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Show 46G SENSITIVENESS '1'0 LIGHT. CHAP. IX. detected in ~he acc~racy of their bending, whether they. stood w~th thm~· broad ?r narrow si<los facing the hght, or 1n any 1nterroedmte position; and 80 it was with the cotyledons of Avena sativa, which arl· likewise oval in section. Now, a little reflection will show that in whatever position tho cotyloJ.on. mar stand, there will be a line of greatest illnminatio1 ; , exactly fronting the light, and on each si<le of this line an equal amount of light will be rocoivod; but if the oval stttncls obliquely with respect to tho lirrht, this will be diffused over a wider surface on one :ide of the central line than on the other. We may therefore infer that the same amount of light, 'rhether diffused over a wider s1uface or concontratccl on a l:lmaller surface, produces exactly the same effect; for the cotyledons in the long nttrrow box stood in all sorts of positions with reference to the light, yet all pointed truly towards it. That the bending of the cotyledons to tho light depends on the illumination of one whole si<le or on the obscuration of the whole opposite side, anclnot on n nanow longitudinal zone in the line of tho light being affected, was shown by the effects of painting longitud.inally with Indian ink one side of five cotyledons of Phalaris. ']~hose were then place<l on a tabh· nom· to a south-west window, and the painted half wa: directed either to the right or left. Tho result was that instead of bending in a direct line towards the window, they were deflected from the window an<l towards the unpainted side, by the following angles, 3:'?, 83°, sr, 43°, and. 39°. It should be remarked that it was hardly possible to paint one-half accnn'Ltcly, or to plncc all the seedlings which are on1l in section in qni.to t~1e BiUile position relatively to the light; and tlus wdl ar euuut for the differences in the angles. ]fire coty- CuAP. IX. SENSITIVENESS '1'0 LIGHT. 4G7 Iedons of Avena were also painted in the sttme mttnner, but with greater care; ancl they wore latemlly deflected from the line of the window, towards th unpainted sicle, by the following angles, 44°, 44°, 55°, 51°, and 57°. This deflection of the cotyledons from the window is intelligibl , for tho whole nnpaintecl side must have received some light, whereas the opposite and painted side receive<l none ; but a nttrrO\Y zone on the unpttinted side dir ctly in front of th<· window will have received most light, and all the hinder parts (half an oval in section) less and loss light in varying degrees; an(l we may conclude that thr angle of deflection is the resultant of the action of tlw light over the whole of the un pttinted side. It should httve been premised that painting Yv"ith Indian ink does not injure plants, at least within several hours; and it coul<l injure them only by stopping respiration. To ttscertain whether injury was thus soon caused, the upper halves of 8 cotyledons of Avena were thickly coated with transparent matter,-4 wit]t gum, and 4 with gelatine; they were pbced in tlH-' morning before a window, and by the evening they were normally bowed towards tho light, although the coatings now consisted. of dry crusts of gum and gelatine. Moreover, if the soecllings which vvere paintecl longitudinally with Indian ink had been injured on the painted side, the opposite side would have gone on growing, and they would consequently have become bowed towards the painted side ; whoretts the curvature was always, as we have seen, in the opposite direction, or towards the unpainted side which was exposed to the.light. We witnessed the effe cts of injuring longitudmally one side of the cotvledons of Avena ancl ~~alaris; for before we know that grease was highly lllJnrious to them, several were painted down one si<l<~ 2 II 2 |
