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Show 130 SENSITIVENESS OF RADICLES. CHAP. III. it, pressing on it with very little force. H?w far such abrupt changes in its former course are mdod by the circumnutation of the tip must be left doubtful. rrhin slips of wood were cemented on more or less steeply inclined glass-plates, at right angles to the radicles which were gliding down them. Straight lines had been painted along the growing terminal part of some of these radicles, before they met the opposing slip of wood; and the lines became sensibly curved in 2 h. after the apex had come into contact with the slips. In one case of a radicle, w11ich was growing rather slowly, the root-cap, after encountering a rough slip of wood at right angles, was at first slightly flattened transversely: after an interval of 2 h. 30 m. the flattening became oblique; and after an additional 3 hours the flattening had wholly clisnppcarcd, and the apex now pointed at right angles to its former course. It then continued to grow in its now direction alongside the slip of wood, until it came to the end of it, round which it bent re ·tangnlnrly. Soon afterwards when coming to the edge of the plato of glass, it was again bent at a large angle, and de· scended perpendicularly into the damp srmcl. When as in the above cases, radicl cs encountered an obsta~le at ~jght angles to their course, the terminal growing part became curved for a length of between · 3 and · 4 of an inch ( 8-10 mm.), measured from ~he apex. This was well shown by the black lines wJuch had been previously painted on them. The first an? most obvious explanation of the curvature is, tbat 1t results merely from the mechanical resistance to the growth of the radicle in its original direction. Never· .theless, this explanation did not seem to us satisfact~ry. The radicles did not present the app aranco of havmg been subjected to a sufficient pressure to account for CHAP. III. SENSITIVENESS OF RADICLES. 131 their curvature; and Sachs has shown* that the growing part is more rigid than the part immediately above which has ceased to grow, so that the latter might have been expected to yield and become curved as soon as the apex encountered an unyielding object; whereas it was the stiff growing part which became curved. Moreover, an object which yields with the greatest ease will deflect a radicle : thus, as we have seen, when the apex of the radicle of the bean encountered the polisheu surface of extremely thin tin-foil laid on soft sand, no impression was left on it, yet the radicle became deflected at right angles. A second explanation occurred to us, namely, that even the gentlest pressure might check the growth of the apex, and in this case growth could continue only on one side, and thus the radicle '\.vould assume a rectangular form; but this view leaves wholly unexplained the curvature of the upper part, extending for a length of 8-10 mm. vVe were therefore led to suspect that the apex was sensitive to contact, and that an effect was transmitted from it to tho upper part of the radicle, which was th~s excited to bend away from the touchino· object. As a httle loop of fino thread hung on a tendril or on the petiole of a leaf-climbing plant, causes it to bend, w~ thought that any small hard object affixed ~o the tip. of ~ radicle, freely suspended and growing m damp au, might cause it to bend, if it were sensitive, ~nd yet would not offer any mechanical resistance to Its growth. Full details will be given of the ex periments which were tried, as tho result proved remarkable. The fact of tho apex of a radicle bein()' sensitive to contact has never be~n observed, though, ~s we shall "' 'Arbeiten Bot. Inst. Wiirzburg,' Heft iii. 1873, p. 398. K 2 |