34 CHAP. V. PHOTOSYNTHESIS AND INTENSITY OF LIGHT 



being uninterrupted, the successive bubbles then occurred at 

 intervals of 53", 47", 47", 47", physiological adjustment to 

 steady condition being attained in the course of 53 seconds. 

 When the intensity was increased to 1500 lux, the successive 

 bubbles appeared at intervals of 30", 30", 30", 31". Here 

 the readjustment was attained very quickly (fig. 6). It is 

 seen that the period of adjustment does not take more than 

 53 seconds, and an allowance of 1 minute is therefore more 

 than sufficient. As the steady rate subsequently attained 

 is practically uniform, it is only necessary to count the 

 number of bubbles, N, for, say, four minutes. The activity 

 in c.mm. per hour is then : 



N X - -X the constant of the Bubbler. 



Fig. 6. Records showingthe periods^of physiological adjustment 

 under rapid change from 750 to 1000, and then to 1500 lux 

 (7-5, 10 and 15 units) 



The rate became uniform at 1000 lux in the course of 53" ; 

 at 1500 lux the adjustment was practically immediate. 



The period for each individual observation is therefore 

 five minutes only. Eight different observations at different 

 intensities of light are sufficient for the construction of the 

 photosynthetic curve, the total duration of the experiment 

 being thus less than an hour. 



Photosynthetic Curve under Increasing 

 Intensity of Light 



A very important element affecting the photosynthetic 

 activity is the tonic factor depending on the season. I will 

 describe the relative photosynthetic activities of the spring- 

 and winter-specimens obtained from the same pond, the 



