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 an 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, the movement towards the light was extremely unequal in rate, and the cotyledons often remained almost stationary for some minutes, and two of them retrograded a little.  Another seedling which was circumnutating transversely to the line of light, moved towards it in 4 m. after the solution was removed; it then remained [page 463] almost stationary for 10 m.; then crossed 5 divisions of the micrometer in 6 m.; and then 8 divisions in 11m.  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 24 m., and during the next 1 h. 33 m. 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 15 m., probably for about 27 m.  The doubt arose from the necessity of not looking at the seedlings often, and thus exposing them, though momentarily, to the light.  This same seedling was now kept in the dark, until 2.18 P.M., by which time it had reacquired 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 completely 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 started from at 2.18 P.M.  These observations show that the cotyledons of Phalaris, after being exposed to a lateral [page 464] light, continue to bend in the same direction for between a quarter and half an hour.