The Controlling Influence of Carbon Dioxide. 147 



In a consideration of this curve (fig. 3) the fact stands out that the regular 

 retardation of CO2 production does not appear to be towards zero. The 

 simplest interpretation of this, perhaps, is to assume two processes concerned, 

 one of which is regularly retarded to inhibition, the other being differently or 

 not at all affected. This assumption is in line with our present knowledge 

 with regard to " floating " and " protoplasmic " respiration. It is possible 

 that only floating respiration as opposed to protoplasmic respiration is 

 suppressed by carbon dioxide. The results of experiments described in a 

 following section lend support to this view. It is clear in this case that 

 until the magnitude of the protoplasmic respiration is known we cannot 

 determine the base line, and therefore the exact form of the curve 

 expressing the depression of floating respiration considered alone. The 

 statement may be made at present as a useful formula that up to about 50 per 

 cent. CO2 the depression of CO2 production is roughly proportional to the square 

 root of the CO2 concentration. 



Section IV. — The Influence of Carbon Dioxide upon Normal Respiration in the 

 Presence of Oxygen {in Seeds). 



Experiments are next described which were directed to determine whether 

 a similar depression is produced in the case of aerobic respiration under the 

 influence of increased concentrations of carbon dioxide. 



The method adopted in the following experiments was the same as that already 

 described, save that 20 per cent, of oxygen was always present. The amount of 

 respiration was measured both by oxygen intake and CO2 output estimated 

 from analyses, giving the respiratory quotient. The percentage of oxygen in 

 the atmosphere, it will be seen, must decrease throughout the experiments. 

 This has not appeared, however, to distort the results obtained. As has been 

 shown by Stich* and others, the percentage of oxygen must be greatly 

 decreased before the rate of respiration is affected. The experiments were 

 terminated before the percentage of oxygen had fallen to such a degree as to 

 limit respiration in this way. 



The following Tables give the results : — 



* 2 was decreased to 3 per cent, before any effect was observed with seedlings. 



