HELIOTROPISM OF ANIMALS 5 



It was formerly believed that the bending of the positively 

 heliotropic parts of plants was due to the fact that the side 

 which was turned away from the light grew more rapidly, 

 because plants when brought into the dark at first grow more 

 rapidly than they do in the light. But it was proved in 

 Sachs's laboratory that negatively heliotropic organs also 

 grow more rapidly in the dark. Because of the similarity 

 of the geotropic and heliotropic movement in plants, Sachs 

 came to the conclusion that the direction in which the rays 

 of light penetrate the plant tissue determines the orientation 

 of the plant toward light. He also proved that not all the 

 rays of the visible sun spectrum bring about heliotropic 

 movements, but only, or at least chiefly, the more refrangible 

 rays. The less refrangible rays, which are of importance in 

 assimilation, are ineffective heliotropically. If the light be 

 previously passed through a dark-blue ammoniacal solution 

 of copper, which absorbs all the red, yellow, and a part of 

 the green rays, the heliotropic bending occurs in the same 

 way as in completely white light. If, however, the light 

 passes through a saturated solution of potassium bichromate, 

 which lets through only red, yellow, and a part of the green 

 rays, " the heliotropic shoots remain straight and vertical, no 

 matter how intense the light is which passes through the 

 solution." Finally, if the light "is passed through a solu- 

 tion of quinine sulphate, the fluorescence of which completely 

 absorbs the ultra-violet rays, the heliotropic curvatures 

 nevertheless appear a proof that they are caused princi- 

 pally by the visible blue and violet rays." 



The best proof of the theory that the direction of the 

 rays of light controls the orientation of plants was found by 

 studying freely moving plant organs, the swarm-spores of 

 algse. These swarm-spores make progressive movements 

 like animals, and Strasburger 1 proved that they move in the 



i STBASBUEGEB, Wirkung des Lichtes und der Wdrme auf Schwarmsporen 

 (Jena, 1878). 



