TEE SIGNIFICANCE OF TROPISMS in 



lighted side, others in the opposite direction, and the majority perhaps 

 pay no attention to the light. 



This condition changes instantly if we add to the water some acid, 

 preferably carbonic acid, which easily penetrates the cells. This is done 

 by slowly adding to every 50 c.cm. of the fresh water a few cubic centi- 

 meters of water charged with carbondioxide. If the correct amount of 

 carbonic acid is added all the individuals become actively positively 

 heliotropic in a few moments and they move in as straight a line as 

 the primitiveness of their swimming movements permits, toward the 

 source of light, and remain there closely crowded together on the lighted 

 side of the vessel. If the vessel is turned around 180°, they go directly 

 back again to the lighted side of the vessel. Every other acid acts like 

 carbonic acid and alcohol acts in the same manner, only more feebly 

 and much more slowly. Animals which were previously indifferent to 

 light become, under carbonic acid treatment, complete slaves of the 

 light. 8 



How does the acid produce this result? "We will assume that it 

 acts as a sensitizer. The light produces chemical changes, for in- 

 stance, oxidation on the surface of the animal, especially in the eye, 

 as was suggested in the case of the aphids. The mass of photochemical 

 substance which is acted upon by the light is often relatively small, so 

 that even when the light strikes the crustacean (copepod) on one side 

 only, the difference in the chemical changes on the two sides of the 

 body remains still too small to call forth a difference in tension or 

 action, in the muscles of the two sides of the body, sufficient to turn 

 the animal toward the source of light. But if we add an acid this could 

 act as a catalyzer, as, for instance, in the catalysis of esters. In the 

 catalysis of esters, the acid acts, according to Stieglitz, only to the extent 

 of increasing the active mass of the substance which undergoes a 

 chemical change. In order to provisionally fix our ideas, we will assume 

 that the acid makes the animal more strongly positively heliotropic by 

 increasing the active mass of the photosensitive substance. By this 

 means it becomes possible for the same intensity of light which before 

 produced no heliotropic reaction now to cause a very pronounced posi- 

 tively heliotropic reaction; because if now the animal is struck on one 

 side only by the light, the difference in the reaction product in both 

 retinae becomes rapidly great enough to cause automatically a difference 

 in the action of the muscles of both sides of the body and a turning of 

 the head towards the source of light. 



A second consideration must also be mentioned here. In certain 

 forms, for instance, in daphnia and in certain marine copepods, a de- 

 crease in temperature also increases the tendency to positive helio- 

 tropism. If the mere addition of acid is not sufficient to make the 



8 Loeb, Pfliigers Archiv, Bd. 115, S. 564, 1906. 



