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Show 458 SENSITIVENESS TO LIGHT. CHAr. IX. After 8 h. 40 m. the chords of their arcs were deflected from the perpendicular by an average anglo of only 16°. Had the light been bright, they would have become much more curved in between 1 and 2 h. Several trials were made with seedlings placed at various distances from a small lamp in a dark room ; but we will give only one trial. Six pots were placed at distances of 2, 4, 8, 12, 16, and 20 feet from the lamp, before which they were left for 4 h. As light decreases in a geometrical ratio, the seedlings in the 2nd pot received i th, those in the 3rd pot -foth, those in the 4th :futh, those in the 5th 0'-:rth, and those in the 6th tlroth of the light received by the seedlings in the first or nearest pot. Therefore it might have been expected that there would have been an immense differ· ence in the degree of their heliotropic curvature in the several pots; and there was a well-marked difference between those which stood nearest and furthest from the lamp, but the difference in each succ~ssive pai: of pots was extremely small. In order to avmd preJudice, we asked three persons, who knew nothing abo:1t the experiment, to arrange the pots in order accorclmg to the degree of curvature of the cotyledons. The first person arranged them in proper order, but doubted long between the 12 feet and 16 feet pots; yet these two received light in the proportion of 36 to 64. The second person also arranged them properly, but doubted between the 8 feet and 12 feet pots, which received light in the proportion of 16 to 36. The third person arrano·ed them I· n wrong order ' and doubted about four bo f the pots. Th1· s ev1·1c once sh.o m conclusively how little the curvature of tho se~dlmgs d1• Jure re d I. n t h e success1. ve po t s, m. co mpar·ison with the great di.f ference I. n the amount of 11· g· ht which they· received; and it should be noted that there was no CHAP. IX. SENtli'l'IVENESS TO LIGHT. 459 excess of superfluous light, for the cotyledons became but little and slowly curved even in the nearest pot. Close to the 6th pot, at the distance of 20 feet from the lamp, the light allowed us just to distinguish a dot 3·56 mm. (·14 inch) in diameter, made with Indian ink on white paper, but not a dot 2·29 mm. (·09 inch) in diameter. The degree of curvature of the cotyledons of Phalaris within a given time, depends not merely on the amount of lateral light which they may then receive, but on that which they have previously received from above and on all sides. Analogous facts have been given with respect to the nyctitropic and periodic movements of plants. Of two pots containing seedlings of Phalaris which had germinated in darkness, one was still kept in the dark, and the other was exposed (Sept. 26th) to the light in a greenhouse during a cloudy day and on the following bright morning. On this morning (27th), at 10.30 A.M., both pots were placed in a box, blackened within and open in front, before a north-east window, protected by a linen and muslin blin~ and by a towel, so that but little light was admitted, though the sky was bright. Whenever the ~ots were looked at, this was done as quickly as posSI? le, and the cotyledons were then held transversely With respect to the light, so that their curvature could not have been thus increased or diminished. After ~0 m. the seedlings which had previously been kept m. darkness, were perhaps, and after 70 m. were cer~ mly, curved, though very slightly, towards the wmdow. After 85 m. some of the seecllin()'s which had · 1 . . b' previous y been Illuminated were perhaps a little ~e c ted , and after 100m. some' of the youno·er ones were t · 1 . o th' ?er a1_n Y ~ httle curved towards the light. At IS tune (1.e. after 100m.) there was a plain difference |
