498 PHYSIOLOGY OF GROWTH 



is caused to curve by brief contact with a solid body, and 

 then left to itself, it straightens again completely, and is once 

 more sensitive to contact. At a high summer temperature, the 

 straightening of the tendrils of Sicyos proceeds comparatively 

 quickly, and even in experiments with Cyclanthera tendrils I 

 found that a tendril strongly curved owing to stimulation, had, at 

 a temperature of about 22 C., extended again in the course of an 

 hour, and was once more sensitive to stimulation. 



If a branch of a Sicyos tendril is touched at different points 

 with a thin wooden rod, we shall easily make out that the irrita- 

 bility of the organ is especially great in its anterior third, but 

 diminishes considerably as we pass thence towards the base. We 

 can further determine that in the sensitive zone, only one flank of 

 the branch reacts to the stimulus of contact, and in fact only that 

 side is sensitive to contact which, when the branch is still very 

 young, and therefore still spirally coiled, forms the convexity. It 

 may here be remarked, however, that there are other plants in 

 which the tendrils are not sensitive merely on one side, but on all 

 sides. To convince ourselves further of the very great sensitive- 

 ness of the tendrils of Sicyos or Cyclanthera, it is to be recom- 

 mended to place gently on the tips of the tendrils, fragments of 

 cotton yarn or small paper riders, a few mg. in weight ; curvatures 

 very quickly take place. Other tendrils, e.g. those of Vitis, which 

 we shall carefully examine later, are far less sensitive than those 

 we have been considering ; loads a few mg. in weight do not, as a 

 rule, cause them to bend inwards at all. 



It is of essential importance, and easily proved, that tendrils 

 are not simply sensitive to pressure, impact, or contact, but that 

 only a particular form of contact acts as a stimulus. 2 If tendril- 

 bearing shoots of Sicyos are violently shaken, care being taken 

 to avoid any contact of the tendrils with a solid body, shock 

 curvatures may appear in the tendrils, but the striking effect 

 which is brought about by contact with a solid body is absent. 

 This experiment teaches also thnt friction against the air does not 

 act on the tendrils as a stimulus. Further, a stream of water 

 directed, e.g., by means of a wash bottle against the irritable side 

 of the tendrils of Sicyos, does not stimulate them. The sensitive 

 leaves of Mimosa pndica, as I satisfied myself, behave quite dif- 

 ferently from tendrils, both towards a stream of water and to 

 simple shock \_Erscliiitterung~]. These leaves are stimulated by 

 any kind of mechanical shock whatever, and if, e.g., we direct a 



