The Controlling Influence of Carbon Dioxide. 



137 



importance both in physiology and medicine. In studying the phenomenon 

 of carbon dioxide inhibition, as exhibited by dormant seeds, we seem to be 

 in the presence of that fundamental question in physiology, the question of 

 the cause of growth. 



Our problem, then, in these researches is clearly to determine what 

 physiological changes accompany the inhibition of growth under the influence 

 of carbon dioxide. Two hypotheses may be put forward. The inhibitory 

 action of carbon dioxide may be the indirect result of a carbon dioxide effect 

 upon the physical state of the protoplasm, for example, a change in its 

 colloidal structure, its water-holding capacity, its permeability. On the 

 other hand it may be of the nature of a direct chemical action producing 

 a change in some phase of metabolism. "Work to determine how far effects 

 of the first class actually exist is still in progress. In the present paper one 

 striking effect of the second class will be dealt with, namely, the retarding 

 effect of carbon dioxide upon respiration. That oxidation and respiration 

 are necessary for growth by cell division in plants is known. With regard 

 to the animal ovum Loeb finds that oxidations are a controlling factor in 

 the stimulus to growth, whether by natural or artificial fertilisation. The 

 breaking up of the dormant condition of the egg and the beginning of cell 

 division is accompanied by an essential increase in respiration. Should this 

 increase be prevented either by a small dose of potassium cyanide or by the 

 absence of oxygen, fertilisation, whether artificial or natural, has no effect, 

 and growth does not ensue. 



It is a simple assumption at the outset that the presence of carbon dioxide 

 may interfere with the forward movement of the respiratory process 

 producing C0 2 . From much recent work it is clear, however, that respiration 

 is by no means a simple process, but that it includes a complicated chain of 

 reactions. A short reference therefore of some results already obtained in 

 work upon plant respiration and of the conclusions based upon them will 

 be useful before proceeding to consider the results described in the following 

 pages. 



Section I. — The Complexity of the Hespiratory Processes m Plants. 



The discovery, first made by Kollo in 1798 but not widely accepted for 

 many years, that plant tissues will produce CO2 in the absence of oxygen as a 

 vital process, opened up a promising line of enquiry under the influence of a 

 leading idea, namely, that this anaerobic C0 2 production was intimately 

 associated with processes of normal respiration. Pfeffer believed that this 

 anaerobic CO2 production, which be termed intramolecular respiration, was a 

 normal process in plant respiration, being genetically connected with sequent 



