Page:Darwin - On the movements and habits of climbing plants.djvu/30

 all. The clasped petiole in the course of two or three days swells greatly, and ultimately becomes nearly twice as thick as the opposite leaf-stalk which has clasped nothing. When thin transverse slices of the two are placed under the microscope their difference is conspicuous: the side of the footstalk which has been in contact with the support is formed of a layer of colourless cells with their longer axes directed from the centre of the petiole, and very much larger than any cells found in the opposite or unchanged petiole; the central cells, also, are in some degree enlarged, and the whole is much indurated. The exterior surface generally becomes bright red. But a far greater change takes place in the nature of the tissues than that which is externally visible: the petiole of the unclasped leaf is flexible, and can be easily snapped, whereas the clasped footstalk acquires an extraordinary toughness and rigidity, so that considerable force is required to pull it into pieces. With this change, great durability is probably acquired; at least this is the case with the clasped petioles of Clematis vitalba. The meaning of these changes is plain, namely, that the petioles may firmly and durably support the stem.

Clematis microphylla, var. leptophylla.—The long and thin internodes of this Australian species revolve sometimes in one direction and sometimes in an opposite one, describing long, narrow, irregular ellipses or large circles: four revolutions were completed within five minutes of the same average rate of 1 h. 51 m.; so that this species moves more quickly than any other of the genus. The shoots, when placed near a vertical stick, either twine round it or clasp it with the basal portions of their petioles. The leaves whilst young are nearly of the same general shape, and act in the same manner like a hook, as will be described under C. viticella; but the leaflets are more divided, as in C. calycina, and each segment whilst young terminates in a hardish point, and is much curved downwards and inwards; so that the whole leaf readily catches and becomes entangled with any neighbouring object. The petioles of the young terminal leaflets are acted on by loops of thread weighing th and th of a grain: the basal portion of the main petiole is much less sensitive, but will clasp a stick against which it presses.

The whole leaf, whilst young, is in continual, spontaneous, slow movement. The stem was secured close to the base of the leaves, and, a bell-glass being placed over the shoot, the movements of the leaves were traced on it during several days. A very