A rigorous examination of other young plants would probably show slight spontaneous movements in their stems, petioles or peduncles, as well as sensitiveness to a touch. We see at least that the Maurandia might, by a little augmentation of the powers which it already possesses, come first to grasp a support by its flower-peduncles, and then, by the abortion of some of its flowers (as with Vitis or Cardiospermum), acquire perfect tendrils.
There is one other interesting point which deserves notice.We have seen that some tendrils owe their origin to modified leaves, and others to modified flower-peduncles; so that some are foliar and others axial in their nature.It might therefore have been expected that they would have presented some difference in function.This is not the case.On the contrary, they present the most complete identity in their several characteristic powers.Tendrils of both kinds spontaneously revolve at about the same rate.Both, when touched, bend quickly to the touched side, and afterwards recover themselves and are able to act again.In both the sensitiveness is either confined to one side or extends all round the tendril.Both are either attracted or repelled by the light.The latter property is seen in the foliar tendrils of Bignonia capreolata and in the axial tendrils of Ampelopsis.The tips of the tendrils in these two plants become, after contact, enlarged into discs, which are at first adhesive by the secretion of some cement.Tendrils of both kinds, soon after grasping a support, contract spirally; they then increase greatly in thickness and strength.When we add to these several points of identity the fact that the petiole of Solanum jasminoides, after it has clasped a support, assumes one of the most characteristic features of the axis, namely, a closed ring of woody vessels, we can hardly avoid asking, whether the difference between foliar and axial organs can be of so fundamental a nature as is generally supposed?
We have attempted to trace some of the stages in the genesis of climbing plants.But, during the endless fluctuations of the conditions of life to which all organic beings have been exposed, it might be expected that some climbing plants would have lost the habit of climbing.In the cases given of certain South African plants belonging to great twining families, which in their native country never twine, but reassume this habit when cultivated in England, we have a case in point.In the leaf-climbing Clematis flammula, and in the tendril-bearing Vine, we see no loss in the power of climbing, but only a remnant of the revolving power which is indispensable to all twiners, and is so common as well as so advantageous to most climbers.In Tecoma radicans, one of the Bignoniaceae, we see a last and doubtful trace of the power of revolving.
With respect to the abortion of tendrils, certain cultivated varieties of Cucurbita pepo have, according to Naudin, either quite lost these organs or bear semi-monstrous representatives of them.In my limited experience, I have met with only one apparent instance of their natural suppression, namely, in the common bean.
All the other species of Vicia, I believe, bear tendrils; but the bean is stiff enough to support its own stem, and in this species, at the end of the petiole, where, according to analogy, a tendril ought to have existed, a small pointed filament projects, about a third of an inch in length, and which is probably the rudiment of a tendril.
This may be the more safely inferred, as in young and unhealthy specimens of other tendril-bearing plants similar rudiments may occasionally be observed.In the bean these filaments are variable in shape, as is so frequently the case with rudimentary organs; they are either cylindrical, or foliaceous, or are deeply furrowed on the upper surface.They have not retained any vestige of the power of revolving.It is a curious fact, that many of these filaments, when foliaceous, have on their lower surfaces, dark-coloured glands like those on the stipules, which excrete a sweet fluid; so that these rudiments have been feebly utilized.
One other analogous case, though hypothetical, is worth giving.
Nearly all the species of Lathyrus possesses tendrils; but L.
nissolia is destitute of them.This plant has leaves, which must have struck everyone with surprise who has noticed them, for they are quite unlike those of all common papilionaceous plants, and resemble those of a grass.In another species, L.aphaca, the tendril, which is not highly developed (for it is unbranched, and has no spontaneous revolving-power), replaces the leaves, the latter being replaced in function by large stipules.Now if we suppose the tendrils of L.
aphaca to become flattened and foliaceous, like the little rudimentary tendrils of the bean, and the large stipules to become at the same time reduced in size, from not being any longer wanted, we should have the exact counterpart of L.nissolia, and its curious leaves are at once rendered intelligible to us.
