CHEMOTAXIS 255 



tract tlic spermato/oids of a certain I'oliacoous moss. In the case of 

 the malic acid, it seems to be the anion that produces tlie effect, since 

 salts of malic acid have exactly the same property. 



Stahl's •"' ex])eriment with a large jelly-like Plasmodium {Aethal- 

 twn scpticu)))) p'rowing on hark in wet places, has become classical. 

 He found that if the plasmodium was placed on a moist surface, and 

 nearby was placed a drop of an infusion of oak bark, the organism 

 moved by the process of ])rotoplasmic streaming toward and into the 

 infusion. If a piece of oak bark was placed in the water, plasmodial 

 arms were sti'ctched out to it and the piece of bark was soon com- 

 pletely surrounded by the organism. These movements were found 

 to occur in any direction, even exactly against the force of gravity. 

 Other substances, as acids or strong solutions of salt or sugar, were 

 found to cause the phismodium to move away from them, although 

 when sufficiently dilute they exerted an attraction. A plasmodium 

 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 concentrated sugar solution 

 previously had been. 



Temperature was also found to exert a marked thermotactic effect. 

 If a Plasmodium was placed on a filter-paper, on^ 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 to be an orientation of the organism 

 until it is headed in the axis along which it is to move. This is 

 ascribed by J. Loeb ** to the existence of a certain degree of equality 

 of irritability of sjrmmetrical parts of the body. The stimulant, 

 Avhether it be rays of light, or diffusing 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 organ- 

 ism 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 stimulant. Not 

 only protozoa, but much higher forms, including some vertebrates 

 are believed to react in this way to stimuli — e. g., the maintenance bj 

 fish of a position heading up stream. The above constitutes the so- 

 called 'theory of tropism," and we have such reactions to stimuli of 

 all sorts, not only chemotropism and thermotropism, but also helio- 



sBotanisehe Zeitunp, 1884 (42), 145 and 161. 



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



