LETTER X. 



" Ubi stimulus, ibi fluxus." 



" It is evident that the force, whatever be its nature, by which 

 the continued movement is kept up, must be developed by the pro- 

 cesses to which that movement is subservient ; in other words, that 

 the changes involved in the acts of nutrition and secretion are the 

 real source of the motive power." Dr Carpenter. 



" If a piece of bladder be tied over the surface of a vine-stump, 

 when the sap is rising rapidly, it soon becomes tightly distended, and 

 will ultimately burst." Professor Henslow. 



January 7, 1855. 



My Dear Sons, 



1. I concluded my last letter by observing, that 

 although (as we there saw) the old stems and roots 

 undergo no organic change of any kind after the year 

 they are formed, — neither an increase in length or 

 thickness, nor a renewal of substance, — the sap, never- 

 theless, moves through them during the next and 

 several subsequent years. And I remarked, that this 

 might be regarded as a proof that they retain their 

 vitality for a much longer period than I allow they do. 



2. It does not therefore follow, however, that the 

 parts in question are alive. To warrant such an infer- 



