PHYSIOLOGICAL STUDIES 43 



factor is simplified if all the other factors are kept at some 

 chosen constant level. 



The concentration of carbon dioxide. The problem of keep- 

 ing the concentration of carbon dioxide (or of oxygen) sen- 

 sibly constant during measurements of photosynthesis was 

 solved by the use of a flow technique first used by Kreusler 

 in 1885. In this method a stream of gas is continually passed 

 over the assimilating material, and the amount of photo- 

 synthesis is calculated from the change in carbon dioxide or 

 oxygen concentration of the gas stream. The rate of respira- 

 tion may be similarly determined when the plant is kept in 

 the dark. Blackman and his collaborators used water plants, 

 but instead of a gas stream circulated a flowing water stream 

 containing dissolved carbon dioxide. The eff"ect of varying 

 the concentration of carbon dioxide was determined, using 

 a light intensity which was so great that further increase did 

 not appreciably increase the observed maximum rate of 

 photosynthesis. 



2-3 4-6 6-9 MX 10^ 



Concentration of carbon dioxide 



FIG. 4.1. The rate of photosynthesis at high Hght intensity with 



different concentrations of carbon dioxide for Fontinalis anti- 



jpyretica (•) and Elodea canadensis (x). (Data from Blackman 



and Smith.) Ord. gm. CO^ihrJi^'] cm.^ illuminated surface. 



