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Show 476 TRANSMIT'l'ED EFFECTS OF LIGH'l'. CHAP. IX. crack or furrow all round their bases, which would admit a little light on all s ides; but this would not happen when they were illuminated laterally, for we know that they quickly bend towards a lateral light, and they then press so firmly against tho sand on the illuminated side as to furrow it, and this would effectually exclude light on this Ride. Any light admitted on the opposite and shaded Ride, where an open furrow is formcu would tend to count met the curvatnre towards tho lamp 0; otl1cr source of tho light. It may be acltlod, that tho usc of fine moist sand, which yields easi ly to presRure, was indlspcnstthlo in the above experiments; for seedlings r::tised in common oil, not kept espedally clamp, and exposed for 9 h. 30 m. to a strong lateral light, did not form an open furrow at their hasos on the shaded side, and were not bowed beneath the Rurfaco. Perhaps tho most strikinp; proof of the action of the upper on tho lower part of tho cotyledons of Phalaris, when httcrally illuminated, was afforded by tho blackened glass-tnbcs (before alluderi. to) with very narrow stripes of the varnish scraped off on one side, through which a little light was admitted. Tho breadth of those stripes or slits varied between ·01 and · 02 inch (' 2.:J and ·51 mm. ). Cotyledons with their upper halves enclosed in such tubes were placed before a south-we·t window, in such a position, that the . crapocl stripes di<l not directly face the window, but obliquely to one side. Tho seedlings were left exposed for 8 h., before tho close of which time the many free seedlings in the same pots had become greatly bowed towards the window. Under those circumstances, the whole lower halves of tho cotyledons, which had their summits encloRocl in tho tubes, were fully exposed to tho light of the sky, whilst their upper halves received exclusively or chiofty difl'used light from the room, and thi.· only through a very narrow slit on one side. Now, if tho cmvature of tho lower part had been determined by the illumination of this part, ttll the cotyledons assuredly would have become curved towards the window; but this was far from being the case. T~bes of the kind just described wore placed on several occa,:ons over the upper halves of 27 cotyledons; 14 of them rema~ned all the time quito vertical; so that sufficient diffuseu hgbt did not enter throuo·h tho narrow slits to produce any offe~t t> 'f tt Cll' whatever; and they behaved in the same manner as 1 1 upper halves had been enclosed in completely blackened tubeR. The lower halves of the 13 other cotyledons became bowed CHAP. IX. TRANSMITTED EFFEC'l'8 OF LIGHT. 477 not directly in the line of the window, but obliquely towardR it; one pointed at an angle of only 1S0 , but the remaining 12 at angles varying botweon 45° and G2° from the line of the window. At the commencement of tho experiment, pins hacl been laid on the earth in the direction towards which tho slits in the varnish faced; and in this direction alone a small amount of diffused light entered. At the close of the experiment, 7 of the bowed cotyledons pointed exactly in the liue of the pinR, aud 6 of them in a line between that of the pins and that of the window. This intermediate position is intolbgible, for any light from the sky which entered obliquely through tho slits would be much more efficient than the diffused light which entered directly through them. After the 8 h. exposure, the contrast in appes.trance between these 13 cotyledons and tho many other seedlings in the same pots, which were all (excepting the above 14 vertical ones) greatly bowed in straight and parallel lines towards the window, was extremely remarkable. It is therefore certain that a little weuk light striking tho upper halves of the cotyledons of Phalaris, is far more potent in determining the direction of the curvature of the lower halves, than tho full illumination of tho latter during tho whole time of exposure. In confirmation of the above results, the effect of thickly painting with Indian ink one side of tho upper })art of three cotyledons of Phalaris, for a length of · 2 inch from their tips, may be worth giving. These were placed so that the unpainted surfaee was directed not towards the window, but a little to one side; and ~hey all became bent towards tho unpainted side, and from the hne of the window by angles amounting to 31°, 35°, and 83°. The curvature in this direction extended clown to their bases, although the whole lower part was fully exposed to the b O'ht from the window. 0 Finally, although there can be no doubt that the illumination of the upper part of the cotyledons of Phalaris greatly affects the pow~r and manner of bonding of tho lower part, yet some o~servat10ns seemed to render it probable that the simultaneouR stimulation of the lower part by light greatly favours, or is almost necessary, for its well-marked curvature; but our experi~ ents were not conclusive, owing to tho difficulty of excluding hg~t from the lower halves without mechanically p1·eventing theu curvature. Avenu sativa.-The cotyledons of this plant become quicldy bowed towards a lateral light, exactly like those of Phalaris. |