286 Dr Alison on the Principle of Vital Affinity. 



ultimate atoms, partly to differences in the minute quantities 

 of inorganic matter, and of other organic compounds not yet 

 mentioned, which enter into their composition, that so many 

 varieties are found, not only in these compounds themselves, 

 but in the qualities which they present as found in different 

 species of plants, and even in different individuals of the same 

 species. In the case of a graft inserted on the stem of an 

 individual, or even of a species, different from that which fur- 

 nishes the shoot, we see that the vital affinities of the par- 

 ticles composing the shoot are capable, not only of extract- 

 ing from the nourishing fluid of the stock all the compounds 

 required for its development, but of imparting to the living 

 textures formed of those compounds which they extract, all 

 those peculiar properties of form, of colour, of smell, of rough- 

 ness, smoothness, &c., by which species, and even individuals 

 of the same species, are characterized. And when we con- 

 sider these facts, I apprehend we must admit that, under 

 the influence of the vital affinities which operate in the cells 

 of living vegetables, much more minute differences of com- 

 pounds are produced, than can be detected and explained by 

 any chemical analysis. 



4. An important question here is. Whether the carbonic 

 acid of the air is decomposed in the leaves where it is chiefly 

 taken in, the amylaceous compounds immediately formed 

 with the help of water, and the oxygen set at liberty, or 

 whether that acid is taken into the juices of the plant, as we 

 now know that oxygen is into the blood at the lungs, and 

 gradually decomposed there, letting its oxygen escape gra- 

 dually, and aiding in the formation of different com[)ounds, 

 besides the varieties of starch ? That the latter is the more 

 probable supposition may be inferred, partly from the analogy 

 of the action at the lungs of animals, but chiefly from the 

 fact, that a separation of oxygen is equally required for the 

 elaboration, which certainly takes place in vegetables, of 

 other compounds, of the varieties of oil, and of protein, which 

 are chiefly deposited in other parts of their structures. 



5. The relations of compounds of this class to sugar, de- 

 mand more special notice. It seems doubtful whether this 

 is ever the first compound formed ; it appears in the sap of 



