l8 CHAP. III. RATE OF EVOLUTION OF OXYGEN 



escapes into the air ; the pressure inside the Bubbler is then 

 restored to the normal. The successive bubbles in the Bubbler 

 thus represent equal volumes of gas. The occurrence of the 

 bubble is independent of any change in the frequency and the 

 size of the bubblets given out at the cut end of the stem. Each 

 bubble given out under light is a definite volume of oxygen, 

 indicating a corresponding amount of photosynthetic action. 

 The rate of photosynthesis and its induced variations may 

 therefore be measured by determining the successive periods 

 of evolution of equal volumes of gas. 



Measurement of Photosynthetic Activity 



Photosynthesis may be measured by the total volume 



of gas, NV, given out in a unit of time, where N represents 



the number of bubbles and V the constant volume of each 



bubble. In order to avoid fractions we take an hour as the 



unit of time. Since the volume of the bubbles is constant, 



the photosynthetic activity is measured by the number of 



bubbles N given out in an hour. We need not, however, 



wait an hour for this determination ; for if t be the interval 



,,,, XT 60 X 60 



in seconds between the successive bubbles, JN , = - — — -> 



represents the activity. 



Determination of the Constant of the Bubbler 



In carrying out investigations on the effect of change 

 in any particular factor, all that is necessary is to determine 

 the induced variation in the period between successive 

 bubbles. For ordinary purposes it is not necessary to know 

 the absolute volume of each bubble, though the constant 

 of each Bubbler can be determined without any difficulty 

 by forcing in a definite volume of air into the plant-vessel 

 and counting the number of bubbles given out by the 

 Bubbler. The sensitiveness of the Bubbler can be varied 

 within wide limits, by an appropriate change in its capacity. 



