^^ Dv Harvey's Observations on the 



be quite distinct from them,* being, in fact, the roots and 

 stems of the new plants attached to and growing upon them. 



2. In the second place, no removal, by interstitial absorp- 

 tion, of their substance, and replacement of this by new 

 vegetable tissue, ever takes place. The tissue composing 

 them undergoes no subsequent change of this kind. Once 

 formed, it is never afterwards the seat of any change corre- 

 sponding to the renewal of substance, which is continually 

 going on in the living tissues of animals-t 



And, with reference to this, it may be remarked, that the 

 absence of any such change goes far to shew that, on their 

 growth being completed, the parts in question are really dead. 

 Judging from what obtains in animals, many of which truly 

 live for years, it is not unreasonable to infer that a continual 

 or frequent change of substance is essential to the mainten- 

 ance of the vitality of any structure which really continues 

 for any length of time to be the seat of vital action. The 

 brain, for example, of an animal is possessed of vitality, and 

 performs important vital actions during the whole time that 

 the animal lives ; but the maintenance of its vitality, and the 

 performance of its vital actions, appear to be dependent on, 

 and to involve, a continual change in the substance of the 

 organ. Moreover, the rapidity of that change seems to be 

 exactly proportioned to, and to afford a measure of, the fre- 

 quency and energy wherewith the vital actions of the organ 

 are carried on, — to be more rapid when these are often and 

 actively performed, and less rapid when they are seldom and 



* Lindley, Op. cit, p. 228 and p. 241, et seq. 



t " The economy of vegetables is fitted for their office of constantly convert- 

 ing inorganic into organized matter, by this peculiarity, that their nutrition is 

 maintained without any such function as the interstitial absorption of animals ; 

 and necessarily involves, during the whole time that any living 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 ; whilst in animals there is a constant pro- 

 cess going on, by which the old matter is taken away and the new deposited, 

 and the organs thus renewed."'— Dr Ware, in Smellie's Phil, of Nat. Hist., 

 Introduction, chap. ii. 



