12 CHAP. II. EVOLUTION OF PURE OXYGEN UNDER LIGHT 



condition the quantity of gas forced out must be equal to 

 the quantity which is formed in the plant under light. The 

 quantitative results, to be given later, establishing a strict 

 proportionality between the oxygen and the other photo- 

 synthetic products (Chap. XXIV), afford conclusive proof 

 that photosynthetic activity can be accurately determined 

 from the volume of oxygen given out by the plant. 



A few words may be said here about the loss of oxygen 

 by respiration. I shall give later a detailed account of two 

 independent methods (Chap. XVII) by which it has been 

 ascertained that the loss by respiration is relatively small, 

 being about 4 per cent, at a temperature of 22° C. This 

 small correction has to be added in obtaining the true rate 

 of photosynthesis. Most of the experiments on the effect 

 of variation of a single factor were carried out at a constant 

 temperature ; hence it is only necessary to determine the 

 relative variation of the rate, the small loss due to respiration 

 being the same in all. The loss by respiration has, however, 

 to be taken fully into account in certain other quantitative 

 determinations. 



Having secured the evolution of oxygen gas unmixed 

 with other gases, we have to determine the activity of photo- 

 synthesis by measuring the volume of oxygen given out in 

 a unit of time. For this the counting of bubblets, as will 

 be presently explained, gives very fallacious results. It is 

 therefore necessary to determine the periods not of successive 

 bubblets, but of successive equal volumes of gas. I have 

 used the term ' bubblets ' advisedly, to distinguish them 

 from the ' bubbles ' of equal volume given out by the special 

 Bubbler which will be described in the next chapter. 



Summary 



In the determination of the photosynthetic activity by 

 the volume of oxygen evolved, errors are introduced from 

 the admixture of nitrogen with the oxygen and the partial 

 absorption of the oxygen by the water. 



