Page:Darwin - On the movements and habits of climbing plants.djvu/77

 of a grain. So it would appear that the 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 curling after having been touched. When one of the lateral branches of a tendril has firmly clasped any object, the middle branch continues to revolve. When a stem is bent down and secured, so that its tendril depends but is left free to move, its previous revolving 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 movement is independent of the action of light, for it took place twice in the dark, and another time with the light coming in on one side alone. The movement no doubt is guided by opposition to the force of gravity, as in the case of the ascent of the plumules of germinating seeds.

A tendril does not long retain its revolving power; as soon as this ceases, it bends downwards and contracts spirally. 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 travels, under favourable circumstances, at about the rate of an inch in two minutes and a quarter, yet its sensitiveness to contact is so great that it hardly ever fails to seize a thin stick placed in its path. The following case surprised me much: I placed a thin, smooth, cylindrical stick (and I repeated the experiment seven times) so far from a tendril, that its extremity could only curl half 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