THE NATURE OF PHOTOSYNTHESIS 117 



to. It will be recalled that these investigators found that the unicellular 

 alga used by them, when cultivated in light of high intensity, converted 

 but a small amount of the light absorbed in the process of photosynthesis, 

 while the plants cultivated in light of low intensity converted a large portion 

 of the absorbed energy in the photosynthetic process. It is essential to 

 bear in mind that \\'arburg and Negelein worked with light of low inten- 

 sity only, they did not expose their "light" plants to illumination of high 

 intensity. Thus their results as far as they go are really in agreement 

 with those of Harder. Finally, it should be pointed out that the results of 

 Harder and those of Warburg and Negelein are not strictly comparable. 

 The latter investigators were concerned with photosynthetic efficiency, 

 namely, the proportion of radiant energy, absorbed which is converted into 

 chemical energy. This is a strictly quantitative and physical method of 

 measurement. Harder was satisfied with the relative amounts of oxygen 

 liberated by the two types of plants under different conditions of illumina- 

 tion ; there is no measure of the ratio of energy absorbed to that converted. 

 Thus there is no information on the efficiency of the two types of plants 

 under the different conditions. The investigations of Harder and those 

 of Warburg and Negelein were carried out from different viewpoints 

 and had not the same goal in view. 



Aquatic plants lend themselves better to experimentation of the nature 

 just discussed than land plants. Particularly on account of errors due to 

 temperature changes within the leaf and variations in the stomatal open- 

 ings which give rise to alterations in the gaseous exchange and water loss ; 

 thus, experimentations with land plants are associated with many diffi- 

 culties. While the principles underlying the reactions involved in photo- 

 synthesis are probably the same in aquatic and land plants, it is in the 

 latter that greatest interest centers. Lubimenko ^* has worked with a 

 number of land plants in relation to their behavior in light of different 

 wave-lengths. His results are difficult to interpret on the basis of other 

 findings. Thus, he concludes that while the reduction of carbon dioxide 

 is greater in red light than in blue or violet the increase of dry substance 

 is greater in blue light than in red. 



Concerning the role of ultraviolet rays in the photosynthesis process, 

 it may be stated that it is highly improbable that these are of significance 

 under natural conditions. The intensity of ultraviolet light at sea-level is 

 exceedingly low. The solar spectrum ends very abruptly near wave- 

 length 300 ji |i ; even at great heights above sea-level there is no appre- 

 ciable energy of wave-lengths shorter than 290 j-ii-i.**^ Moreover photo- 

 synthesis is carried on normally by plants under glass which is opaque 

 to ultraviolet light. Whether it is possible for photosynthesis to take 

 place in ultraviolet light is as yet uncertain. Bonnier and Mangin's "'' ex- 



** Lubimenko, Zentr. Biochcm. Bioph\s., 10, 803 (1910); Rev. gen. hot., 23, 1 

 (1911); Compt. rend., 145, 1191 (1907): 177, 606 (1923). 



"'Luckiesh, "Ultraviolet Radiation," 1922, p. 24. 

 • "* Bonnier and Mangin, Compt. rend., 102, 123 (1886). 



