OCR Text |
Show 152 SENSITIVENESS OF THE APEX CHAP. III. of the Bean, compared with that of Geotropism.-We know that when a little square of card or other object is fixed to one side o~ the tip of a verticall.y dependent radicle, the .growmg . ~art bends fro~ It often into a semicircle, In opposition to geotropism, which force is conquered by the effect of the irritation from the attached object. Radicles were therefore extended horizontally in damp air, kept at the proper low temperature for full sensitiveness, and squares of card were affixed with . shellac on the lower sides of their tips, so that If the squares acted, the terminal growing part would curve ~pwards. Firstly, eight beans were so placed that then ~hort, young, horizontally extended radicle.s would be simul: taneously acted on both by geotropism and by Sachs curvature, if the latter came into play; and they all eight became bowed downwards to the centre ?f the earth in 20 h., excepting one which was only slightly acted on. '1\vo of them were a little bowed downwards in only 5 h.! Therefore the cards, affixed to the lower sides of their tips, seemed to produce no eff:c~; ~nd geotropism easily conquered the effects of the u:1tat1~n thus caused. Secondly, 5 oldish radicles, 1-! mch m length, and therefore less sensitive than the above· mentioned young ones, were similarly placed and similarly treated. From what has been seen o~ many other occasions, it may be safely infened that 1f they had been suspended vertically they would have bent away from the cards; and if they had been extended horizontally, without cards attached to them, they would have quickly bent vertically downwards through geotropism; but the result was that two of these radicles were still horizontal after 23 h. ; two were curved only slightly, and the fifth as much as 40° beneath the horizon. Thirdly, 5 beans were fastened CHAP. III. OF THE RADICLE OF THE DEAN. 153 with their flat surfaces parallel to the cork-lid, so that Sachs' curvature would not tend to make the horizontally extended radicles turn either upwards or downwards, and little squares of card were affixed as before, to the lower sides of their tips. The result was that all five radicles were bent down, or towards the centre of the earth, after only 8 h. 20 m. At the same time and within the same jars, 3 radicles of the same age, with squares affixed to one side, were suspended vertically; and after 8 h. 20 m. they were considerably deflected from the cards, and therefore curved upwards in opposition to geotropism. In these latter cases the irritation from the squares had overpowered geotropism ; whilst in the former cases, in which the radicles were extended horizontally, geotropism had overpowered the irritation. Thus within the same jars, some of the radicles were curving upwards and others downwards at the same tim&these opposite movements depending on whether the radicles, when the squares were fiTst attached to them, projected vertically down, or were extended horizontally. This difference in their behaviour seems at first inexplicable, but can, we believe, be simply explained by the difference between the initial power of the two forces under the above circumstances, combined with the well-known principle of the after-effects of a stimulus. When a young and sensitive radicle is extended horizontally, with a square attached to the lower side of the tip, geotropism acts on it at right angles, and, as we have seen, is then evidently more efficient than the irritation from the square · and the power of aeo-t . . ' b rop~sm Will be strengthened at each successive period by Its previous action-that is, by its after-effects. On .the other hand, when a square is affixed to a vertically dependent radicle, and the apex begins to |