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Show 462 SENSITIVENESS TO LIGHT. CHAP. IX. placed under a microscope provided with an eye-piece micrometer, of which en.ch division equalled "1lhth of an Inch (0·051 mm.) ; and they were at :first illuminated by light from a paraffin lamp passing through a solation of bichromate of potassium, which docs not induce heliotropism. Thus the direction in which the cotyledons were circumnutating could be observed independently of any action from the light; and they could be made, by turning round the pots, to circumnutate transversely to the line in which the light would strike them, as soon as the solution was removed. 'Jlhe fact that the direction of the circumnutating movement might change at any moment, and thus the plant might bend either towards or from the lamp inuependently of the action of the light, gave an element of uncertainty to the results. After the solution had been removed, :five seedlings which were circumnutating transversely to the line of light, began to move towards it, in 6, 4, 7!, 6, and 9 minutes. In one of these cases, the apex of the cotyledon crossed five of the eli visions of the micrometer (i.e. Th-th of an inch, or 0·254 mm.) towards the light in 3 m. Of two seedlings which were moving directly from the light at the time when the solution was removed, one began to move towards it in 13 m., and the other in 15 m. This latter seedling was observed for more than a.n hour and continued to move towards the light; It crossed at one time 5 divisions of the micrometer (0·254 mm.) in 2 m. 30 s. In all these cases, t~e movement towards the light was extremely unequal m rate, and the cotyledons often remained almost sta· tionary for some minutes, and two of them retrogra~ed a little. Another seedling which was circumnut~ti~g transversely to the line of light moved towards It m ' . . d 4 m. after the solution was removed ; 1t then remame OuAP. IX. SENSITIVENESS TO LIGHT. 463 almost stationary for 10 m.; then crossed 5 divisions of the micrometer in 6 m. ; and then 8 eli visions in 11 m. This unequal rate of movement, interrupted by pauses, and at :first with occasional retrogressions, accords well with our conclusion that heliotropism consists of modified circumnutation. In order to observe how long the after-effects of light lasted, a pot with seedlings of Phalaris, which had germinated in darkness, was placed at 10.40 A.M. before a north-east window, being protected on all other sides from the light; and the movement of a cotyledon was traced on a horizontal glass. It circumnutated about the same space for the :first 24m., and during the next 1 h. 33m. moved rapidly towards the light. The light was now (i.e. after 1 h. 57 m.) completely excluded, but the cotyledon continued bending in the same direction as before, certainly for more than 15m., probably for about 27 m. The doubt arose from the necessity of not looking at the seedlings often, and thus exposing them, though momen~ ily, to the ligh~. This same seedling was now kept m the dark, until 2.18 P.M., by which time it had rea~~uired through apogeotropism its original upright positiOn, when it was again exposed to the light from a.clouded sky. By 3 P.M. it had moved a very short distance towards the light, but during the next 45 m. travelled quickly towards it. .After this exposure of 1 h. 27 m. to a rather dull sky, the light was again compl~tely excluded, but the cotyledon continued to bend In the same direction as before for 14 m. within a very small limit of error. It was then placed in the dark, and it now moved backwards so that after 1 h. 7 m. it stood close to where it had ;tarted from at 2.18 P.M. These observations show that the cotyledons of Phalaris, after being exposed to a lateral |