Page:Darwin - On the movements and habits of climbing plants.djvu/93

 the Vitaceæ, Sapindaceæ, and Passifloraceæ, are modified flower-peduncles. This is likewise the case, according to De Candolle (as quoted by Mold), with the tendrils of Brunnichia, one of the Polygonaceæ. In two or three species of Modecca, one of the Papayaceæ, the tendrils, as I hear from Prof. Oliver, occasionally bear flowers and fruit; so that at least they are axial in their nature.

.—This movement, which shortens the tendrils and renders them elastic, commences in half a day or in a day or two after their extremities have caught some object. There is no such movement in any leaf-climber, with the exception of an occasional trace of it in the petioles of Tropæolum tricolorum. On the other hand, it occurs with all tendrils after they have seized some object, with the few following exceptions,—namely Corydalis claviculata, but then this plant might still be called a leaf-climber; Bignonia unguis and its close allies, and the Cardiospermum; though these tendrils are so short that the contraction could hardly take place, and would be quite superfluous; and Smilax aspera, the tendrils of which, though rather short, offer a more marked exception. In the Dicentra, whilst young, the tendrils are short and do not contract spirally, but only become slightly flexuous; the longer tendrils, however, borne by older plants contract spirally. I have seen no other exceptions to the rule that all tendrils, after clasping by their extremities a support, contract spirally. When, however, the tendril of any plant of which the stem happens to be immoveably fixed, catches some fixed object, it does not contract, simply because it cannot; this, however, rarely occurs. In the common Pea only the lateral branches, and not the central stem of the tendril, contract; and with most plants, such as the Vine, Passiflora, Bryony, the basal portion never contracts into a spire.

I have said that in Corydalis claviculata the end of the leaf or the tendril (for this part may be indifferently thus designated) does not contract into a spire. The branchlets, however, of the tendril, after they have wound round thin twigs, become deeply sinuous or zigzag; and this may be the first indication of the process of spiral contraction. Moreover the whole end of the petiole or tendril, if it seizes nothing, ultimately bends abruptly downwards and inwards, showing that its inferior surface contracts; and this may be confidently looked at as the first indication of the power of spiral contraction. For with all true tendrils when they contract spirally, it is the lower surface, as Mohl (S. 52) has remarked, which contracts. If the inferior surface of