316 BOTANY part i 



in roots, though it cannot be said that it plays an important role in 

 their life. 



In the examples of chemotropism given above the stimulating 

 substances were solid substances in solution. When on the other 

 hand the plant is induced to perform directive movements by the 

 unequal distribution in a space of aqueous vapour or gases, a distinct 

 name has been required, though no distinction of principle can be 

 drawn. Irritable movements caused by differences in moisture are 

 termed hydrotropic {^-'^), while those brought about by gaseous differ- 

 ences are termed aerotropic (^°^). Aerotropism has been proved for 

 pollen tubes, roots, and shoots, and hydrotropism for roots and moulds. 

 Thus roots are positively hydrotropic and seek out the damper spots 

 in the soil by reason of this irritability. The sporangiophores of the 

 Mucorineae are negatively hydrotropic and thus grow out from the 

 substratum. These reactions may be so energetic as to overcome other 

 {e.g. geotropic) stimuli. 



Chemotaxis plays the same part in the case of freely motile 

 organisms or organs as does chemotropism in those that are attached. 

 Here also chemotaxis in the narrower sense may be distinguished from 

 hydrotaxis and aerotaxis. Aerotaxis determined by oxygen is found 

 in the case of Bacteria ; on this account these organisms have been 

 used to demonstrate the assimilation of carbon dioxide (p. 214). A 

 positive hydrotaxis is shown by the plasmodia of Myxomycetes, and 

 this passes into negative hydrotaxis at the time of spore-formation. 

 The chemotaxis of Bacteria and spermatozoicls Q^'') is especially well- 

 known owing to Pfeffer's fundamental investigations ; this leads to 

 the Bacteria seeking out favourable nutrient substances and to the 

 spermatozoid seeking out the egg-cell. 



The spermatozoids of the Fenis are attracted by malic acid or malates to the 

 neck of the archegonium ; in the case of the spermatozoids of Lycopodiiim, citric 

 acid, iu Mosses, cane sugar solution, and in the Marchantieae proteid substances are 

 the respective attractive substances. Often extremely minute quantities of the sub- 

 stance will bring about active irritable movements ; thus even a 0'001% solu- 

 tion of malic acid will attract the numerous spermatozoids of a Fern swimming 

 in pure water. 



In part these tactic irritable movements are brought about by the organism, 

 when it has reached the area of diffusion of the specific stimulating substance, at 

 once turning so that its long axis is in the direction of diffusion ; it then, according 

 to the special circumstances, swims either to or from the centre of diffusion. Often, 

 however, as occurs, for instance, iu the accumulation of Bacteria at a particular spot, 

 no such rotation and arrangement in the line of diffusion takes place. Tlie organisms 

 move about in the fluid without object, but hasten back when their movements have 

 carried them beyond a certain concentration of the stimulating substance (phobic 

 reaction, cf. p. 302). 



We may perhaps connect with chemotaxis the movements which have been 

 observed in nuclei and ])rotoplasts after wounding (trauniotaxis). 



