44 CHAP. VI. RELATION BETWEEN LIGHT AND PHOTOSYNTHESIS 



ently of C0 2 -concentration, was demonstrated in experi- 

 ments, made with a different plant, in which the C0 2 -con- 

 centration employed was being only 3 mg. per 100 c.c. : 

 the results under increasing intensities of Pointolite and 

 sunlight are given in Table VIII. 



These results are practically the same as those with a 

 C0 2 -concentration of 8 mg. per 100 c.c. Here also, as in 

 that case, the maximum under strong sunlight, 0-5 S, was 

 nearly double that under 4000 lux of Pointolite. This will 

 be seen clearly by comparison of the photosynthetic curves, 

 S and P, under the two lights (fig. 8). The effect of 0-12 S 

 is found by interpolation to be 257, which is about the same 

 as the effect of 1500 lux. On the previous day, with a 

 different specimen (and probably with some difference in the 

 intensity of sunlight), o • 12 S was found to be physiologically 

 equivalent to 1750 lux. The results on the two days are seen 

 to be of the same order. 



Dependence of maximum response on the intensity of 

 stimulus. — The above results show that the maximum re- 

 sponse is relative, and that it increases with the intensity 

 of stimulus. The absolute limit is only reached when the 

 intensity of stimulus is so great as to cause injury and 

 the resulting physiological depression. This will be inde- 

 pendently demonstrated by results obtained with other 

 modes of response (cf. fig. 10). 



The Hydrilla Plant as a Photometer 



The results already described show that there is a 

 physiological equivalence between sunlight and Pointolite. 

 An interesting question arises whether the photosynthetic 

 activity of Hydrilla could be employed in the measurement of 

 the intensity of the incident sunlight. The following experi- 

 ment on the subject was carried out at the end of March at 

 11 a.m., the sunlight being comparatively bright. The light 

 was reflected into the experimental room by a heliostat and 

 rendered divergent by a double convex lens, the various 



