210 TROPIC MOVEMENTS 



Lepidium sativum, when successively laterally illuminated for one second 

 and darkened for two seconds during a period of twenty-five minutes, 

 performed as strong a heliotropic curvature as when continuously illumi- 

 nated for the same time from the side. It will probably also be possible 

 by using super-optimal intensities of illumination to produce more rapid 

 curvature by intermittent than by continuous illumination. In Wiesner's 

 experiment the same result was obtained in both cases owing to the fact 

 that the reaction is only increased up to a certain limit by increasing 

 intensities of light. Naturally when the intervals between the successive 

 periods of stimulation are unduly prolonged no response may be shown, 

 although periods of one second of strong illumination and fifteen to thirty 

 seconds darkness ultimately prove effective. A striking instance of the 

 varying degrees of summation is afforded by the sensitive leaflets of various 

 Leguminosae. Thus the leaflets of Mimosa pudica fold together fully 

 when alternately exposed to strong sunlight for two seconds and shaded 

 for two seconds, although the movement is slower than under continuous 

 exposure. If for two seconds in sunlight and four seconds in the shade 

 in regular succession, the leaflets rise up through angles of 15 to 20 only, 

 while under alternating periods of one second exposure and ten seconds 

 shade the leaflets remain fully expanded 1 . If an opaque wheel with an 

 indented rim is rotated between the object and the source of illumination, 

 the alternating periods of exposure and darkness may be made excessively 

 short, but nevertheless a response is still shown if the light is sufficiently 

 intense, so that the shortest flash of light can be perceived by the plant. 



Similar summation appears to be possible in all the tropic reactions 

 hitherto investigated. Noll 2 found, for instance, that geotropic induction 

 lasting for five minutes produced no effect, but that a curvature was induced 

 when for three hours the seedling was placed alternately horizontally for 

 five minutes and vertically for twenty-five minutes. It can, indeed, hardly 

 be doubtful that a feeble continuous tropic stimulus which is unable to 

 produce any perceptible response is, nevertheless, perceived as a feeble 

 sensory excitation, which is incapable of overcoming the autotropic tenden- 

 cies and self-regulatory activities of the organism. 



Minimal stimuli. The minimal intensities of light required to produce a helio- 

 tropic response have been investigated by Darwin, and subsequently by Wiesner and 

 Figdor 3 , who placed the plant in a dark room at varying distances from a candle- 

 flame. Under favourable conditions Figdor found that the sensitive hypocotyls of 



1 Ewart, The Effects of Tropical Insolation, Annals of Botany, 1897, Vol. xi, p. 449. 



2 Noll, Jahrb. f. wiss. Bot., 1900, Bd. xxxiv, p. 463. Cf. also Czapek, Jahrb. f. wiss. Bot, 

 1898, Bd. xxxn, p. 206 ; Sitzungsb. d. Wien. Akad., 1895, Bd. civ, Abth. i, p. 1217 ; Darwin and 

 Pertz, Annals of Botany, 1892, Vol. vi, p. 245; 1903, Vol. xvn, p. 93; Jost, Biol. Centralbl., 1902, 

 Bd. xxn, p. 175. 



3 Darwin, The Power of Movement in Plants ; Wiesner, Die heliotropischen Erscheinungen, 

 1878, Bd. i, p. 40; Figdor, Sitzungsb. d. Wien. Akad., 1893, Bd. en, i, p. 45. 



