CHAPTER XVII 



THE RELATIVE EFFECT OF DIFFERENT RAYS ON THE 

 REACTIONS OF UNICELLULAR FORMS 



The first observations on the effect of different colors 

 on the movement of unicellular forms were made by 

 Cohn in 1865, nearly fifty years after similar observations 

 had been made on sessile plants by Poggioli (18 17). 

 Cohn's account of his observations is very brief. He 

 studied the movements of swarm spores in colors differ- 

 entiated by means of colored glass and concluded that the 

 blue rays are the most effective and that the red act like 

 total darkness. He says (1865, p. 222), '' Die Organismen 

 werden von den blauen Strahlen am starksten angezogen, 

 wahrend sich die rothen wie totale Finsterniss verhalten." 



I. Strashiirger's Experiments 



Much more extensive and conclusive results were 

 obtained by Strasburger (1878), w^ho also studied swarm- 

 spores of various kinds, but principally Botrydium. 

 Strasburger exercised the utmost precaution in his experi- 

 ments. Many of the observations were made in a dark 

 room in light of different colors produced by a quartz 

 prism in a horizontal beam of direct sunlight. The slit 

 in the opaque screen over the prism was only 0.4 mm. 

 wide and the spectrum at the point of exposure 55 mm. 

 long. The Fraunhofer lines could be clearly seen. It is 

 therefore evident that there was but little intermingling of 

 rays of different lengths in the spectrum. In addition to 

 the spectrum, colored glass and various solutions were 

 used. The results led Strasburger to conclude that the 

 blue, indigo and violet light alone cause orienting reactions, 

 but that yellow, red and green cause a quivering movement 



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