Page:Darwin - On the movements and habits of climbing plants.djvu/3

 memoirs by Dutrochet. Nevertheless I believe that my observations, founded on the close examination of above a hundred widely distinct living plants, contain sufficient novelty to justify me in laying them before the Society.

Climbing plants may be conveniently divided into those which spirally twine round a support, those which ascend by the movement of the foot-stalks or tips of their leaves, and those which ascend by true tendrils,—these tendrils being either modified leaves or flower-peduncles, or perhaps branches. But these subdivisions, as we shall see, nearly all graduate into each other. There are two other distinct classes of climbing-plants, namely those furnished with hooks and those with rootlets; but, as such plants exhibit no special movements, we are but little concerned with them; and generally, when I speak of climbing plants, I refer exclusively to the first great class.

This is the largest subdivision, and is apparently the primordial and simplest condition of the class. My observations will be best given by taking a few special cases. When the shoot of a Hop (Humulus Lupulus) rises from the ground, the two or three first-formed internodes are straight and remain stationary; but the next-formed, whilst very young, may be seen to bend to one side and to travel slowly round towards all points of the compass, moving, like the hands of a watch, with the sun. The movement very soon acquires its full ordinary velocity. From seven observations made during August on shoots proceeding from a plant which had been cut down, and on another plant during April, the average rate during hot weather and during the day was 2 h. 8 m. for each revolution; and none of the revolutions varied much from this rate. The revolving movement continues as long as the plant continues to grow; but each separate internode, as it grows old, ceases to move.

To ascertain more precisely what amount of movement each internode underwent, I kept a potted plant in a well-warmed room to which I was confined during the night and day. A long inclined shoot projected beyond the upper end of the supporting