552 PLANT RESPONSE 



(2) Ordinary tonic condition. — It will thus be seen that 

 inferences drawn from experiments on plasmolysis may lead 

 us to very wrong conclusions unless we take full account of 

 the possible excitatory action of the particular solution. 

 This is made strikingly evident by the following experiment. 

 If we apply a strong solution — say 10 per cent. — of sodium 

 chloride to the pulvinus of Mimosa, then, arguing entirely 

 from the theory of plasmolysis, we must expect that the 

 withdrawal of water will cause flaccidity of the tissue, and 

 so bring about the fall of the leaf. This flaccidity, further 

 increasing with the duration of application, would tend also to 

 increase the fall of the leaf progressively. But a similar fall 



Fig. 229. Response of Leaf of Mimosa in Ordinary Tonic Condition 

 to the Chemical Stimulus of 10 per Cent. Solution of Salt 



Under continuous stimulation the normal responsive fall is followed by 

 fatigue-relaxation. 



may, on the other hand, be due to an excitatory reaction 

 caused by strong salt, and there is nothing at first sight 

 which would enable us to distinguish the one from the other. 

 We know, however, as regards Mimosa, that under a con- 

 tinuous application of stimulus— as, for instance, rapidly 

 succeeding mechanical or electrical shocks — the first fall 

 of the leaf is succeeded by a return to the erect posi- 

 tion. If the predominant effect of salt, therefore, in the 

 given instance, be to induce a responsive fall by excitation, 

 then its continuous operation should give rise to a subse- 

 quent erection. And from the automatic curve recorded on 

 the smoked drum this is found to be the case (fig. 229) ; 



