92 PLANT RESPONSE 



spiral tendrils, on receiving electric stimulation, began 

 instantly to uncurl — their free ends, as they did so, sweeping 

 through large arcs — and then straightened themselves out. 

 Startling as this was, however, it was not all. I have already 

 alluded to the phenomena of successive excitations and 

 alternating fatigue, of the different sides of an anisotropic 

 organ, under continuous stimulation. Often, owing to this 

 peculiarity, the tendrils after their first uncoiling action 

 showed, though with less vigour, the movement of recurling. 

 These violent contortions were strongly suggestive of the 

 writhing of a worm under torture. 



Though the response to the stimulus of strong electrical 

 shock is the most vigorous, yet this responsive movement of 

 uncoiling can also be obtained by other forms of stimulation, 

 such as the thermal and chemical. For this purpose we may 

 dip the spiral tendril into hot water, or into dilute sulphuric 

 acid. The differential contractile response may then be 

 observed. 



We have thus traced out in unbroken continuity the 

 various types of mechanical response as seen in plants. To 

 begin with, we have observed the responsive longitudinal 

 contraction, pure and simple, of a strictly radial organ. Next, 

 in the case of plagiotropic stems, the same longitudinal 

 contraction, but acting differentially, produced lateral move- 

 ment, the differential action being the result of induced 

 molecular anisotropy and consequent difference in the 

 excitabilities of the two halves, as a result of which, a 

 plagiotropic stem functions as a diffuse pulvinoid. From 

 this we pass to the anatomical anisotropy, which may be 

 observed in the dorsi-ventral petioles of ordinary leaves. 

 Here we find a tendency in the diffuse pulvinoid to become 

 contracted to certain definite areas ; and the responsive 

 movement, in such cases, also, is brought about by differential 

 longitudinal contraction of the upper and lower halves. And, 

 finally, we discover the culminating type of such differentia- 

 tion for the purpose of motile efficiency, in the pulvinus of the 

 so-called ' sensitive ' plants, where also responsive movement 

 is brought about by differential longitudinal contraction. 



