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Show 70 ACTION OF THE RADICLE. CHAr. II. growing part continues to. do so, probabl~ for as l~ng as growth continues. Th1s movement of the l'athcle has been described in Brassica, lEsculus, Phaseolus, Vicia, Cucurbita, Quercus and Zea. The probability of its occurrence was inferred by Sachs,* from radicles placed vertically upwards being acted on by geotropism (which we likewise found to be. the case), for if they had remained absolutely perpendicular, the attraction of gravity could not have caused then~ to bend to any one side. Circumnutation was observed 1n the above specified cases, either by means of extremely fine filaments of glass affixed to the radicles in the manner previously described, or by their being allowed to grow down wards over inclined smoked glass-plates, on which they left their tracks. In the latter cases the serpentine course (see Figs. 19, 21, 27, 41) showed unequivocally that the apex had continually moved from side to side. This lateral movement was small in extent, being in the case of Phaseolus at most about 1 mm. from. a medial line to both sides. But there was also movement in a vertical plane at right angles to the inclined glass-plates. This was shown by the tracks often being alternately a little broader and narrower, due to the raclicles having alternately pressed with greater and less force on the plates. Occasionally little bridges of soot were left across the tracks, showing that the apex had at these spots been lifted up. 'rhis latter fact was especially apt to occur * 'Ueber das Wacbsthum der Wurzeln: Arbeiten des bot. In· stituts in Wurzburg,' Heft iii. 1873, p. 460. '!'his memoir, besides its intrinsic and great interest, deserves to be studied as a model of careful investigation, aud we shall have oceusion to refer to it repeatedly. Dr. Frank had previously remarked. (' B.ei: trage zur Pfinnzenphyswlogie, 1868, p. 81) on the fact of raJi~lcs placed vertically upwards bewg acted on by geotropism, an~ . he explained it by the suppotntwn that their growth was uot equal on all sides. CHAP. II. AU'l'ION OF THE RADICLE. 71 when the radicle instead of travelling straight down the glass made a semicircular bend; but Fig. 52 shows that this may occur when the track is rectilinear. rrhe apex by thus rising, was in one instance able to surmount a bristle cemented across an inclined glassplate; but slips of wood only,(-0 of an inch in thickness always caused the radicles to bend rectangularly to one side, so that the apex did not rise to this small height in opposition to geotropism. · In those cases in which radicles with attached fila) nents were placed so as to stand up almost vertically, they curved down wards through the action of geotropism, circumnutating at the same time, and their oourses were consequently zigzag. Sometimes, however, they made great circular sweeps, the lines beinO' like"vise zigzag. 0 Radicles closely surrounded by earth, even when this is. thoroughly soaked and softened, may perhaps be qmte prevented from circumnutating. Yet we should remember that the circumnutating sheath-like cotyledons of Phalaris, the h ypocoty ls of Solanum, and the epicotyls of Asparagus formed round themselves little circular cracks or furrows in a superficial layer of damp argillaceous sand. They were also able, as well as the hypocotyls of Brassica, to form straight furrows in damp sand, whilst circumnutatinO' and bending towards a lateral light. In a futur~ chapt~r it will be shown that the rocking or circumnutatmg movement of the flower-heads of Trifolium subterraneum aids them in burying themselves. It is therefore probable that the circumnutation of the tip of the radicle aids it slightly in penetrating the ground; and it may be observed in several of the previously given diagrams, that the movement is more strongly pronounced in radicles when they first |