44 PLANT RESPONSE 



wholly insensitive, ordinary mechanical stimulation having no 

 effect on its leaves. But if we apply strong thermal or 

 electrical stimulus to the end of the twig bearing the leaves, 

 then they begin to close very slowly, in serial succession. 

 But whereas with Biophytum the response begins almost 

 instantaneously, the maximum being reached in less than a 

 second, and complete recovery attained in about four minutes, 

 in the leaf of Philanthus urinaria the latent period, for 

 moderate stimulus, is as long as three minutes, the maximum 

 reached in not less than forty-five minutes, and complete 



Fig. 30. Responses of (a) quickly reacting Biophytum, and (b) sluggish 

 Philanthus urinaria, under moderate and (c) under stronger Stimulation 



recovery may require from two to three hours (fig. 30). It is 

 seen, then, that even with regard to the so-called sensitive 

 plants, there is a wide range of sensitiveness. In some, a 

 slight shock produces quick reaction, in others a very intense 

 stimulation is necessary to initiate response, and the reaction 

 itself is very sluggish. 



Anisotropy necessary to lateral response. — We shall 

 now try to understand a little more of the mechanics which 

 cause this responsive curvature. In these pulvinated organs, 

 one thing that is noticeable is that the organ is not isotropic, 

 that is to say, its properties are not the same in all directions. 



