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 
alga. These swarm-spores make progressive movements 
like animals, and Strasburger’ proved that they move in the 
1 STRASBURGER, Wirkung des Lichtes und der Wdrme auf Schwiirmsporen 
(Jena, 1878). 
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