Page:Popular Science Monthly Volume 17.djvu/655

Rh of movement by which its shoot may sometimes point in one direction, sometimes in another. Bat this is only half the phenomenon, and, if we examine closely, we shall find that the movement is constant and regular, the stem first pointing north, then east, then south, then west, in regular succession, so that its tip is constantly traveling round and round like the hand of a watch, making on an average, in warm August weather, one revolution in two hours. Here, then, is a most curious power possessed by the shoots of twining plants, which is worth inquiring further into, both as regards the way in which the movement is produced, and as to how it can be of any service to the plant. Questions are often asked in gardening periodicals as to how hops or other climbing plants always manage to grow precisely in the direction in which they will find a support. This fact has surprised many observers, who have supposed that climbing plants have some occult sense by which they discover the whereabouts of the stick up which they subsequently climb. But there is in reality no kind of mystery in the matter: the growing shoot simply goes swinging round till it meets with a stick, and then it climbs up it. Now, a revolving shoot may be more than two feet long, so that it might be detained in its swinging-round movements by a stick fixed into the ground at a distance of nearly two feet. There would then be a straight bit of stem leading from the roots of the plant, in a straight line to the stick up which it twines, so that an observer who knew nothing of the swinging-round movement might be pardoned for supposing that the plant had in some way perceived the stick and grown straight at it. This same power of swinging round slowly comes into play in the very act of climbing up a stick.

Suppose I take a rope and swing it round my head: that may be taken to represent the revolving of the young hop-shoot. If, now, I allow it to strike against a rod, the end of the rope which projects beyond the rod curls freely round it in a spiral. And this may be taken as a rough representation of what a climbing plant does when it meets a stick placed in its way. That is to say, the part of the shoot which projects beyond the stick continues to curl inward till it comes against the stick; and, as growth goes on, the piece of stem which is projecting is, of course, all the while getting longer and longer; and, as it is continually trying to keep up the swinging-round movement, it manages to curl round the stick. But there is a difference between the rope and the plant in this—that the rope curls round the stick at the same level as that at which it is swung, so that, if it moves round in an horizontal plane at a uniform height above ground, it will curl round the stick at that level, and thus will not climb up the stick it strikes against, But the climbing plant, although it may swing round when searching for a stick, at a fairly uniform level, yet, when it curls round a stick, does not retain a uniform distance from the ground, but by winding round like a corkscrew it gets higher and higher at each turn.