548 TRANSFORMATION OF ENERGY 



s 



yet the absence of any repulsive reaction in the case of such substances as 

 glycerine or urea may be due to their known power of penetrating the pro- 

 toplasm rapidly. An osmotactic effect is absolutely correlated with the imper- 

 meability of the protoplasm to the substance under consideration, while its entry 

 is perhaps essential to a chemotactic response. We shall not go far wrong if 

 we look upon the withdrawal of water as the main agent in inducing perception 

 in osmotaxis. 



Weber's law applies to osmotaxis almost as well as to chemotaxis. In 

 experiments with Spirillum undula it may be clearly shown that as the osmotic 

 pressure of the culture fluid increases the liminal intensity of the stimulus 

 necessary for inducing osmotactic repulsion rises. Osmotactic repulsion is 

 induced, 



in a normal solution by 007 G. M. of sodium chloride 



,, „ + 003 G.M. of sodium chloride by 0-20-0-25 ,, ,, 



„ M +006 „ „ „ 025-030 ,, ,, 



„ u +009 „ „ „ 040-045 „ „ 



In addition to negative osmotaxis there is also a positive osmotaxis in 

 organisms whose natural habitat is a concentrated medium, and to which they 

 are adapted. This has been shown to be the case by Massart (1891 a) in cer- 

 tain marine Bacteria which exhibit positive osmotaxis. The significance of 

 osmotaxis is closely related to that of chemotaxis, for both sensitivities serve 

 to bring the organism under optimum vital conditions or to retain them there. 

 Many lower organisms are known, however, which are positively chemotactic in 

 highly concentrated fluids, and they collapse at once after being placed in them, 

 as a result of osmotic activity ; osmotactic phenomena are not manifested by 

 these forms at all. 



Chemotaxis and osmotaxis are also well illustrated in the plasmodia of 

 Myxomycetes. The fundamental facts were first established by De Bary (1864) 

 and Stahl (1884), still a systematic revision of the phenomena from the point 

 of view gained by similar studies on Bacteria and sperms is as yet non-existent. 

 Since in this case we have to deal not only with alterations in directive move- 

 ments pure and simple under the influence of chemical stimuli, but at the same 

 time with alterations in the form of the plasmodium, we will not discuss the 

 phenomena presented to us further, but merely note that sensitivity in the 

 Myxomycetes is special in character and dependent on their peculiar habit and 

 relation to a solid substratum. The plasmodia are hydrotactic, i. e. they seek 

 situations where a certain amount of moisture prevails or avoid dry substrata. 

 Hydrotaxis is probably very closely allied to osmotaxis. In both series of 

 phenomena it is the withdrawal of the water that leads to perception, and 

 perhaps it is immaterial to the organisms whether the withdrawal is brought 

 about by osmotic activity or by transpiration. 



Hydrotaxis in Myxomycetes is also known to be closely related to rheotaxis 

 (JoNSSON, 1883 ; Stahl, 1884), a fact which may be easily shown by allowing 

 water to flow off from a vertically placed medium, e. g. filter paper, when the 

 Plasmodium moves upwards in the opposite direction to the current. Analogy 

 with rheotropism suggests to us that the motive cause of the stimulus is to be 

 sought for in the mechanical action of the water, that is to say, in the impact 

 of the fluid on the plasmodium. This fact leads us to mention that ' hapto- 

 taxis ' (thigmotaxis), a movement induced by contact stimulus, is also to be 

 recognized as occurring among lower organisms. 



Light and heat, like the chemical and physical properties of bodies, also 

 induce directive movements in motile plants, i. e. we must also recognize both 

 phototaxis and thermotaxis. We shall not go far wrong in regarding such 

 types of taxis as entirely analogous to chemotaxis in their nature, inasmuch as 



