84 



LETTERS ON TREES. 



ance, and its ordinary physical qualities, it is really ! 

 dead. 



10. jSTow, how stands the case with Vegetables? It ; 

 is on all hands admitted, that such a process has no 

 place in their economy. No removal of their sub- 

 stance and replacement of this by new tissue ever 

 takes place. The tissues composing them undergo no 

 change of that kind. Once formed, they are never 

 afterwards the seat of any change corresponding to 

 the renewal of substance which is continually going on 

 in the hving tissues of animals.* 



11. And this is true of all vegetables without excep- 

 tion — of the perennial tree equally with the annual 

 plant. They stand in this respect on precisely the 

 same footing. Are we not warranted then in saying, 

 that the absence of all change of that kind in the stems 

 and roots that remain after the fall of the leaves, and 



" The economy of vegetables is fitted for their office of con- 

 stantly converting inorganic into organised matter, by this peculi- 

 arity, that their nutrition is maintained without any such function as 

 the interstitial absorption of animals ; and necessarily involves, 

 during the whole time that any vital actions are going on, continual 

 additions to their substance." — Alison, Outlines of Physiology, 3d 

 Ed. p. 12. 



" In vegetables there is none of that absorption of the different 

 parts which takes place in animals. The matter of which they are 

 composed, being once deposited, is never taken up again ; v/hilst in 

 animals there is a constant process going on, by which the old matter 

 is taken away and the new deposited, and the organs thus renewed." 

 — Dr Ware of Philadelphia, m his Edition of Smellie's Phil of Nat. 

 Hist., Introduction, chap. ii. 



