352 



FACTORS AFFECTING PHOTOSYNTHESIS 



Low light intensities favor stomatal closure and hence may sometimes check 

 photosynthesis by restricting the entrance of carbon dioxide as well as by 

 acting as a direct limiting factor. Similarly high light intensities often cause 

 increased rates of transpiration and hence, indirectly, a reduced water con- 

 tent of the leaf cells, and consequent diminished rates of photosynthesis. A 

 high light intensity also has the usual effect of raising leaf temperatures some- 

 what above the prevailing temperatures of the surrounding atmosphere and 

 may thus influence photosynthesis indirectly by its effect on thermal conditions 

 within leaves. Very high light intensities also have a destructive effect upon 

 chlorophyll, as has been shown by Ewart (1898) and others. 



350 



450 550 650 



WAVE LENGTHS IN MILLIMICRONS 



750 



Fig. 90. Relative rates of photosynthesis in different wave lengths of light of equal 



intensity. Data of Hoover (1937). 



3. Effects of Different Light Qualities upon Photosynthesis. — Hoover 

 (1937) has recently investigated the effect of different wave lengths of radia- 

 tion upon the rate of photosynthesis of wheat plants (Fig. 90). By the use 

 of suitable filters he was able to irradiate the plants with narrow spectral 

 bands. All measurements were made with equal intensities of radiation 

 incident upon the plants. The results of his investigation indicate the occur- 

 rence of maximum photosynthesis at a wave length of 655 mix in the red, and 

 a secondary maximum of 440 //z/x in the blue. The green region of the spectrum 

 is relatively less effective in photosynthesis, presumably because of the smaller 

 proportion of the radiation absorbed by leaves in this range of wave lengths. 



There are a number of conditions under which plants growing in their 

 natural habitats are exposed more or less continuously to light of a very dif- 



