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Show 548 SUMMARY AND CnAP. XII. modified, and the part bonds either to or from the exciting cause; or it may occupy a new position, as in the so-called sleep of leaves. The influence which modifies circumnutation may be transmitted from one part to another. Innate or constitutional changes, independently of any external agency, often modify tho circumnutating movements at particular periods of the lifo of the plant. As circumnutation is universally present, we can understand how it is that movements of the same kind have been developed in the most distinct members of the vegetable series. But it must not be supposed that all tho movements of plants arise from modified circumnutation; for, as we shall presently see, there is reason to believe that this is not the case. Having made those few preliminary remarks, we will in imagination take a germinating seed, and consider the part which the various movements play in the life-history of the plant. The first change is the protrusion of the radicle, which begins at once t? circumnutate. 'rhis movement is immediately modi· fied by the attraction of gravity an~ rendered geotropic. The radicle, therefore, supposmg the seed to be lying on the surface, quickly bends downwards, following a more or less spiral course, as was s~en .on the smoked glass-plates. Sensitiveness to gravitatiOn :esides in the tip; and it is the tip which .transmits some influence to the adjoining parts, causmg them to bend. As soon as the tip, protected by the roo~cap, teaches the ground, it penetrates the surf~ce, ~f this be soft or friable ; and the act of penetratiOn. IS apparently aided by the rocking or circumnutatmg movement of the whole end of the radicle. If the sur· face 1. s compact, an d cannot easi·1 y· b e pen etrated ' then CHAP. XII. CONCLUDING REMARKS. 549 the seed itself, unless it be a heavy one, is displaced or lifted up by the continued growth and elongation of the radicle. But in a state of nature seeds often get covered with earth or other matter, or fall into crevices, &c., and thus a point of resistance is afforded and the tip can more easily penetrate the o-ronnd: But even with seeds lying loose on the surfa~e there is another aid: a multitude of excessively fine hairs are emitted from the upper part of the radicle, and these attach themselves firmly to stones or other objects lying on the surface, ~nd can do so even to glass; and thus the upper part Is held down whilst the tip presses against and penetrates the ground. The attachment of the root-hairs is effected by the liquefaction of the outer surface of the cellulose walls, and by the subsequent setting hard of the liquefied matter. This curious process probably takes place, not for th~ sake of ~he attachment of the radicles to superficial obJects, but m order that the hair may be brought into the closest contact with the particles in the soil by which means they can absorb the layer of water ~urrounding them, together with any dissolved matter. After the tip has penetrated the ground to a little depth, the increasing thickness of the radicle too-ether w'rht th e root-hairs, hold it securely in its pl' aceb ; and now the_ force .exerted by the longitudinal growth of ~he. radicle dnves the tip deeper into the ground. his force, combined with that due to transverse growth, give~ to the radicle the power of a wedge. Even a gr?wmg root of moderato size, such as that of a seedlm~ bean, can displace a weight of some ~ounds. It Is not probable that the tip when buried 1~ co~pact earth can actually circumnutate and thus ard 1ts downward passage, but the circumnutating movement will facilitate the tip entering any lateral |
