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.[140] [140] For the above facts: Pfeffer, _Pflanzen-Physiologie_, vol. 2, pp. 423-8; Green, _Vegetable Physiology_, p. 389; Kerner, _l.c._, p. 697; Bonnier, _l.c._, p. 305. 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).[141] [141] Henslow, _Origin of Plant Structures_, p. 223.