IRRITABILITY. 435 



thermotropic. Maize-seedlings, on the contrary, curved to- 

 wards the source of heat, and thus proved themselves to be 

 positively thermotropic. Similar results have been obtained 

 by Barthelemy. In his experiments with Hyacinths, he found 

 the roots to be positively, and the leaves to be negatively, 

 thermotropic. 



It was mentioned in the previous lecture (p. 374), that the 

 effect of the action of an external agent upon an irritable 

 organ is not at once exhibited, but that there intervenes be- 

 tween the commencement of the action and the first manifesta- 

 tion of a response, a latent period : and also that the effect 

 of the action persists for a time after the action has ceased. 

 This is very evident in the action of light in producing helio- 

 tropic heterauxesis. Wiesner has shewn, namely, that an 

 organ exposed for a time to unilateral illumination, during 

 which it exhibits no indication of curvature, will, on being 

 placed in darkness, undergo heliotropic curvature. For in- 

 stance, the epicotyls of some seedlings of Phaseohis multiflorus 

 were exposed for one hour to unilateral illumination ; during 

 that time they exhibited no trace of curvature. They were 

 then placed in darkness, and at the end of two hours they 

 had undergone a well-marked positively heliotropic curva- 

 ture. The exposure had induced in them the heliotropism 

 which was subsequently manifested by curvature. This 

 Wiesner speaks of as photomechanical induction. From his 

 further investigation of this subject, Wiesner came to the 

 conclusion that when photomechanical induction has once 

 taken place, any further exposure to light is entirely without 

 effect upon either the rapidity or the extent of curvature. 

 In some cases (Cress, Vetch) Wiesner ascertained the length 

 of exposure necessary for photomechanical induction ; in both 

 these plants, which are very sensitive, it amounted to one-third 

 of the latent period, the latent period being 25 minutes for 

 the Cress and 35 for the Vetch. The latent period and the 

 persistent effect can, in fact, only be determined, or even per- 

 ceived at all, in plants which respond rapidly to the stimulus 

 of light. 



We will now direct our attention to the heliotropic organ, 



282 



