MEASURING PHOTOSYNTHETIC ACTIVITY 233 



creases more slowly than the actual rate of photosynthesis. Thus for 

 example the following analyses are given by Kniep : 



Cabomba caroliniana. 



12 : 30. 20 bubbles in 8.8 seconds, gas occupying 79.2 scale divisions 

 in 2 minutes. The percentages of O, and No are based upon 

 the total volume after deduction of the COg. 

 CO2 2.2 Per cent 



O, 40.2 " " 



N, 59.8 " " 



Photosynthesis was allowed to continue uninterruptedly. 



3:40. 20 bubbles in 11 seconds. Gas occupying 63.1 scale divisions 

 in 2 minutes. 

 CO2 1.9 Per cent 

 O2 35.5 " " 

 N2 64.5 " " 

 4: 15. Further reduction in light intensity. 



4:45. 20 bubbles in 30 seconds. Gas occupying 72.3 scale divisions 

 in 8 minutes. 

 CO2 1.5 Per cent 

 O2 22.8 " " 

 N2 77.2 " " 



The emission of oxygen is made possible because of an increase of 

 pressure thereof in the intercellular spaces, a pressure greater than can 

 be taken care of by the diffusion of the gas into the surrounding water 

 which is already saturated with oxygen. The gas bubbles at first con- 

 tain a per cent of oxygen not much greater than is contained in the 

 intercellular air before photosynthesis started. Nitrogen and carbon diox- 

 ide thereafter are drawn out with the oxygen. If these two gases were 

 not again replaced, the escaping gas would soon be composed of pure 

 oxygen. But both the nitrogen and carbon dioxide are replaced as soon 

 as their percentage in the intercellular spaces drops below their respec- 

 tive tensions in the water surrounding the plant. In the case of the car- 

 bon dioxide, which is very low. the conditions are somewhat dift'erent 

 from nitrogen, both on account of its being used in the photosynthetic 

 process and because of its relatively high solubility in water. When the 

 rate of bubble emission is high, relatively large quantities of nitrogen 

 are carried away in a given time interval. The composition of the emitted 

 gas will thus decrease in nitrogen and increase in oxygen. But the less 

 nitrogen there is in the intercellular spaces, the greater is the tendency 

 of this gas to diffuse in from the surrounding water. A condition is 

 then arrived at in which the relation of No to Oo remains constant at a 

 constant rate of bubble emission. This represents a state where the rate 

 at which diffusion of nitrogen into the intercellular spaces is equal to 



