142 PHOTOSYNTHESIS 



opinion as to whether all plant parts containing chloroplasts exhibit the 

 capacity for photosynthesis. In view of the fact that frequently the 

 chloroplasts are rendered inactive by injurious external agencies, it is 

 difficult to establish this point experimentally.^*^ 



While, then, it is generally accepted that chlorophyll is essential for 

 photosynthesis, the quantitative determination of the role of this factor is 

 exceedingly difficult, because it is impossible to vary experimentally the 

 concentration of the chlorophyll in the plants under investigation. In 

 considering chlorophyll alone as a factor in photosynthesis, it is well 

 to bear in mind that this is done only for the sake of analysis; photo- 

 synthesis can take place only when the chlorophyll is associated with the 

 living plasma in the chloroplast. We shall discuss, then, the effect on 

 photosynthesis of plants containing varying amounts of chlorophyll, bear- 

 ing in mind always that chlorophyll is but one part of a complex mechanism 

 to the successful functioning of which other parts are essential. 



On account of the impossibility of varying the concentration of chloro- 

 phyll experimentally in any one plant, the method which has been fol- 

 lowed most generally has been to compare leaves of the same species 

 but of different chlorophyll-content, as for example, normal varieties, 

 rich in chlorophyll, with the yellow varieties, poor in chlorophyll. While 

 this is probably the best method of approach now available to the prob- 

 lem of the influence of variation in chlorophyll-content on photosynthe- 

 sis, it is not altogether satisfactory. Two varieties of the same species 

 of a plant may differ in chlorophyll-content according to differences in 

 certain environmental conditions, e.g., light intensity and soil, or this 

 dift'erence in chlorophyll-content may arise from more deep-seated heredi- 

 tary causes. The question arises whether associated with the differences 

 in chlorophyll-content there may not be other differences which also 

 affect photosynthesis. Chlorophyll is itself a product of the metabolism 

 of the plant ; for its formation, in most plants, light is essential as is also 

 oxygen. But the respiratory and metabolic activity of the plant is of con- 

 siderable significance in the photosynthetic activity thereof. It is. there- 

 fore, not entirely improbable that the factors which produce differences 

 in chlorophyll-content affect other portions of the photosynthetic mecha- 

 nism. Thus the differences in photosynthetic activity which in some 

 cases have been ascribed to differences in chlorophyll-content may actually 

 be due to a complex of factors. 



Lubimenko ^^^ found that the light requirements of "shade" plants for 

 photosynthesis is considerably lower than that of "Hght" plants, i.e. the 

 former type of plant can accomplish the same amount of photosynthetic 

 work with a lower illumination intensity than the latter type. This fact 



"'Dehnecke, Dissertation, K51n (1880). Friedel, Comt>t. rend.. 142, 1092 (1906). 

 F.wart, Journ. Linnean Soc, 31, 436 (1896). 



"*Lubimenko, Rev. gen. Bot.. 17, 381 (1905); 20, 162, 217, 253, 285 (1908); 

 Ann. Sci. Nat. (9), 7, 321 (1908) ; Compt. rend., 145, 1347 (1907). Griffon, Ann. 

 Sci. Nat. (8), 10, 1 (1899). 



