Page:Popular Science Monthly Volume 17.djvu/664

646 to obey, and therefore remained motionless. It was in reality extremely sensitive to a touch on any one of its sides.

There is a remarkable movement which occurs in tendrils after they have caught an object, and which renders a tendril a better climbing organ than any sensitive leaf. This movement is called the "spiral contraction," and is shown in Fig. 4, which represents the spirally contracted tendril of the wild bryony; it may also be seen in Fig. 5, which represents the tendrils of the Virginia creeper. When a tendril first seizes

 A. Tendril fully developed, with a young leaf on the opposite side of the stem. B. Older tendril, several weeks after its attachment to a wall, with the branches thickened and spirally contracted, and with the extremities developed into disks. The unattached branches of this tendril have withered and dropped off.

an object it is quite straight, with the exception of the extreme tip, which is firmly curled round the object seized. But in a day or two the tendril begins to contract, and ultimately assumes the corkscrew-like form represented in the figures. It is clear that in spirally contracting the tendril has become considerably shorter; and, since the end of the tendril is fixed to a branch, it is obvious that the stem of the bryony must be dragged nearer to the object which its tendril has caught. Thus, if a shoot of bryony seizes a support above it, the contraction of the tendril will pull up the shoot in the right direction. So