284 Dr Alison on the Principle of Vital Affinity, 



Having so far considered the general nature of the chemi- 

 cal changes which are peculiar to living bodies, and the kind 

 of apparatus provided by nature for carrying on these changes, 

 we may next take a more special view of the different che- 

 mical changes themselves, beginning with the greatest and 

 most fundamental of all, the formation of the amylaceous 

 matters by vegetables, acting on the water and carbonic acid 

 with which they are supplied, both in the liquid form by their 

 roots, and in the gaseous form by their leaves, — and the con- 

 sequent evolution of oxygen. In regard to this grand func- 

 tion of living plants, the following facts seem the most im- 

 portant that have been ascertained. 



1. We see this change effected, in the present order of 

 things, only by the agency of one of the amylaceous princi- 

 ples themselves, although the quantity of that pre-existent 

 matter, in the case of the seeds of many vegetables, is exceed- 

 ingly minute. We need not enter on the question how far, 

 besides the pre-existence of matter capable of forming cells, 

 in the textures of the plant itself, previously existing organ- 

 ized matter, in the dead state, is essential as part of the nu- 

 triment of vegetables, — farther than to observe, that, as the 

 seed of every plant contains a store of organic compounds 

 already formed, there is certainly a strong presumption that 

 a certain quantity of such compounds, formed by previous 

 living processes, is highly useful, if not necessary, to the 

 nourishment of vegetables, as well as animals. This, how- 

 ever, appears most important in the early period of the exist- 

 ence of plants, when their power of decomposing the carbo- 

 nic acid has not yet attained its full intensity. The evidence 

 of the greater part of the nourishment of vegetables being 

 from carbonic acid, water, and ammonia, applied to their 

 leaves, or absorbed by their roots, is quite conclusive ; and 

 when we consider that vegetables preceded the appearance 

 of animals on earth, that the first vegetables (as is well ob- 

 served by Liebig) were of the kind which depend least on 

 their roots and most on their leaves for subsistence, and that 

 the kind of animals which first inhabited the earth, were 

 those which consumed the smallest quantity of oxygen, and 

 can live, therefore, in air highly charged with carbonic acid, 



