Page:Darwin - On the movements and habits of climbing plants.djvu/70

 modified lateral leaflets; but Mohl (S. 41) ranks them as modified stipules. These tendrils are from 1½ to 1¾ inch in length, are thin, and have slightly curved, pointed extremities. They diverge a little from each other, but stand at first nearly upright. When lightly rubbed on either side, they slowly bend to that side, and subsequently become straight again. The back or convex side of a tendril placed in contact with a stick became just perceptibly curved in 1 h. 20 m., but did not completely surround the stick till 48 h. had elapsed; the concave side of another tendril became considerably curved in 2 h., and fairly clasped the stick in 5 h. As the tendrils grow old, they diverge more from each other and slowly bend towards the stem and downwards, so that they project on the opposite side of the stem to that on which they arise; they still retain their sensitiveness, and can clasp a support placed behind the stem. Owing to this movement, the plant can ascend a thin upright stick, clasping it with the tendrils which arise from the leaves placed alternately on opposite sides of the stem. Ultimately the two tendrils belonging to the same petiole, if they do not come into contact with any object, cross each other (as at B in fig. 7) behind the stem and loosely clasp it. This movement of the tendrils towards and round the stem is, to a certain extent, guided by the action of the light; for when the plant stood so that one of the two tendrils in thus slowly moving had to travel towards the light, and the other from the light, the latter always travelled, as I repeatedly observed, more quickly than its fellow. The tendrils do not contract spirally in any case. Their chance of finding a support depends on the growth of the plant, on the wind, and on their own slow backward and downward movement, which is guided, to