I96 CHAP. XXII. EFFICIENCY OF THE SPECTRAL RAYS 



and blue-violet were taken for the equal exposures of seven 

 minutes' duration ; the amplitude of these responses is 

 given at the upper part of fig. 51. 



From these experiments it appears that the rays corre- 

 sponding to the absorption-band between F and G of the 

 chlorophyll-spectrum are chiefly active in phototropic 

 stimulation. This naturally raises the general question as 

 to the absorption of phototropically active rays by plants, 

 more specially by parts of plants without chlorophyll, but 

 it does not fall within the scope of the present work. 



Summary 



Photosynthetic activity is reduced or abolished by 

 prolonged exposure to the feeble radiation of the spectral 

 rays. 



In order to measure photosynthesis under the spectral 

 rays, which are relatively feeble, it is necessary to enhance 

 the sensitiveness of the specimen. This may be partially 

 effected by raising the temperature short of the optimum, 

 constancy of temperature being maintained by a vacuum 

 jacket. The sensitiveness may also be increased by pro- 

 ducing depletion of the deposit of starch by prolonged 

 previous maintenance of the plant in partial darkness. 



The best means for increasing the effective sensitiveness 

 to radiation is the Heterostatic Method. By means of an 

 Auxiliary Light- stimulator, the plant is brought to the verge 

 of photosynthesis. The incident rays now induce C0 2 - 

 assimilation in proportion to their respective photosynthetic 

 efficiency. 



By the device of the Auxiliary Stimulator the sensitive- 

 ness of the plant can be maintained constant for a long time. 



The relation between photosynthesis and the energy of 

 the incident radiation was ascertained by the simultaneous 

 determination of the photosynthetic activity by the 

 Bubbler, and of the intensity of radiation by the Magnetic 

 Radiometer. Curves were obtained showing the relation 



