ABSORPTION OF THE EVOLVED OXYGEN BY WATER II 



a production of vacuum causes injury to the plant. The 

 water is next charged with the C0 2 , the best proportion of 

 which I find to be from 7 to 9 mg. per 100 c.c. of water. A 

 small quantity of pure oxygen is also bubbled through the 

 water, as its presence is necessary for the normal life-activity 

 of the plant. Before collecting the gas for photosynthetic 

 measurements, it is advisable to expose the plant for a short 

 time for the disappearance of the last trace of nitrogen. 

 Analysis of the gas subsequently evolved under light shows 

 that it is pure oxygen. • 



The special plant-vessel — -to be described in the next 

 chapter — is filled up with the nitrogen-free water charged 

 with a proper quantity of carbon dioxide and oxygen. A 

 special valve prevents the introduction of nitrogen from the 

 outside air. 



Absorption of the Evolved Oxygen by Water 



There is a possibility of loss of oxygen through absorp- 

 tion before it is given out of the water in the form of bubbles. 

 As already stated, the water contains a small proportion of 

 oxygen ; if the channel through which the oxygen arises 

 were saturated with oxygen there could then be no further 

 absorption. The absorptive power of water for different 

 gases has been carefully determined ; in the case of oxygen, 

 100 c.c. of water absorb about 2-8 c.c. at 22 C. The cut 

 end of the plant is fixed in the plant-vessel (see fig. 2) so that 

 it is about 2 mm. below the surface of the water ; hence 

 a continuous stream of oxygen rises through a definite and 

 short length of water which must be soon saturated with it, 

 and the loss of oxygen by absorption therefore becomes 

 quite negligible. 



There may be some misgiving that the intercellular 

 spaces might divert a proportion of the oxygen produced. 

 It is obvious that this cannot affect the rate of evolution ; 

 for under the action of light the intercellular spaces become 

 completely filled, and on the attainment of the steady 



