Mar. 1901.] BOTANICAL SOCIETY OF EDINBURGH 57 



any appreciable quantity of either oxygen or carbon dioxide 

 from a piece of stem of Opuntia which had absorbed about 

 80 c.c. of oxygen from the air contained in a receiver" 

 (Vines, loc. cit.). It would thus appear that the absorbed 

 oxygen was chemically fixed in the plant, and not in a 

 loose chemical combination, or in the form of intramolecular 

 oxygen. 



How is the evolution of carbon dioxide to be explained ? 

 The view of botanists is given by Vines (" Physiology of 

 Plants," p. 200): "The presence of free oxygen promotes 

 certain processes of destructive metabolism, of which the 

 exhalation of carbon dioxide is an expression." This 

 coincides with the views of our author, which he expresses 

 as follows : — " The oxidation brings about atomic diplace- 

 ments in the biomolecule, which, consequently, becomes 

 liable to undergo new transformations by other reactions. 

 Dnring the accomplishment of these reactions atomic 

 displacement occurs, and it is possible that two atoms of 

 oxygen may be linked to one of carbon, thus saturating its 

 affinities, when it would be unlinked from the biomolecule 

 and exhaled as carbon dioxide." 



" The oxygen which is fixed to the biomolecule and brings 

 about these chemical changes, in the course of which there 

 is exhalation of carbon dioxide, plays the role of a true 

 chemical stimulus. The assimilatory reactions which 

 follow oxidation are evidently provoked by the action of 

 the oxygen which produces a whole series of chemical 

 changes, of which the point of departure is oxidation, 

 and the final effect the disengagement of carbon dioxide." 

 The above affords us an easy explanation of the pheno- 

 menonon of " intramolecular respiration." " Admitting 

 that oxvgen is fixed to the biomolecule, and remains there 

 for some time; admitting that the disengagement of carbon 

 dioxide is not a direct consequence of oxidation but of 

 another assimilatory reaction, — it is obvious that the dis- 

 encjasement of carbon dioxide may go on for some time 

 after the oxygen has ceased to act." 



Eespiration is characteristic of destructive metabolism. 

 If atoms of oxygen and carbon, as carbon dioxide, are being 

 continually given off from the biomolecule, how is the 

 destruction of the latter averted, and how does the 



