ClIEMOTAXIS AM) TUOPISMS 249 



Stahl's'^ oxpcriiucnl with a laij;(' jelly-like ijjasmodiuin (Aethal- 

 ium septicum) growing on l)aik in wet places, has become classical. 

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

 nearl)y was placetl a drop of an infusion of oak hark, the organism 

 moved by tiie process of protoplasmic streaming 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 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, wercT 

 found to cause the plasmocUum 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 

 previousl}' had been. 



Temperature was also found to exert a marked ihermofactic 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 stimuU of all sorts. The first step in their 

 change of direction of movement is considered by many obser\'ers to be an orien- 

 tation 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 symmetrical parts of the body. The stimulant, whether it be rays 

 of light, or diffusing chemicals, or heat-waves, moves along definite lineb, 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 stimulant. Not only protozoa, Init much higher 

 forms, including some vertebrates are believed to react in this way to stimuli — 

 e. g., the maintenance by fish of a position heading up stream. The above con- 

 stitutes the so-called ^^ theory of tropisms," and we have such reactions to stimuli 

 of all sorts, not only chemotropism and thermotropism, but also heliotropism (reac- 

 tion to light); geotropism (to gravitjO, eledropism (to electricityj, thig-motropism 

 (reaction to contact), etc. 



The work done upon tropisms applies particularly to ciliated, freely motile 

 organisms, and interests us less in connection with leucocytes than do the obser- 

 vations on such forms as Amoeha.^ In passing may be mentioned the thigmotaxis 

 or thigmotropism (reaction to mechanical stimuli) shown by spermatozoa, which 

 explains their apparently difficult feat of advancing in opposition to the cilia of 

 the epithelial lining of the female generative tract. It may also be noted that 

 the nature of reactions of organisms to various stimuli is not con.'^tant for even 

 the same organism. Copepods (minute Crustacea) may be negatively heliotropic 

 in the day and go away from the bright surface of the water, whereas at night 



« Botanische Zeitung, 1884 (42), 145 and 161. 

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

 * For full details see Jennings (Publication No. 16, Carnegie Institute. Wash- 

 ington, 1904; also J. Loeb, "Studies in General Physiology." 



