76 MB. DARWIN ON CLIMBING PLANTS. 



of a grain. So it would appear that tlie tendrils are habituated 

 to drops of water or to rain ; for artificial rain made by violently 

 flirting a wet brush produced not the least effect on them. I 

 repeatedly rubbed rather roughly the lower part of a tendril, 

 but never caused any curvature; yet this part is sensitive to 

 prolonged pressure, for when it came into contact with a stick, it 

 would slowly bend round it. 



The revolving movement is not stopped by the extremity curl- 

 ing after having been touched. When one of the lateral branches 

 of a tendril has firmly clasped any object, the middle branch con- 

 tinues to revolve. "When a stem is bent down and secured, so 

 that its tendril depends but is left free to move, its previous re- 

 volving movement is nearly or quite stopped ; but it begins to 

 rise in a vertical plane, and as soon as it has become horizontal 

 the revolving movement recommences. I tried this four times ; 

 generally the tendril rose to a horizontal position in an hour or an 

 hour and a half ; but in one case, in which the tendril depended 

 at an angle of 45° beneath the horizon, the movement took two 

 hours ; in another half-hour the tendril rose to 23° above the 

 horizon and recommenced revolving. This upward vertical move- 

 ment is independent of the action of light, for it took place twice 

 in the dark, and another time with the light comi^j in on one' side 

 alone. The movement no doubt is guided by opposition to the force 

 of gra\dty, as in the case of the' ascent of the plumules of germi- 

 nating seeds. 



A tendril does not long retain its revolving power ; as soon as 



this ceases, it bends downwards and contracts spiraUy. But after 



the revolving movement has ceased the tip still retains for a short 



time its sensitiveness to contact, but this can be of little service to 

 the plant. 



Though the tendril is highly flexible, and though the extremity 



an 



is so great tha 

 its path. The 



quarter 



smooth, cylindrical stick (and I repeated the experiment seven 

 times) so far from a tendril, that its extremity could only curl hal£ 

 or three-quarters round the stick ; but I always found in the 

 course of a few hours afterwards that the tip had managed to 

 curl twice or even thrice quite round the stick. I at first thought 

 that this was due to rapid growth ; but by coloured points- and 

 measurements I proved that there was no sensible increase of 



