100 PHO TOS YN THESIS 



by the chloroplasts. However, under these circumstances photosynthesis 

 does not become evident by an evolution of oxygen, but only by an appar- 

 ently lower rate of respiration, that is, the amounts of carbon dioxide 

 given off and of oxygen absorbed are less. There is no evidence that 

 feeble illumination affects the rate of respiration itself, so that it has been 

 concluded that the lower rate of carbon dioxide evolution and of oxygen 

 absorption are due to the fact that in the very low light intensity a por- 

 tion of the carbon dioxide evolved in the process of respiration, is taken 

 up by the chloroplasts and a small amount of oxygen is evolved which in 

 turn is immediately utilized in respiration again. This results in a dimin- 

 ished gas interchange of the plant with the atmosphere. With increasing 

 illumination a point is finally reached where the rate of photosynthesis 

 just balances that of respiration and there is no gas interchange whatso- 

 ever. A discussion of this state, the so-called compensation-point, will be 

 taken up later. Exact determinations of the minimum intensity of light 

 which is necessary for carbon dioxide reduction have not yet been made.*** 

 Most of the measurements consider the point of oxygen evolution, that is, 

 where photosynthesis overbalances respiration. But this, of course, is not 

 the lowest intensity of light necessary for photosynthesis, but merely an 

 intensity where the rate of photosynthesis is greater than that of 

 respiration. 



Early experiments showed that photosynthetic activity increases pro- 

 portionately to the intensity of light and that there is a limit to the in- 

 crease above which increase in the light intensity results in no augmenta- 

 tion of the photosynthetic rate. Moreover, every plant has its own specific 

 requirements and the capacity to use light as well as the efficiency varies 

 widely in different species. The underlying principles for these phe- 

 nomena have been brought to light by more recent investigations. 



Some reference to the influence of light on the rate of photosynthesis 

 has already been made under the discussion of the effect of carbon dioxide. 

 It has repeatedly been stated that none of the factors affecting the rate 

 of photosynthesis can be studied alone or without regard to the influence 

 of other factors. From a consideration of the principles which have been 

 given expression in the theory of limiting factors, it is evident that 

 changes of light intensity have an effect on the rate of photosynthesis 

 only when light is the limiting factor or "in minimum." Considering first 

 Blackman's conclusions in formulating this conception we can summarize, 

 that where carbon dioxide supply and temperature are in excess the rate 

 of photosynthesis is proportional to the intensity of illumination. This, 

 of course, assumes that other factors affecting photosynthesis, as 

 chlorophyll-content and water supply are constant. Blackman and 

 Matthaei '•' used sunlight and perforated screens to reduce the light in- 



•" For Moonlight: Boussingault, J. B., Ann. d. sci. nat. (Ser. V), 10, 335 (1869). 

 Ursprung, A., Ber. bot. Ges.. 35, 62 (1917). Midnight-sun: Curtel, G., Rev. gen. 

 bot 2 7 (1902). Kostytschew, Ber. bot. Ges., 39, 334 (1921). 



^Blackman and MaUhaei, Proc. Roy. Soc. London, B 76, 402 (1905). Black- 

 man and Smith, ibid., B 83, 389 (1911). 



