " fatal to Mr. Knight's hypothesis.*' Syst. of phys. Bot. vol. 

 ii. p. 21. 



