20 CHAP. III. RATE OF EVOLUTION OF OXYGEN 



volume of the bubblets are entirely removed by the em- 

 ployment of the Bubbler. This consists of a small U-tube 

 closed at one end by an oil-valve. The oxygen evolved 

 by the plant, entering the U-tube, produces an increase of 

 pressure, and a definite volume of gas escapes by the lifting 

 of the oil-valve. 



The successive bubbles given out by the Bubbler in- 

 dicate the photosynthetic production of equal volumes of 

 gas, and are independent of any irregularity in the evolution 



of bubblets at the cut end of the plant. 



The capacity of the Bubbler may be determined by 



forcing in a known volume of gas into the plant-vessel and 



counting the number of bubbles. The sensitiveness of the 



Bubbler may be varied within wide limits from 0-2 to 



5 cubic mm. per bubble. 



The absolute rate of photosynthetic evolution is found 



by multiplying the constant of the Bubbler by the number 



of bubbles per unit time. 



