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Show 154 SENSITIVENESS OF THE RADICLE. CHAP. III. curve upwards, this movement will be opposed by geotropism acting only at a very oblique anglo, and the irritation from the card will be strengthened by its previous action. We may therefore conclude that the initial power of an irritant on the apex of the radicle of the bean, is less than that of geotropism when acting at right angles, but greater than that of geotropism when acting obliquely on it. Sensitiveness of the tips of the Secondary R adicles of tlw Bean to contact.-All the previous observations relate to the ma]n or primary radicle. Some beans suspended to cork-licls, with their radicles dipping into water, had developed secondary or lateral radiclos, which were afterwards 1 opt in very clamp air, at the proper low temperature for full sensitiveness. They projected, as usual, almost horizontally, with only a slight downward curvature, and retained this position during several days. Sachs has shown* that these secondary roots are acted on in a peculiar manner by geotropism, so that if displaced they reassume their former sub-horizontal position, and do not bend verti· cally downwards like the primary radicle. Minute squares of the stiff sanded paper wore affixed by means of shellac (but in some instances with thick gum-water) to the tips of 39 secondary racliclos of different ages, generally the uppermost ones. Most of the squares were fixed to the lower sides of the apex, so that if they acted the radicle would bond upwards; but some were fixed laterally, and a few on the upper side. Owing to the extreme tenuity of these raclicles, it was very difficult to attach the square to the actual apex. Whether owing to this or some other circumstance, only nine of the squares induced any * 'Arbciten Bot. Inst., Wiirzburg,' Heft iv. 1874, p. 605-617. CHAP. III. SENSITIVENESS OF THE RADICLE. 155 curvature. Tho curvature amounted in some cases to about 45° above the horizon, in others to 90°, and then the tip pointed to the zenith. In one instance a distinct upward curvature was observed in 8 h. 15 m., but usually not until 24 h. had elapse<!. Although only 9 out of 39 radiclos were affected, yet the curvature was so distinct in several of them, that there could be no doubt that tho tip is sensitive to slight contact, and that the growing part bends away from the touching object. It is possible that some secondary radicles are more sensitive than others ; for Sachs has proved* the interesting fact that each individual secondary radicle possesses its own peculiar constitution. Sensitiveness to contact of the Primary Radicle, a little above the apem, in the Bean ( Vicia faba) and Pea (Pisum sativum ).-'fhe sensitiveness of the apex of the radicle in the previously described cases, and the consequent curvature of the upper part from the touching object or other source of irritation, is the more remarkable, because Sachs t has shown that pressure at the distance of a few millimeters above the apex causes the radicle to be~d, li~e a tendril, towards the touching object. By fixmg pms so that they pressed against the radicles o~ beans suspended vertically in damp air, we saw this kmd of curvature; but rubbing the part with a twig or needle for a few minutes produced no effect. Haberlandt remarks,+ that these radicles in brcakino- through the seed-coats often rub and press against the 5 ruptured edges, and consequently bend round. th m. As little squar.es of the card-like paper affixed with shellac to the t1ps were highly efficient in causing the radicles to bend away from them, similar pieces (of about ~th b • ',Arbciten Bot. Instit., Wtirz- t 'DieSchutzoinrichtungen der utrg , ~eft. iv. 1874, p. G20. K . fl Ib1d. Heft iii. Ul73, p. 437 . e1mp anze,' 1877, p. 25. |