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Mr. F. Kidd. The Controlling Influence of 



obtained. The fact must be borne in mind that we are dealing in this case 

 with a gaseous agent more soluble at low than at high temperatures. This 

 implies that to maintain in solution in the tissues the same concentration of 

 carbon dioxide at a low temperature as at a high temperature necessitates a 

 greater partial pressure of CO2 in the atmosphere at the low temperature. 

 Further work upon this relation of carbon dioxide inhibition in seeds to 

 temperature is needed.* 



Section II. — Relation to Oxygen Pressure of the Inhibitory Effect of Carbon 

 Dioxide on Germination. 



The presence of the testa between the embryo and its gaseous environment, 

 as a membrane, only permeable with some difficulty, will, as has been pointed 

 out, cause (1) a reduction in the amount of oxygen reaching the embryo, 

 and (2) a relative rise in the actual C0 2 pressure in the embryo tissues. 



It has been shown by the removal of the testa that temperature has, 

 nevertheless, a direct effect in determining the inhibitory value of a given 

 pressure of C0 2 . 



The following experiments were made to determine whether a varying 

 oxygen supply might not also influence the inhibitory action of carbon 

 dioxide. A large number of experiments were conducted at the same 

 temperature, but with varying pressures of oxygen and carbon dioxide in the 

 atmospheres used. 



The testas were not removed in these cases. With a given pressure of 

 CO2, the temperature being fixed throughout, no variation in permeability in 

 the testa was looked for. It is possible that an increased oxygen supply may 

 cause a corresponding increase in the actual C0 2 pressure in the embryo 

 tissues. The results show, however, that for the main purpose of the 

 experiments this possibility may be neglected, as it clearly appears that an 

 increase of oxygen supply decreases the inhibitory value of any given 

 pressure of C0 2 , while correspondingly a decrease in oxygen supply intensifies 

 it, so that with small amounts of oxygen very low percentages of C0 2 will 

 induce complete inhibition. 



* In a critical consideration of the actual pressures of C0 2 in the embryo tissues at any 

 temperature we should -have to take into account not only the external partial pressure 

 of C0 2 , but also the rate of C0 2 production in the tissues and the rate of the escape of 

 this C0 2 from the tissues by diffusion. Eoughly, in relation to different temperatures, 

 these two processes tend to cancel one another, and their combined effect to give the 

 same value at all temperatures. In the above experiments no account has, therefore, been 

 taken of any change with temperature of the rate of C0 2 production in the tissues or of 

 the rate of diffusion from the tissues. 



