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Show 544 SBNSI'l'IVENESS TO GRAV11'A'l'ION. C HAP. Xt. part which is sensitive to geotropism in the members of such distinct farnil ies as tho Loguminosro, Malvaeca; Cuenrbitacem and Graminem, we may infer that thi~ eharactoT is cornm m to the roots of most seedling plants. Whilst a root is penetrating the oTonnd, the tip must travel first; and we can soc the advantage of its being sensitive to geotropism, as it has to determine the course of tho whole root. vVhenever th tip is deflected by any subterranean obstacle, it will also he an ad vantage th<tt a considerable lrngth of the root should Lo able to bend, more especially as tho tip itself o·rows slowly mul bonds but little, so that the proper downward course may be soon recovered. But it appears at first sight immaterial whether this were effected by the whole growing part bcjng sensitive to g "otropism, or by an influon e transmitted rxclnsively from the tip. vVe should, howov r, romc.mber that it is the tip which is sensitive to tho contuct of hard objects, causing the radicle to bend away from them, thus guiding it along the lines of least resistance in the soil. It is again the tip which is alone sensitive, at least in some cases, to moisture, causing the radicle to bend towards its source. These two kinds of sensitiveness conquer for a time the sensitiveness to geotropism, which, however, ultimately prevails. 'fherefore the throe kinds of sensitiveness must often ' come into antagonism ; first one prevailing, and then another; and it would be an advantage, perhaps a necessity, for the interweighing and reconciling of these three kinds of sensitiveness, tb<tt they should be all localised in the same group of cells which have to transmit the command to the adjoining parts of the radicle, causing it to benu to or from tho source of irritation . .Finally, the fact of the tip alone being sensitive to CHAP. XI. TRANSMITTED EFFECTS : OONOL USION. 545 the attraction of gravity has an important bem·ing f . on the theory o geotropism. Authors seem genorall t look at the bendin~ of a radicle towards the cent1~ 0~· the earth, as the duect result of gravitation which · believed .to modify the growth of the uppo:· or low~; surfaces, m such a manner as to induce curvature in the proper direction. But we now know that it is the tip alone which is acted on, and that this part transmits some influence to the adjoi~ing parts, causing them t? curve do\V~wards. Gravity does not appear to act m a more dueot manner on a radicle, than it does on any lowly organisec.l animal, which moves away when it feels some weight or pressure. 2 N |