THE LIGHT FACTOR IN PHOTOSYNTHESIS 1033 



The opposite conclusion is reached by van der Paauw (126), who 

 also worked with this alga. He obtained a logarithmic relationship 

 with algae cultivated in daylight and states, "An approximation of 

 the Blackman scheme is apparently out of the question." When these 

 organisms are cultivated in weak light an apparent inhibition of the 

 photosynthetic action is produced when the assimilation is measured 

 at high light intensities. The results of Montfort (80) on the alga 

 Fucus vesiculosus agree very well with the limiting factor hypothesis. 

 On the other hand, the results of experiments made on the shade fern 

 Trichomancs radicans at low light intensities agree better with the 

 relativity law. At higher intensities there is a complete inhibition 

 of photosynthesis. Boysen-Jensen (7) investigated these relations 

 in Sinapis alba and Miiller (81) in Chamaeneriiim latifolium and in 

 Salix glauca. While these investigators drew no conclusions as to the 

 validity of either hypothesis, the curves published conform more nearly 

 to the Blackman principle. 



Maskell (74) attempted to find a quantitative expression which 

 would represent the march of photosynthesis under different conditions 

 of light intensity, carbon dioxide concentration, and also with considera- 

 tion of the diffusion resistance of carbon dioxide. Even though the 

 equation derived met with some success in depicting the rates of assimila- 

 tion, Maskell concludes: "It is impossible, however, to suggest any general 

 picture of more than limited validity. Regarded and used as a clue 

 to the interpretation of the phenomena, the general principle of limiting 

 factors suggested by Blackman in 1905 cannot as yet be replaced." 



INHIBITION BY HIGH LIGHT INTENSITIES 



The inhibitory effect of high light intensities has been investigated 

 more fully by Montfort (80), who found that the rate of photosynthesis 

 of the shade fern Trichomanes radicans increased up to one-eighth full 

 daylight, and then decreased until at one-half full daylight the assimila- 

 tion ceased. Similar results (80a) were found for the red alga Rhodymenia 

 and for the green alga Cladophora. When returned to favorable condi- 

 tions, the plants regained their original photosynthetic rate. 



An adequate explanation of this inhibitory effect has not been found. 

 It is not known whether it is an inhibition of the photosynthetic process 

 itself, an accelerating effect on some other process such as respiration, 

 or temporary injury of the protoplasm. 



Nutritional Factors. — The rate of photosynthesis at different light 

 intensities is considerably influenced by the nutrition of the plant. 

 Van der Paauw (126) has shown that increase in the sugar concentration 

 of the nutrient solution for Hormidium increases the photosynthesis. 

 The effect is greater at low intensities of light than at high. Miiller 

 (81a) found that addition of small amounts of potassium nitrate in the 



