RATE UNDER CONSTANT CONDITIONS 



879 



of photos3^lthesis of two twigs of Elodea canadensis, one adapted to strong 

 light and another to weak Hght, in a steadily renewed medium. Both show 

 good constancy for many hours of uninterrupted illumination (after an 

 initial induction period in the shade plant) . Some of Gessner's experiments 

 were extended over 6 days, with rate variations remaining within ±25%. 

 If to these results of Gessner with aquatic plants we add those of War- 

 burg and his successors with unicellular algae, and of Hoover, Brackett, 

 and Johnston (1933), Mitchell (1936) and Bolming (1949) with higher 

 land plants, there appears to be no fundamental difference between plant 



300- 



N 

 O 



n 

 E 



e 



in 



>- 

 <n 

 o 

 h- 

 o 



X 

 Q. 

 U. 



o 



UJ 



a: 



200- 



100 - 



20 



40 60 80 100 120 

 INCIDENT INTENSITY 



140 



Fig. 26.10. Gas exchange of Chlorella cells from cultures of 

 different ages (in air, temperature 29° C.) (after Wassink and 

 Katz 1939). 



classes with respect to their capacity to carry out uniform photosjiithesis 

 for considerable periods of time. 



Experiments with carefully treated plant material show a simple, di- 

 rect and reversible response of the rate of photosynthesis to at least two 

 external factors, light intensity and concentration of carbon dioxide (and 

 within certain narrow limits, also to changes in temperature). 



Obviously the "internal factors" are in no way eliminated, even in such 

 selected material. Their importance is revealed in the induction phenom- 

 ena, in the permanent effects of age and previous treatment and in the 

 fluctuations of the rate, by as much as 10 or 20%, which occur without any 

 apparent external reason. Different plants or algal suspensions of the same 



