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Show 4GO SENSITIVENESS TO LIGII'r. CIIAP.1X. in the curvature of the seedlings in the two pot After 2 h. 12 m. tho chords of tho arcs of four ;f the most strongly cmTod seedlings in each pot were measured, a~d tho moan ano-Io from tho perpendicular of those whwh had previously boon kept in darkness was 19°, and of those which had previou ly been illuminated only 7°. Nor did this difference <.limini h during two additional hours. As a chock, the seedlings in both pots wore then placed in complete darkness for two hours, in order that apogootropism should a_ct on them ; and those in the one pot which were htt~e curve~ became in this time almost completely npnght, whilst tho more curved ones in the other pot still remained plainly curved. . Two days afterwards the experiment was repeated, with the sole difference that oven loss light was admitted through the window, as it was protected by a linen and muslin blind and by two towels ; tho sky, moreover, was somewhat loss bright. The result was the same as before, excepting that everything occurred rather slower. 'rhe seedlings which had been previously kept in darkness wore not in tho least cuncd after 54 m., but were so after 70 m. Those which bad previously been illuminated were not at all affected until 130 m. had elapsed, and then only slightly. After 145 m. some of the seedlings in this latter pot were certainly curved towarcls the light; and there was now a plain difference between tho two pots. After 3 h. 45 m. the chords of the arcs of 3 seedlings in each pot were measured, and the moan angle from the perpendicular was 16° for those in the pot which bad previously been kept in darkness, and o~1ly 5o for those which had previously been illuminated. The curvature of tho cotyledons of Phalaris towards a lateral light is therefore certainly influenced by the CHAP. IX. SENSITIVENESS TO LIGII'r. 461 degree to which they have been previously illuminated. vVe shall presently sec that the influence of light on their bending continuos for a short time after the light has been extinguished. These facts, as well as that of the curvature not increasing or decreasing in nearly the same ratio with that of the amount of light which they receive, as shown in the trials with the plants before the lamp, all indicate that light acts on them as a stimulus, in somewhat the same manner as on the nervous system of animals, and not in a direct manner on the cells or coli-walls which by their contraction or expansion cause the curvature. It has already been incidentally shown how slowly the cotyledons of Phalaris bend towards a very dim light; but when they were placed before a bright paraffin lamp their tips were all curved rectangularly towards it in 2 h. 20 m. The hypocotyls of Solanum lycopersicum had bent in the morning at right angles towards a north-east window. At 1 P.M. (Oct. 21st) the pot was turned round, so that the seedlings now pointed from the light, · but by 5 P.M. they had reversed their curvature and again pointed to the light. They had thus passed through 180° in 4 h., having in the morning previously passed through about 90°. But the reversal of the first half of the curvature will have been aided by apogeotropism. Similar cases were obse:ved with other seedlings, for instance, with those of Smapis alba. . We attempted to ascertain in how short a time light .acted on the cotyledons of Phalaris, but this was dtfficult on account of their rapid circumnutating move~ent; moreover, they differ much in sensibility, ~cordmg to age ; nevertheless, some of our o bservattons are worth giving. Pots with seedlings wero |