CHAPTER VI 



RELATION BETWEEN THE QUANTITY OF LIGHT AND THE 

 AMOUNT OF PHOTOSYNTHESIS 



Comparison of photosynthetic effectiveness of sunlight and Pointolite — 

 The limiting maximum, relative and not absolute — Dependence of 

 the maximum on intensity of stimulus — Physiological equivalence of 

 Pointolite and sunlight — The Hydrilla plant as a photometer — Effect 

 of duration, intensity and directive angle of light on activity — 

 Relation between the quantity of light and activity. 



I will in the present chapter deal with the important 

 question of the relation between the quantity of light and 

 the activity of photosynthesis as estimated by the volume of 

 oxygen evolved. Befoie doing this it is necessary to equate 

 the relative effects induced by different sources of light, 

 Pointolite and sunlight for example. Is a given intensity 

 of sunlight photosynthetically equivalent to a particular 

 intensity (in lux) produced by Pointolite ? The Pointolite 

 is brilliantly white and its spectral components are not very 

 different from those of sunlight ; the latter contains perhaps 

 a larger proportion of the more refrangible rays. But this 

 difference would not produce any great variation, since the 

 more refrangible rays are found to be relatively ineffective 

 in photosynthesis. 



Comparison of Photosynthetic Effectiveness 

 of Sunlight and Pointolite 



The experiment was carried out with one and the same 

 specimen, placed in water containing 8 mg. C0 2 per ioo 

 c.c. Observations of photosynthetic activity were made first 

 with Pointolite at intensities varying from 400 to 3000 lux. 

 After a suitable period of rest, the plant was subjected to 



