90 



MK. DARWIN" OX CLIMBING! PLANTS. 



millegramines) placed most geutly on the tip, thrice plainly caused 

 it to curve ; as twice did a bent bit of thin platina wire weighing 

 ^th of a grain ; but this latter weight, when left suspended, did 

 not suffice to cause permanent curvature. These trials were made 

 under a bell-glass, so that the loops of thread and wire were not 

 agitated by the wind. The movement after a touch is very rapid : 

 I took hold of the lower part of several tendrils and then touched 

 with a thin twig their concave tips, and watched them carefully 

 through a lens ; the tips plainly began to bend in the following 

 times— 31, 25, 32, 31, 28, 39, 31, and 30 seconds; so that the 

 movement was generally perceptible in half a minute after the 

 touch, but once plainly in 25 seconds. One of the tendrils which 

 thus became bent in 31 seconds had been touched two hours pre- 

 viously and had coiled into a helix ; thus in this interval it had 

 straightened itself and had perfectly recovered its sensibility. 



I repeated the experiment made on the Echinoci/stis, and placed 

 several plants of this Passiflora so close together that the tendrils 

 were repeatedly dragged over each other ; but no curvature ensued. 

 I likewise repeatedly flirted small drops of water from a brush on 

 many tendrils, and syringed others so violently that the whole 

 tendril was dashed about, but they never became curved. The 

 impact from the drops of water on my hand was felt far more 

 plainly than that from the loops of thread (weighing ^nd of a 

 grain) when allowed to fall on it ; and these loops, which caused 

 the tendrils to become curved, had been placed most gently on 

 them. Hence it is clear, either that the tendrils are habituated to 

 the touch of other tendrils and to that of drops of rain, or that 

 they are sensitive only to prolonged though excessively slight 

 pressure. To show the difference in the kind of sensitiveness in 

 different plants and likewise to show the force of the syringe used, 

 I may add that the lightest jet from it instantly caused the leaves 

 of a Mimosa to close ; whereas the loop of thread weighing ^nd 

 of a grain, when rolled into a ball and gently placed on the glands 

 at the bases of the leaflets of the Mimosa, caused no action. Had 

 I space, I could advance much more striking cases in plants both 

 belonging to the same family, of one being excessively sensitive to 

 the lightest pressure if prolonged, but not to a brief impact ; and 

 of another plant equally sensitive to impact, but not to slight 

 though prolonged pressure. 



Passiflora punctata. — The intcrnodes do not move ; but the 

 tendrils regularly revolve. One that was about half-grown and 

 very sensitive made three revolutions, opposed to the course of 



