yoo PLANT RESPONSE 



111 Desinodiui/i,Vir\der the continuous stimulation of strong 

 light, these reversals are often recurrent. The dovvnstroke, 

 which is at first quicker, becomes less quick than the upstroke, 

 and this may be again and again reversed. 



In a ciliated organism the swimming movements are 

 explicable by similar unequal up and down strokes of the 

 anisotropic cilia. When the downstroke is quicker, the 

 organism propels itself forward. When the upstroke is quicker, 

 there is a movement backwards. Such swimming movements, 

 due to multiple response, are initiated by stimuli of various 

 forms. There are two natural types of these responses : 

 positive movement, or swimming towards, and negative, or 

 away from, stimulus. These are determined by the relative 

 excitabilities of the upper and lower halves of the cilium. 



Unilateral light exerts a directive action on responsive 

 swimming movements, because the only stable position is one 

 in which pairs of cilia are equally excited. The axis of the 

 organism is thus orientated till parallel with incident light. 



Strong stimulation of light, for reasons described, causes a 

 reversal of the normal movement. Alternate periodic fatigue 

 causes a responsive movement of the organism to and fro. 



Under moderate unilateral thermal stimulus the organism 

 exhibits a positive swimming movement, and under stronger 

 a negative movement. 



Similar responses to stimulus are exhibited under galvanic 

 excitation. The normal response here also undergoes reversal 

 under long-continued stimulation ; since the actions of acids 

 and alkalis are antagonistic, organisms reared under acid 

 and alkali cultures tend to exhibit opposite reactions under 

 galvanotaxis. 



Under chemical stimulation, swimming organisms exhibit 

 multiple responses. Acid and alkali, owing to their an- 

 tagonistic actions, bring about opposite responses. The. 

 same reagent again occasions opposite responses, according 

 to the amount of the dose. Thus antherozoids of ferns are 

 attracted by the dilute solution of a malate, but stronger 

 solutions exert a repellent action. 



