CHAPTER XLIX 



ON PULSATORY RESPONSE AND SWIMMING MOVEMENTS 

 AS INITIATED AND MODIFIED BY LIGHT AND OTHER 

 FORMS OF STIMULUS 



Investigations on the influence of light on the lateral leaflet of Desmodium gyrans : 

 (a) in sub-tonic condition ; (b) in normal tonic condition - Changes induced in 

 existing anisotropy of Desmodium leaflets — Reversal under intense stimulation 

 seen in all forms of response — The swimming movements of ciliated organisms 

 — Fundamental resemblance between the swimming responses and the ordinary 

 heliotropic responses in radial organs — Phototactic movements : (a) Two 

 natural types of responsive movements ; {b) Responsive movement positive, 

 negative, or intermediate, according to intensity of stimulation — Directive 

 action of light— Thermotaxis — Galvanotaxis — Chemotaxis. 



We have already studied in detail the autonomous responses 

 of the leaflets of Desmodium ; but there are also other 

 leaves which execute similar pulsatory movements, and we 

 have now to investigate the effect of the stimulus of light on 

 such multiply-responding organs, either in initiating these 

 movements or in modifying them. We have also another 

 instance of such multiple pulsations, in the swimming move- 

 ments of ciliated organisms, of which the causes are at 

 present regarded as very obscure. The treatment of these I 

 include within the present chapter, because I hope to show 

 that they really belong to a class of phenomena essentially 

 similar to the autonomous movements of Desmodium, and 

 that from this point of view all their seeming peculiarities 

 may be very satisfactorily explained. 



In the first place, I shall take up the question of multiply- 

 responding leaves or leaflets. Concerning the effect of light 

 on these, there is a certain amount of discrepancy of observa- 

 tion. Strasburger, for example, referring to such movements 



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