The Controlling Influence of Carbon Dioxide. 1 49 



•conditions. The depressant action of C0 2 up to 50 per cent, varies roughly 

 "with the square root of the concentration. Sufficient work along this line 

 has not yet been done to allow us to draw any definite quantitative con- 

 clusions. The general result, however, seems of importance, since in much 

 experimental work upon respiration there must have been present sufficient 

 CO2 in the respiring tissues to depress the function appreciably and give 

 misleading evaluations of the respiration. They have further a significant 

 bearing upon the hypothesis of Crocker and others already referred to, which 

 imputes dormancy when it occurs in certain moist seeds to lack of oxygen, 

 on the grounds that their respiration is found to be reduced by the presence 

 of the testa, the removal of which causes a rise in the rate of respiration and 

 immediate germination. For, as has been shown, the C0 2 content of such 

 seeds — in which, even in the absence of oxygen, CO2 production would 

 proceed anaerobically — will be raised by the presence of the testa limiting 

 gaseous diffusion, and for this reason alone, even in excess of oxygen, the 

 respiration would be reduced, as was found in the experiments of these 

 workers. 



Section V. — The Influence of Carbon Dioxide upon Respiration (Aerobic and 

 Anaerobic) in Leaves. Floating and Starvation Respiration. 



It has been pointed out that while carbon dioxide regularly retards respira- 

 tion it does not, even in the highest concentration used, completely inhibit it. 

 An interpretation of this result has been suggested above on the basis of 

 Blackman's distinction between " protoplasmic " and " floating " respiration. 



Blackman has shown that in the general respiration of a plant tissue we 

 have at least two quite distinct types of respiration proceeding simul- 

 taneously : in the first place, a floating respiration, involving essentially an 

 oxidation of carbohydrate or fat to carbon dioxide and water ; secondly, a 

 permanent substratum of " protoplasmic " respiration which is the necessary 

 minimum of life. This is revealed, acting alone, in conditions of starvation. 



Seeds cannot be reduced to a condition of starvation. Leaves have been 

 used, therefore, in order to find if any difference exists between the action of 

 CO2 upon floating and carbohydrate respiration respectively. Most leaves 

 can be quickly reduced to a starvation condition in the absence of light, and 

 then show the typical protoplasmic respiration of Blackman. 



The same general method was used here as already described, except that 

 seeds were now replaced by cut leaves in the flasks. Several difficulties 

 were encountered, the disturbance of cutting the leaf, the rapidity of starch 

 disappearance, which renders it difficult to maintain leaves fully fed for the 

 desired period of time, the variable nature of the material with regard to 



