STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 



85 



healthy condition during the course of the experiment. A consider- 

 able number of experiments was carried out; but as these in each 

 case yielded consistent results, only one, representative of the altered 

 conditions, is described here. 



2. The Apparatus. 



The method employed for determining the rate of photosynthetic 

 activity was that based upon the differential determination of 

 carbon dioxid. A stream of air containing a known concentration 

 of carbon dioxid was passed over the leaf. The rate of carbon- 

 dioxid emission from the leaf when this was in the dark was first 

 determined; this gave the value for the rate of respiratory activity. 

 The leaf was then illuminated and the amount of carbon dioxid in 

 the gas-stream after it had passed over the leaf was determined. 



Figure 20. 

 Arrangement of apparatus for study of the rates of photosynthesis. The entire apparatus 

 is in a constant-temperature room. The leaf-container is in a water-thermostat, which is elec- 

 trically heated, controlled, and stirred. 



Thus the total amount of carbon dioxid fixed by the leaf could be 

 calculated. The rate of the gas-stream was maintained absolutely 

 constant throughout the course of the experiment. The entire 

 apparatus was in a constant-temperature room which was ther- 

 mostatically controlled. The leaf was in a container which was 

 placed in an electrically controlled water-thermostat, usually at 24° ; 

 the room was kept at 20°. 



For these experiments a single excised sunflower or bean leaf was 

 used. The same advantages and disadvantages of working with 

 excised leaves for determining rates of respiration which have already 



