904 CONCENTRATION FACTORS CHAP. 27 



and co-workers (1941-1942) for purple bacteria, is characterized by con- 

 tinued increase of the rate of photosynthesis with increasing carbon dioxide 

 concentration until the latter has reached 50, 80, 200 (Singh and Kumar) 

 or even 400 X 10""^ mole/1. (Blackman and Smith) ; the last value corre- 

 sponds to 12% carbon dioxide in the air! Earlier measurements of Kreusler 

 (1885, 1887) and Brown and Escombe (1902), not included in the table, fall 

 into the same category. 



An intermediate group of results, including Harder's (1921) and Smith's 

 (1937, 1938) on higher aquatic plants, and Emerson and Green's (1934) 

 on Gigartina, place carbon dioxide saturation at 20-30 X 10"^ mole/1. CO2. 

 Finally, in several careful investigations, the rise of photosynthesis with 

 increasing carbon dioxide concentration was found to cease as early as be- 

 tween 0.5 and 5 X 10"^ mole/1. CO2 (Hoover and co-workers 1933, and 

 Singh and Lai 1935, higher plants; van der Honert 1930 and van der 

 Paauw 1932, Hormidium; Emerson and Green 1938, Chlorella; and 

 Barker 1935, diatoms). It will be noted that results of this low order of 

 magnitude have been obtained both with land plants in rapidly circulating 

 gas, and with algae in well-stirred acid or alkaline solutions. 



There is little doubt that most if not all results of the first type were due 

 to insufficient circulation and consequent depletion of carbon dioxide in 

 the medium surrounding the plants. It is by no means certain that con- 

 centration gradients in the external medium did not affect significantly 

 also the results in group 2, or even in group 3. And, in addition to gradi- 

 ents in the external medium (which can be reduced by intense circulation), 

 we also must consider those in the stomata, air channels, cell walls and 

 cytoplasm. 



The importance of rapid circulation can be understood by considering 

 that green cells, such as Chlorella, can consume, in strong light, up to one 

 half their own volume in carbon dioxide each minute. In cell suspensions, 

 the volume of the cells usually is from 0.1 to 1% of the volume of the 

 medium. Consequently, the suspension as a whole will use up its own 

 volume in carbon dioxide in from 200 to 2000 min. In other words, the 

 rate of consumption of carbon dioxide will be from 2 X 10 "Ho 2 X 10"* 

 mole CO2/I. min. Consequently, if the concentration of carbon dioxide 

 in the medium is a; X 10"^ ilf, it will be all used up in from 0.05 x to 0.5 

 X minute, or from 3 a; to 30 x second. Consulting Table 8.II, we note that 

 in water equilibrated, at 25° C, with an atmosphere containing 0.01% 

 CO2, X = 0.4; 0.1% CO2, a: = 4.1; 1% CO2, x - 41, and so on. Conse- 

 quently, a cell suspension in an acid medium that contains no significant 

 amounts of HCO3- ions, containing 0.1 to 1% cells by volume, will con- 

 sume all its carbon dioxide in from 1.2 to 12 seconds, if it has been equili- 

 brated with air containing 0.01% CO2; in from 12 to 120 seconds, if the 



