CHEMO-TROPISM AND GALVANO-TROPISM 561 



was carried out using an E.M.F. of ten volts, the two elec- 

 trodes being applied at a distance of 10 cm. from each other. 

 This meant a potential gradient of one volt per cm. This 

 relatively strong voltage was applied for the sake of obtaining 

 measurable response within a short time. But a much 

 smaller E.M.F. is found to produce similar effects, though 

 the action is slower. The application of a strong E.M.F. has 

 the disadvantage of inducing fatigue, which is a drawback 

 not present in the use of a feeble E.M.F. It is, however, 

 shown that, during the continuation of a current through the 

 soil, the rate of growth of a plant is enhanced. 



Summary 



The effects of unilateral chemical and galvanic excita- 

 tion may be studied, both from pulvinar and growth 

 movements. 



The unilateral application of alkali gives rise to positive 

 curvature, that is to say, a movement towards the stimulat- 

 ing agent. 



Acids, under similar circumstances, give rise to negative 

 curvature. 



The unilateral application of copper sulphate gives rise, by 

 retardation of growth, to a concavity or positive responsive 

 movement. 



The unilateral action of dilute solution of sugar, by 

 enhancing the rate of growth, induces negative response. A 

 strong solution, however, gives the reversed or positive 

 response. 



Effects fundamentally similar are expressed by swimming 

 organisms in appropriate movements to and from sources of 

 chemical stimulation. 



The assumption that the variation of curvature which 

 occurs when an already curved organ is placed in strong 

 solutions of salt is due to the action of plasmolysis is not 

 always justified, for such solutions also exert characteristic 

 excitatory effects. 



The pulvinus of Mimosa in a favourable tonic condition 



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