INFLUENCE OF LIGHT UPON LIFE-PROCESSES 429 



by de Vries to be increased from three to five times in velocity by a 

 rise of 10 in temperature. Still more remarkable is the fact that the 

 Assimilation of Carbon Dioxide by green plants in sunlight which 

 underlies the Photosynthesis of Carbohydrates is also doubled or more 

 than doubled by a rise of ten degrees in temperature; the following 

 results are compiled from the measurements of Gabrielle Matthsei, the 

 absorption measured being that of leaves of Prunus laurocerasus 

 exposed to gaslight of constant intensity: 



Carbon dioxide Temperature-coefficient 

 Temperature. assimilated. per 10 C. 



-6 0.2 28.7 



1.75 2.40 



10 4.2 2.12 



20 8.9 1.76 



30 15.7 1.81 



37 23.8 0.23 



40.5 14.9 



Heat-injury already appears at 30, but below this temperature the 

 coefficients clearly indicate that the rate of assimilation is not deter- 

 mined by the photochemical process but by a reaction of the ordinary 

 type. These results may be interpreted by supposing that the photo- 

 chemical reaction (transformation of CO 2 and H 2 O into formaldehyde) 

 is retarded by its product, and that the speed of photosynthesis is 

 therefore determined by the rate at which this product is removed by 

 a secondary reaction (condensation of formaldehyde into glucose). 



THE INFLUENCE OF LIGHT UPON LIFE-PROCESSES. 



Photosensitive Substances are of very widespread occurrence in living 

 tissues. This is evidenced by the fact that the effects of light upon 

 organisms are not by any means confined to the specialized cells which 

 comprise the visual organs in the higher metazoa. The synthesis of 

 Carbohydrates in plants is brought about by the action of sunlight upon 

 vegetable tissues which contain Chlorophyll or some analogous pigment 

 and are exposed to an atmosphere containing carbon dioxide, but quite 

 independently of this, light additionally exerts an effect upon the proto- 

 plasm of the cells of most plants, leading to a bending of sessile forms or 

 an actual migration of motile forms toward or away from the source 

 of light, a phenomenon known as Phototropism or Heliotropism. This 

 phenomenon is also very generally displayed by animals, and the 

 investigations of J. Loeb have demonstrated that the mechanism of 

 heliotropism in animals and in plants is essentially the same, nor is it 

 invariably associated, even in animals, with the possession of specific 

 light-sensitive organs. For many of the unicellular forms of life sun- 

 light is very definitely toxic, and this is true not only for pigmented but 

 also for colorless cells. The most toxic portion of the spectrum lies 

 in the ultraviolet region, a fact which bacteriologists have attempted 



