CHEMO TAXIS 209 



ing toward and into the infusion. If a piece of oak bark was 

 placed in the water, plasmodial arms were stretched out to it 

 and the piece of bark was soon completely surrounded by the 

 organism. These movements were found to occur in any direc- 

 tion, even exactly against the force of gravity. Other sub- 

 stances, as acids or strong solutions of salt or sugar, were found 

 to cause the plasmodium to move away from them, although 

 when sufficiently dilute they exerted an attraction. A plasmo- 

 dium might, however, move into a strong sugar solution if kept 

 with a scanty supply of moisture for some time, and after it 

 had lived in such a strong solution (2 per cent.) for some time, 

 a weaker solution or pure water .was as injurious as the con- 

 centrated sugar solution previously had been. 



Temperature was also found to exert a marked thermotactic 

 effect. If a plasmodium was placed on a filter-paper, one end 

 of which was in water at 7, and the other in water at 30, it 

 would move toward the warmer end. 



The Theory of Tropisms. Ciliated protozoa, which can 

 move about freely in water, show very distinct reactions to 

 stimuli of all sorts. The first step in their change of direction 

 of movement is considered by many observers l to be an orienta- 

 tion of the organism until it is headed in the axis along which 

 it is to move. This is ascribed by Loeb 2 to the existence of a 

 certain degree of equality of irritability of symmetrical parts of 

 the body. The stimulant, whether it be rays of light, or diffus- 

 ing chemicals, or heat-waves, moves along definite lines, and 

 the organism receives at first unequal stimuli on symmetrical 

 parts of the body, unless the axis of the organism is parallel to 

 the lines of motion of the stimulant. As long as the stimulant 

 acts on symmetrical parts of the body unequally, these parts will 

 react unequally until at length the body is swung into a position 

 where the stimulation is equal, when it will stay in this position 

 and move along a line parallel to the line taken by the stimu- 

 lant. Not only protozoa, but much higher forms, including 

 some vertebrates, are believed to react in this way to stimuli 

 e. g.j the maintenance by fish of a position heading up stream. 

 The above constitutes the so-called " theory of tropisms" and we 

 have such reactions to stimuli of all sorts, not only chemotropism 

 and thermotropism, but also heliotropism (reaction to light); 

 geotropism (to gravity), electropism (to electricity), thigmotropism 

 (reaction to contact), etc. 



1 Jennings does not accept this view, but attributes the results to processes of 

 " trial and error." 



* Comparative Physiology of the Brain, New York, 1900, p. 7. 



14 



