INFLUENCE OF LIGHT 127 



effect of alternating light and darkness nor to increased 

 transpiration. 



The following observations of Vogt illustrate the photo- 

 tonic reaction : when the coleoptile of the oat was alternately 

 illuminated by light of the same intensity, and darkened for 

 periods of fifteen, thirty, and sixty minutes, lesser growth was 

 found to obtain only in the two latter periods of illumination. 

 In these instances the greater growth in darkness is considered 

 to be due to the stimulation of the previous exposures to light. 

 The slower growth in periods of illumination is merely a part 

 of the complete reaction, an acceleration being followed by a 

 depression. 



From these investigations it is clear that the reaction of the 

 plant to light is very complex ; that the total effect observed 

 is the balance of the measures of the accelerating and depressing 

 influences of light ; and that the magnitude of the effect depends 

 upon the intensity and the duration of light. 



These observations refer to ordinary white light ; analysed 

 white light has effects on growth according to the particular 

 rays and to the physiological peculiarity of the species. 



Thus Schanz * grew Begonia, Cucumis, Lobelia, Petunia, 

 and other plants in frames arranged in a series of eight ; by 

 means of glass of different opacities, all but the first, which 

 was uncovered, were illuminated by light from which certain 

 rays had been abstracted : 



1. Unaltered day light. 



2. Rays longer than 320 pp. 



3. Rays longer than 380 pp. 



4. Rays longer than 420 pp. 



5. Rays longer than 560 pp. 



6. Yellow light. 



7. Green light. 



8. Blue- violet light. 



The plants mentioned showed a regular increase in length 

 from I to 5 inclusive and a decrease from 6 to 8 inclusive. 

 The behaviour of all plants, however, was not the same ; the 

 beetroot and the potato developed better in the blue-violet 

 than in the green, and better in the green than in the yellow. 



* Schanz : " Ber. deut. hot. Gesells.," 1919, 37, 430. 



