the vertical glass, a little to the left where the taper had been held.  The tracings showed that they had travelled in zigzag lines.  Thus, an exposure to a feeble light for a very short time at the above specified intervals, sufficed to induce well-marked heliotropism.  An analogous case was observed with the hypocotyls of Solanum lycopersicum.  We at first attributed this result to the after-effects of the light on each occasion; but since reading Wiesner’s observations,* which will be referred to in the last chapter, we cannot doubt that an intermittent light is more efficacious than a continuous one, as plants are especially sensitive to any contrast in its amount.

The cotyledons of Phalaris bend much more slowly towards a very obscure light than towards a bright one.  Thus, in the experiments with seedlings placed in a dark room at 12 feet from a very small lamp, they were just perceptibly and doubtfully curved towards it after 3 h., and only slightly, yet certainly, after 4 h.

* ‘Sitz. der k.  Akad. der Wissensch.’ (Vienna), Jan. 1880, p. 12. [page 458]

After 8 h. 40 m. the chords of their arcs were deflected from the perpendicular by an average angle of only 16o.  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 1/4th, those in the 3rd pot 1/16th, those in the 4th 1/36th, those in the 5th 1/64th, and those in the 6th 1/100th 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 difference 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 successive pair of pots was extremely small.  In order to avoid prejudice, we asked three persons, who knew nothing about the experiment, to arrange the pots in order according 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 arranged them in wrong order, and doubted about four of the pots.  This evidence shows conclusively how little the curvature of the seedlings differed in the successive pots, in comparison with the great difference in the amount of light which they received; and it should be noted that there was no [page 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.