MINIMAL STIMULI AND LATENT PERIODS 211 



Lepidium sativum and Lunaria liennis responded heliotropically to light of intensity 

 equivalent to 0-0003 of a standard candle, those of Helianthus annum and Mirabilis 

 jalapa to an intensity of 0-016 of a standard candle, whereas the etiolated shoots of 

 Salix required an intensity of ioa6 units, and still stronger lateral illumination is 

 necessary to produce a perceptible heliotropic curvature in less sensitive plants. 



It is, therefore, not impossible that plants may be capable of a heliotropic 

 response to bright moonlight *, and they are able to detect and react to differences of 

 illumination imperceptible to the human eye. The strongest action is exercised by 

 the blue and violet rays, as well as by the ultra-violet rays, so that in this respect also 

 the photic sensitiveness of the plant surpasses that of the human eye. In addition, 

 the most sensitive plants may show a heliotropic reaction under an intensity of illumina- 

 tion which produces no perceptible browning in a sensitive chloride of silver paper 2 . 

 Wiesner has shown the importance of eliminating the action of gravity, and as well 

 as that the sensitivity varies according to the cultural conditions 3 . 



The geotropic irritability also varies greatly, as can be shown by substituting 

 varying centrifugal forces. In this way Czapek 4 found that sensitive radicles and 

 seedling-stems performed slight curvatures in response to a centrifugal force equivalent 

 to o-ooi g. The extreme sensitivity of certain tendrils to contact-stimuli has already 

 been discussed, and comparatively slow currents of water may excite a rheotropic 

 curvature. The power of many micro-organisms of responding to the presence of 

 the minutest traces of stimulatory substances is in part correlated with their minute 

 size, but it also indicates a high degree of sensitivity. 



Reaction and induction periods. The most rapid tropic responses appear to be 

 shown by tendrils, for a curvature may become perceptible five to twenty seconds 

 after stimulation. The pulvini of Lourea vespertilionis 5 , and of a few other plants, 

 show the commencement of a heliotropic reaction within one minute, and under 

 favourable conditions the sporangiophores of Phycomyces may begin to curve towards 

 the light in one to three minutes 6 . Usually, however, the time required to produce 

 a heliotropic reaction is at least seven to fifteen minutes even in the case of very sen- 

 sitive objects such as the seedlings of Phalaris, Avena, and Sinapis, while more than 

 an hour is required by the strongly reacting seedling-stem of Vicia sativa 11 . The 

 time required for a heliotropic reaction appears, however, to be shorter, on the whole, 

 than that required for a geotropic reaction, which appears never to be less than twenty 

 to thirty minutes 8 . 



1 Musset, Compt. rend., 1890, T. ex, p. 201. Cf. Bay, Bot. Ztg., 1891, p. 178. 



2 Wiesner, Sitzungsb. d. Wien. Akad., 1893, Bd. en, I, p. 347 ; Bot. Centralbl., 1897, Bd. LXIX, 



p. 305. 



3 Wiesner, Die heliotropischen Erscheimmgen, 1878, Bd. I, p. 54; cf. also Figdor, 1. c., p. 58; 

 Oltmanns, Flora, 1892, p. 231. 



* Czapek, Jahrb. f. wiss. Bot., 1895, Bd. xxvu, p. 307 ; 1898, Bd. xxxil, p. 190. 



5 Cf. Pfeffer, Periodische Bewegungen, 1875, p. 63. The leaflets of Mimosa and other 

 Leguminosae may begin to fold up one or two seconds after strong sunlight has fallen upon them. 

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



6 Cf. Oltmanns, Flora, 1897, p. n. 



7 Darwin, The Power of Movement in Plants ; Wiesner, Die heliotropischen Erscheintmgen, 

 1878, Bd. i, p. 37 ; Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxil, p. 185. 



8 Cf. Czapek, 1. c., p. 184 ; Darwin, 1. c., p. 422 ; Sachs, Flora, 1873, p. 321. 



P 2 



