CHAPTER XVI 



EFFECT OF DIFFERENT RAYS ON THE REACTIONS OF 



SESSILE PLANTS 



It Is evident that as in inorganic and organic compounds 

 and in man, so in the lower organisms, the reactions to 

 light may be due to or at least associated with a specific 

 action of the length of the light waves, or with the ampli- 

 tude of the waves, or with a combination of waves of 

 different lengths. In experiments on the effect of colored 

 light on organisms it is therefore essential to know what 

 sort of light is being used as a stimulating agent ; many 

 results are unreliable because this was not known, or at 

 least is not recorded. The colors used were frequently 

 produced by means of solutions or colored glass which 

 transmit waves varying much in length. In case of red 

 glass, e.g., there is usually some orange and yellow and fre- 

 quently a little blue, violet or ultra-violet transmitted as 

 well as the red. A reaction in such light, apparently due 

 to the longer waves may actually be due to the shorter, or 

 to the specific effect of the combination. Then, too, the rela- 

 tive intensity of the different colors was often not consid- 

 ered. There was then the possibility that the reactions 

 were due to intensity rather than to color. 



The bearing of this discussion becomes evident when 

 we consider the fact that some organisms are sensitive to 

 light of extraordinarily low intensity; e.g., Figdor (1893) 

 found the plants Lepidium sativum, Amaranthus melan- 

 cholicus ruber, Papaver paeoniflorum, and Lunularia biennis 

 to respond to light as weak as 0.00033 ca. m. In organ- 

 isms so extremely sensitive it is evidently impossible to 

 be certain as to what causes a reaction if they are not 

 subjected to monochromatic light of known intensity. In 



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