118 PHOTOSYNTHESIS 



perinients would indicate that this was the case, yet there is considerable 

 doubt as to whether their filters excluded all other light of lower fre- 

 quency. The problem is made difficult on account of the harmful action 

 of ultraviolet light on plants and the complications arising from the 

 effect of these rays on enzymes, starch and other components of the 

 plant. ^" 



Similarly, there is no evidence that the infra-red rays play a part in 

 the reduction of carbon dioxide, although it appears that chlorophyll 

 absorbs some infra-red rays. On the other hand active photosynthesis 

 is caused in purple bacteria by the infra-red rays (800-900 \i \i).^^ 



The composition of the light is changed as it penetrates successive layers 

 of chlorophyll so that the chloroplasts in the center of a leaf are exposed to 

 light of different composition from those near the surface. This is one 

 factor which may account for the varying behavior of different species 

 of leaves. Thus the red rays (700-660 \i \i) cause very active photosyn- 

 thesis, but are also largely absorbed by the outer layers of the leaf and 

 can thus act upon fewer chloroplasts than the yellow rays which pene- 

 trate more deeply. It is conceivable therefore that the maximum carbon 

 dioxide reduction in relation to the amount of light of different wave- 

 lengths absorbed will vary in relation to the thickness of the leaf, and 

 in thick leaves maximum photosynthesis will be shifted toward the blue. 

 The reflection as well as the absorption of light by a leaf are both of 

 considerable importance in determinations of photosynthesis. Undoubt- 

 edly some leaves in nature, due to their glossy surface, reflect a large 

 proportion of the light incident on them. 



The absorption spectra of alcoholic solutions of chlorophyll show a 

 slight displacement toward the blue compared with spectra of chloro- 

 phyllous leaves. Although there has been considerable controversy as 

 to whether a leaf is capable of carrying on photosynthesis in light fil- 

 tered through a chlorophyll solution or through another leaf, there 

 appears to be little doubt that this is possible if the intensity of the light 

 is sufficiently high as was shown by Ursprung ^^ with direct sunlight. 



There is some evidence that there is a difference of effect on photo- 

 synthesis between continuous and intermittent light. Warburg ^°'' has 

 tested this by means of rotating sectors which in one revolution cut off 

 one half the light so that the period of illumination was equal to that 

 of darkness. Comparison between the effects of continuous and inter- 

 mittent illumination were made on the basis of equal periods of illumina- 



"'Kluyver, A. J., Oestrcich. Boi. Zcit., 63, 49 (1913) ; Sitsher. Wicn. Akad., 120, 

 I, 1137 (1911). Bovie, W. T., Bot. Gaz.. 59, 149 (1915) ; 61, 1 (1916). Maquenne 

 and Demoussy, Compt. rend., 149, 756 (1909). Stoklasa, Centralbl. f. Bakt., 31, II, 

 477 (1912). Raybaud, L., Rev. gen. hot., 25, 38 (1913). Ursprung and Blum, 

 Ber. bot. Ges., 35, 385 (1917). 



"» Englemann, Bot. Zeitg., 40, 419 (1882) ; 41, 1 (1883) ; 42, 81 (1884). Pfeffer, 

 W., Arb. bot. Itist. IViirsburq., 1, 41 (1871). Ursprung, Ber. bot. Ges., 35, 55 

 (1917). Warburg and Negelein, Zcit. phvsik. Chcm., 106, 191 (1923). 



""Ursprung, Ber. bot. Ges., 35, 64 (1917). 



*'" Warburg, Biochem. Zeit., 100, 260 (1919). 



