Page:Darwin - On the movements and habits of climbing plants.djvu/65

 many trials with black and white glass and cards to prove it, but failed from various causes; yet these trials countenanced the belief. The tendril may be looked at as a leaf split into filaments, with the segments facing in all directions; hence, when the revolving movement is arrested, so that the light shines on them steadily in one direction, there is nothing surprising in their upper surfaces turning towards the light: now this may aid, but will not account for, the whole movement; for the segments would in this case move towards the light as well as turn round to it, whereas in truth the segments or branches of the tendrils not only turn their upper surfaces to the light, and their lower surfaces which bear the hooks to any closely adjoining opaque object (that is, to the dark), but they actually curve or bend from the light towards the dark.

When the Cobæa grows in the open air, the wind must aid the extremely flexible tendrils in seizing a support, for I found a mere breath sufficed to cause the extreme branches of a tendril to catch by their hooks twigs which they could not have reached by the revolving movement. It might have been thought that a tendril thus hooked only by its extremity could not have fairly grasped its support. But several times I watched cases like the following, one of which alone I will describe: a tendril caught a thin stick by the hooks of one of its two extreme branches; though thus held by the tip, it continued to try to revolve, bowing itself out to all sides, and thus moving its branches; the other extreme branch soon caught the stick; the first branch then loosed itself, and then, arranging itself afresh, again caught hold. After a time, from the continued movement of the tendril, a third branch became caught by a single extreme hook; no other branches, as things then remained, could possibly have touched the stick; but before long the main stem, towards its extremity, began just perceptibly to contract into an open spire, and thus to shorten itself (dragging the whole shoot towards the stick), and as it continued to try to revolve, a fourth branch was brought into contact. As the spiral contraction travelled down the main stem and down the branches of the tendril, all the lower branches, one after another, were brought into contact with the stick, and were wound round it and round their own branches until the whole was tied together in an inextricable knot round the stick. The branches of a tendril, though at first so flexible, after having clasped a support for a time, become rigid and even stronger than they were at first. Thus the plant is secured to its support in a perfect manner.