waste all the precious summer weather in climbing a few feet, whereas the same amount of longitudinal growth devoted to twining up a thin stick would have raised them up to the light after which they are striving And as a plant exercises no choice, but merely swings round till it hits against an object, up which it will then try to twine, it seems as if the inability to climb thick stems might be a positive advantage to a plant, by forcing it to twine up such objects as would best repay the trouble.
In the classification of climbing plants, proposed by my father in his book, he makes a subdivision of "hook-climbers." These may be taken as the simplest representatives of that class of climbers which are not twining plants. The common bramble climbs or scrambles up through thick underwood, being assisted by the recurved spines which allow the rapidly growing shoot to creep upward as it lengthens, but prevent it from slipping backward again; the common goose-grass (Galium) also climbs in this way, sticking like a burr to the side of a hedge-row up which it climbs. Most country boys will remember having taken advantage of this burr-like quality of Galium in making sham birds!-nests, the prickly stems adhering together in the desired form. Such plants as the bramble or Galium exhibit none[1] of the swinging-round movement which I have described in twiners: they simply grow straight on, trusting to their hooks to retain the position gained.
In some species of clematis we find a mechanism which reminds one of a simple hook-climber, but is in reality a much better arrangement. The young leaves projecting outward and slightly backward from the stem may remind us of the hooked spines of a bramble, and like them easily catch on neighboring objects, and support the trailing stem. Or the leaf of the species of clematis given in Fig. 1 may serve as an example of a leaf acting like a hook. The main stalk of the leaf is seen to be bent angularly downward at the points where each successive pair of leaflets is attached, and the leaflet at the end of the leaf is bent down at right angles, and thus forms a grappling apparatus. The clematis does not, like the bramble, trust to mere growth, to thrust itself among tangled bushes, but possesses the same powers of revolving in search of a support which simple or true twining plants possess. Indeed, many species of clematis are actually twining plants, and can wind spirally up a stick placed in their way. And the same revolving movement which enables them thus to wind spirally also helps them to search for some holding-place for their hook-or grapple-like leaves, and in many species the search is carried on by the leaves swinging round, quite independently of the revolving movement of the stem on which they are borne.
- ↑ That is to say, the revolving movement is not sufficiently developed to be of practical importance. The same remark is applicable to the other cases in which I have spoken of the absence of revolving movement in the growing parts of plants.
