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Show 482 'rRANSMITTED EFFEC'l'S Ol!' LIGHT. cnAP. rx. the case of Phalaris, whether the skin covered with a rather thick crust of dry Indian ink did not mocbanically prevent their curvature. Btta vttlgaris.-A few analogous experiments wore tried on this plant, which is not very well adapted for the purpo o,as the basal part of the hypocotyl, after it has grown to above half au inch in height, does not bend much on exposure to a lateral light. Four hypocotyls were surrounded close beneath their })etiolos with strips of thin tin-foil, ·2 inch in breadth, and they remained upright all day before a pantffin bmp; two others were surrounded with strips ·15 inch in breadth, and one of these remained upright, the other becoming bowed; tho bandages in two other cases were only ·1 inch in breadth, and both of these hypocotyls became bowed, though one only slightly, towards the light. The free seedlings in tho same pots were all fairly well curved towards the light; and during the follow· ing night became nearly upright. The pots wore now tmned round and placed before a window, so that the opposite sides of the seedlings were exposed to the light, towards which all the unprotected bypocotyls became bent in tho course of 7 h. Seven out of the 8 seedlings with bandages of tin-foil remained upright, but one which had a bandage only ·1 inch in breadth, became curved to the light. On another occasion, the uppe1· halves of 7 hypocotyls were surrounded with painted gold· beaters' skin; of those 4 remained upright, and 0 became a little curved to the light: at the same time 4 other seedlings sur· rounded with unpainted skin, as well as the free ones in the same pots, all became bowed towards the lamp, before which they had been exposed during 22 hours. llad,icles of Sinapis alba.- The radicles of some plants arc indifferent, as far as curvature is concemed, to tho action of light; whilst others bend towards and others from it.* Whether those movements are of any service to the plant is very doubtful, at least in the case of subterranean roots; they probably resu~t from the radicles being seusiti vo to contact, moisture, and gravJ· tation, and as a consequence to other initants which are uever naturally encountered. 'l'he radiclos of Sinapis alba, wh~n immersed iu water and exposed to a lateral light, bend from Jt, or are apholiotropic. They become bent for a length of about -4 mm. from their tips. To ascertain whetllor this movement * Sachs, 'Physiologic V cgctale,' 1868, P· 44. CuAP. IX. TRANSMIT'l'ED EFFEO'l'S 01<' LIGII'l'. 483 generally occm:red, 4.1 ra~iclos, which had germinated in damp sawdust, were Immersed m water and exposecl to a lateral light; and they all, with two doubtful exceptions, became curved from the light. At the same time tho tips of 54 other radiclos similarly exposed, were just touched with nitrate of silve/ They wore blackened for a length of from ·05 to ·07 mm., and probably killed; but it should be observed that this did not check materially, if at all, the growth of tho upper part; for several, which were measured, increased in the course of only 8-9 h. by 5 to 7 mm. in length. Of the 54 cauterised mclicles one case was doubtful, 25 curved themselves from tho light in the normal manner, and 28, or more than half, were not in tho least apheliotropic. There was a considerable difference, which we cannot account for, in the r esults of the experiments tried towards the end of April and in tho middle of September. Fifteen radicles (part of the above 54.) were cauterised at tho former period and were exposed to sunsl1ine, of which 12 failed to be apheliotropic, 2 wore still apheliotropic, and 1 was doubtful. In September, 39 cauterised radiclos were exposed to a uorthern light, being kept at a proper temperature ; and now 23 continued to be apholiotropic in tho normal manner, and only 16 failed to bend from the light. Looking at the aggregate results at both periods, there can be no doubt that the destruction of the tip for less than a millimeter in length destroyed iu more than half the cases their power of moving from the light. It is probable that if the tips had been cauterised for the length of a whole millimeter: all signs of apheliotropism would have disappeared. It may be suggested that although the application of caustic does not stop growth, yet enough muy be absorbed to destroy the power of movement in the upper part; but this suggestion must be rejected, for we have seen and shall again see, that cauterising one side of the 1ip of various kinds of radicles actually excites movement. The conclu ion seems inevitable that sensitiveness to light resides in the tip of the radicle of Sinapis albc~; and that the tip when thus stimulated transmits some influence to tho upper part, causing it to bend. The case in this re8pect is parallel with that of the radicles of several plants, the tips of which are sensitive to contact and to other irritants, and, as will be shown in the eleventh chapter, to gravitation. 2 I ~ |
