These tendrils can be made to curve by a weight exceedingly small. The most sensitive part of our own skins is quite unable to distinguish so small a weight as is perceived by these tendrils. Even the sensation of taste can only be produced by a weight eight times as great as that shown by some of them. Tendrils curve very quickly after they have been touched. In twenty seconds some tendrils curve (Cyclanthera), others (Passiflora) take thirty seconds, and some of them require four to five minutes or even longer before they make up their minds to coil.
Even more remarkable, however, is the fact that they do not coil when raindrops fall on them, giving a much harder blow than small weights. If one tendril touches or rubs against another, it is said not to curve. They are persevering little things also, for Darwin got a passion-flower tendril to curve when struck or rubbed no less than twenty-one times during fifty-four hours.
If one reflects on all these curious facts, it is difficult to help feeling that these plants behave very much in the way that a reasonable animal would do. There are many other cases in which some vegetable does exactly what we should expect of reasonable beings under the circumstances. The tip of the root (see p. [89]), the Sensitive Plant, the Monkey and Barberry flowers, are all well-known cases.
So that it is difficult to find anything in science to contradict the comfortable belief that wide-open flowers and stretched-out leaves of plants as they drink in the warm rays of the sunlight are really enjoying themselves, whilst they are doing their day's work.
All these interesting facts are so beautifully described and so carefully summed up by Charles Darwin, that we shall only earnestly recommend our readers to get first that fascinating book The Power of Movement in Plants, and then read all the rest of his works.[141]
There are an extraordinary number of these plants and the tendrils are formed exactly where they will be most useful. Every part of a leaf may become a tendril. The whole leaf is changed into one in some kinds of Lathyrus. In a very beautiful creeper which is not so often grown in greenhouses as it might be (Gloriosa superba), the tip of the leaf only acts as a tendril. Leaflets are often made into tendrils. The Clematis is the most economical of them all, for the leaf-stalk coils round and forms little woody rings which hold up the plant.
Before leaving the subject of tendrils, it may be interesting to notice the queer corkscrew spirals in which they roll themselves up. These spirals are formed after the end of the tendril has tied itself to the support and become woody. The free part between the end and its own stem goes on revolving; now if you tie a piece of string at both ends and make it revolve, you will see at once that it must coil itself into a double spiral, one part in one direction and the other in the opposite way, with a flat piece between them.
One might be disposed to think no more about these double coils; but here comes in one of the curious, inexplicable coincidences which happen so often in plant life. Such a coil is much stronger than a straight bit of wire or string would be, because if pulled out it yields and is springy. That of course makes it less probable that the tendril will be broken. Attached by a series of wiry springs, the plant yields and sways to the wind, and it is not likely that it will be torn away. Besides this, the coiling of the tendril pulls the stem closer to its support, which is also a great advantage.
Certain Virginian Creepers and Vines behave in quite a different manner. The tendrils grow away from the light and so seek the shadow of the leaves. They are also divided into little branches. At the tip of each little branch is a small knob; if this should touch the wall or the trunk of a tree, etc., it immediately secretes a drop of cement and glues itself firmly to the wall. There is a curious difference in different sorts of Ampelopsis in this respect. There is no adhesive pad in one of them (Ampelopsis hederacea) until it touches, whilst A. Veitchii has them more or less ready for gluing before they touch (though they become much larger and better developed as soon as they rub against the wall).[142]
One of the most interesting of our common climbers, "that rare old plant the Ivy green," has not yet been mentioned. It is exceedingly decorative on walls, especially on ruins and on old tree-trunks in winter time, where its dark, brilliant green is most effective.