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Show 170 SENSI'riVENESS OF THE APEX CHAP. nr. of card, ancl 13 were not a.cted on. Rather brgo squares, though difficult to affix, seemed more efficient than very small ones. We wore much more successful with caustic; Lut in our first trial, 15 radicles wore too much cauterised, and only two boca.me curved from the blackenetl side; the others being either killed on one sicle, or blu.ckened equally all round. In our n xt trial tl.w dr~c<l tips of 11 radicles were touch d momentanly w1th dry caustic, and after a few minutes were inlmerr:;cd in water. The elongated marks thus ca.used were never black, only brown, anu about -l- mm. in length, or even loss. In 4 h. 30 m. after tho cauterisation, 6 of them were plainly curved from the side with the brown mark, 4 slightly, and 1 not at all. The bttcr proved unhealthy, and never grew; and the marks on 2 of the 4 slightly curved radiclos wore excessively minute, one being distinguishable only with the aid of a lens. Of 10 control specimens tried in the same jars at the same time, not one was in the least cm:ved. In 8 h. 40 m. after th9 ca.uterisation, 5 of tho mdwles out of the 10 (the one unhealthy one being omitted) were deflected at about 90°, and 3 at about 45° from tho perveudicuhtr and from the side bearing t.he brown mnrk. After 24 h. ttll 10 radicles had m· creased immensely in length; in 5 of them the curva· ture was nearly the same, in 2 it h<.ul increascu, itnd in 3 it had decreased. 'rhe contrast presontert by the · 10 controls, after both the 8 h. 40 m. and the 24 h. intervais, was very great; for they had cont.inue~ to grow vertically downwards, excepting two whwh, from some unknown cause, ha<l become somewhat tortuous. In the chapter on Geotropism we shall see that 10 radicles of this plant were extended hori~ontal.l~ 011 and beneath damp friable peat, under whieh conchtwns CHAP. III. OF THB RADICLE OF llAPIIANUS. 171 they grow better aud more natm·ally than in tlamp air; and their tips were slightly eautorisctl on the lower side, brown marks about .}, mm. in lenoth - b being thus caused. Uncauteri o<l specimens similarJy placed became much bent downwards through geotropism in the course of 5 or 6 hom·:. After 8 h. only 3 of the cauteriso<l ones wore bowo<l downwards, and this in a slight degree; 4 remain tl horizontal; and 3 were curved upwttrds in opposition to geotropism and from tho sicle bearing thu brown nhtrk. 'ren other specimens had their tip canturi ·eel at the same time and in the same <l•?ree, on the upp r side; and this, if it produced any ·{f ·t, would tun<l to increase the power of geotropism ; an<l all these radicles were strongly bowed downwards after 8 h. From the several foregoing facts, there can be no doubt that tho cauterisation of the tip of the radicle of this Cucurbita on one siue, if done lightly enough, c~uses the whole growing part to bend to the opposite s1de. R~phanu~ sativus: Se·nsitiveness of the apex of the RadM~e.-We here encountered many liffi.culties in our tnals, both with squares of card and with causti · · for .when seeds were pinned to a cork-lid, many of th~ rad1eles, to which nothing had been dono, grew irr - gularly, often curving upwards, as if attnwted by the ~amp surface above; and when they wore immersed 11~ water they likewise often grew irregularly. We did not therefore clare to trust our experiments with attached squares of card; nevertheless some of them seemed to indicate that the tips wore s nsitive to contact. Our trials with caustic goneru.lly failed from tfih e di.f ficult Yo f no t l·1 1·J ur·m g too gr atly the extremely L ne tips. Out of 7 racliclos thus tried one became owed after 22 h. nt an angle of 60°, a ~ecou<l at 40°, |
