242 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



2. Since, in all oxidations effected at the expense of 

 oxygen derived from the air, it is only the amount of the 

 gas in solution which can take part, the necessity for main- 

 taining this quantity constant is obvious. To insure this 

 the following conditions must be observed : (a) The partial 

 pressure of the gaseous oxygen in the reaction flask should 

 be constant throughout any experiment, and the same in 

 any series of experiments. (6) The reaction of the solution 

 should be the same throughout the work, (c) The tem- 

 perature within the flasks should always be the same. 

 (d) The liquid 1 should be kept saturated with oxygen under 

 the above conditions. 



3. The standard conditions should prevail before the 

 reaction begins. This involves the complete saturation 

 of the gases in the apparatus with water vapour. 



4. The change in pressure indicated should be entirely 

 due to oxygen absorption. To secure this, all carbon 

 dioxide produced must be removed as fast as it is formed. 



5. The carbon dioxide formed should be measured, to 

 serve as a check upon the oxygen absorbed. 



6. The rate of oxidation should always be observed. 

 If the reaction is completed in a few hours, or even in a 

 few days, the total quantity of oxygen absorbed should be 

 measured. 



All manometric methods in which the contents of living 

 cells are concerned are of necessity liable to the error that 

 part of the oxygen required may be derived, not from the 

 air at all, but from easily reducible substances in the solu- 

 tion. Supplies from this source do not occasion any change 

 in oxygen pressure, and so escape measurement. 



The possibility of the injury of the enzyme by shaking 

 must also be borne in mind, as this is known to cause some 

 enzymes to become less active. This point was investigated 

 specially. 



