THE NATURE OF PHOTOSYNTHESIS 85 



A practical method of determining the rate of oxygen evolution is the 

 counting of the huhhles which escape from the cut end of a submerged 

 aquatic plant. In this case the gas passes through the gas-filled inter- 

 cellular spaces of the stem thus taking the path of least resistance. The 

 determination of rates of escape of the gases by counting the bubbles 

 is the basis of a method of comparative measurement of the rate of 

 photosynthesis which has been in use for a long time. The description 

 and necessary precautions of this method are given in Chapter 4. The 

 rate at which these bubbles escape varies with the factors which afifect 

 photosynthesis, more particularly the light intensity. It is evident that 

 a direct proportionality between the rate of the escaping bubbles and 

 photosynthesis can exist only if the gas is pure oxygen or contains only 

 negligible traces of other gases, or if the per cent of oxygen in the gas 

 bubbles is constant and independent of their rate of escape. That these 

 conditions ordinarily are not met has been demonstrated by Kniep.^^ He 

 showed that the oxygen-<:ontent of the liberated gas varies as much as 

 30 per cent. 



Below are given Kniep's analyses of the gas escaping from an 

 illuminated plant of Heleodea canadensis. The percentages of oxygen and 

 nitrogen are calculated for the total volume of gas after removal of the 

 carbon dioxide. The rate is figured on the basis of the time required 

 for 20 bubbles to escape from the plant. 



TABLE IS 



Composition of Gas Escaping from an Aquatic Plant with Different Rates of 

 Photosynthesis. (Percentages of O2 and N2 calculated for total 

 volume after removal of CO2.) 



Rate : 20 Bubbles Per Cent Per Cent Per Cent 



in Seconds O2 COs N2 



7.0 49.6 0.9 50.4 



5.2 54.4 0.6 46.6 



14.7 36.6 1.2 63.4 



The oxygen-content of the emitted gas increases with increased rate 

 of bubble formation and vke versa. From this it follows, that with 

 changing light intensity the rate of gas escape increases or decreases 

 more slowly than the true rate of photosynthesis (as determined by the 

 oxygen-content of the gas). A certain amount of the oxygen formed 

 will, of course, escape by diffusion into the surrounding water. This 

 fact, however, hardly affects the foregoing conclusion, because the diffu- 

 sion out will certainly not be less when the intercellular spaces have a 

 high oxygen-content than when they have a low one. 



The explanation of this phenomenon is probably to be sought in the 

 difference of the solubility of the gases rather than in the difference of 

 their diffusion coefficients as is maintained by Kniep. The plant obtains 



"Kniep, Jahrb. miss. Bot., 56, 460 (1915). 



