310 PLANT RESPONSE 



is only after the lapse of an interval that the protoplasm 

 regains its original properties. There is thus an oscillatory 

 variation of conductivity and excitability. It will therefore 

 be seen how, under the circumstances, a constant stimulus, 

 or a stimulus which is latent in the tissue, can find an excitatory 

 expression only in a pulsating manner. Perhaps a physical 

 model will enable us to visualise this process. Imagine a 

 reservoir into which flows a constant supply of water. An 

 elastic conducting pipe is led from the reservoir, and this pipe 

 is constricted by a compressing spring. On the far open end 

 of this pipe abuts the flattened end of an indicating lever. 

 When water has been supplied for some time, its level is 

 gradually raised, producing an increasing pressure. At a 

 certain point, when the pressure becomes sufficiently great, 

 the spring which keeps the elastic tube constricted gives way, 

 and there is an impulsive discharge of water, which, impinging 

 against the lever, gives rise to a visible response. But the 

 yielding spring again closes, the tube is once more constricted, 

 and thus by the oscillation of the spring which regulates the 

 conduction of water, pulsating hydraulic impulses are kept 

 up. On account of the oscillating mechanism, the outflow, 

 and consequent mechanical response, are periodic, though the 

 supply is constant. In this model the first period, before the 

 pressure of water becomes sufficient to force open the spring, 

 corresponds to the latent period in plant response ; the 

 oscillation of the flow of water corresponds to the oscillation 

 of conductivity ; and the responding lever corresponds to the 

 motile leaflet. Similarly, an ill-fitting spring tap is thus often 

 thrown into a pulsating movement, by the constant pressure 

 of water from the main, and there is then seen a rhythmic 

 play of the water-jet. 



1 After-effect ' and its relative persistence. — In the ex- 

 periment on Biophytum under the continued action of light, we 

 saw that for a period of one minute, during which the plant was 

 absorbing light, there was no response. Then after this latent 

 period of one minute, the energy absorbed reached the verge 

 of response. The excitatory discharge was followed by a 



