698 PLANT RESPONSE 



equal in the two cilia, and this can clearly happen only when 

 the organism has so orientated itself that its axis is parallel 

 to the direction of the incident rays ; and it is evident that 

 in this position the equal beats of both cilia must result either 

 in progressive or in retrogressive motion, parallel to the direc- 

 tion of the incident rays. 



Thermotaxis. — Thermal stimulation also causes respon- 

 sive movements towards or away from the source of stimula- 

 tion ; and these reactions, again, are found to change their 

 signs, with varying intensities of stimulus. Thus, Paramcecia 

 swim towards the warmer side of a vessel which is unequally 

 heated, provided the temperature of the warmer side does not 

 exceed 24 C. The response is in this case, then, seen to be 

 positive ; but when the temperature of the heated side is 

 higher than 28 C. the Paramcecia are found to swim away, 

 thus exhibiting a negative response. 



Galvanotaxis. — Similar multiple response finds expres- 

 sion in certain Infusoria, in swimming movements of posi- 

 tive and negative character. Thus Verworn finds, for 

 example, that under the excitation of an electrical current, 

 Polytoma swims away from the kathode, and Pleuronema 

 towards it. 



In these galvanotactic movements, also, we meet with 

 the same recurrent reversals with which we are already 

 familiar in the case of the leaflets of Desmodium. In work- 

 ing with Paramecium, Arthur W. Greely ' found that after 

 being subjected for about half an hour to a moderate current, 

 the organisms which had previously gathered round the 

 kathode reversed their action, and moved towards the anode, 

 only to dart back immediately, again, towards the kathode. 



I have demonstrated in Chapter XXXVI. the opposite 

 responsive changes which occur under the action of acids 

 and alkalis respectively. This explains the appropriate 

 modification of galvanotactic response in Paramecium, 

 according as the organism has been reared in an acid or in 



1 Science has lost a very promising worker in the early death of this investi- 

 gator. His experiments on Parametrium arc very valuable and suggestive. 



