132 



Extensions of photosynthetic experimentation 



A third application may be made to transmission experiments. Increasing amounts 

 of cells, say from 25 to 300 mm. 3 , are suspended in 7 c.c. carbonate mixture, and 

 placed in turn into the same vessel and photosynthesis effected by a given beam 

 of light determined. One thereby obtains a curve showing for each amount of cells 

 the relative photosynthesis obtained up to the maximum that may be reached with 

 X = 546 va.ii at 300 mm 3 , cells where light absorption is virtually complete. Then 

 photosynthesis at any given cell concentration, divided by the maximum photo- 

 synthesis, represents the fraction a of the absorbed light, or (1 — a) is the trans- 

 mission. It is necessary in applying this method that the light intensity be so low 

 at all cell concentrations that photosynthesis is proportional to light intensity. 

 Note. When cells suspended in carbonate mixtures are used as described for com- 

 parative actinometric purposes, it is important to know that in carbonate there is 

 an induction period of minutes until the light action is fully developed mano- 

 metrically. The cause of this induction is chemical; it cannot be merely physical 

 equilibration, because for the same shaking rates and same conditions there is no 

 corresponding induction if the cells are suspended in acid culture medium. With 

 cells in carbonate, therefore, transition periods of at least 5 min. have to be consi- 

 dered, whereas with cells in acid culture medium this is not ordinarily necessary 

 if the light intensity is not extreme. 



VIII. Actinometric Determination of Transmission 



Another method of deterrnining light transmission through cell suspensions may 

 be based on the chemical actinometer. A small vessel, such as used for the two- 



Cell-suspension 



Actinometric liquid 



•Light beam 



Fig. 1. Actinometric vessel for measuring light transmission of. Chlorella cell Suspension, contained 

 in inner separable compartment, under conditions of manometric shaking. 



vessel method, contains the cell Suspension. It is surrounded by a larger vessel 

 that contains the actinometric liquid and is connected with a manometer for measur- 

 ing oxygen consumption. Both vessels are quartz vessels, blown by Lempert, 



