INSTANCES OF SPECIFIC TROPIC IRRITABILITY 229 



to it will here also produce a curvature towards the weaker light. Darwin l 

 found that when one side of a plant was smeared with indian ink the 

 plant curved away from that side in diffuse light owing to the fact that 

 more light penetrated on the unsmeared side. Even this experiment, 

 however, does not afford sure proof that the difference in the intensity, 

 and not the direction of the light-rays, acts as the orienting stimulus. 



It is quite possible that light may induce chemical changes or 

 variations of surface-tension capable of acting as stimuli, but it is by no 

 means certain whether Loeb 2 is correct in ascribing the phototactic 

 movements of animal organisms to the direct action of changes of surface- 

 tension produced by light. Quincke 3 has recently observed that the pre- 

 cipitations produced by alkaline carbonates in solutions of calcium salts turn 

 towards the light, so that light may exercise a direct physical orienting action. 

 No protoplasmic aggregation or displacement has as yet been established as 

 a precedent to phototropic or phototactic response. Vines supposed that 

 light directly depressed the motility of the protoplasm, while Wiesner 

 supposed that it increased the power of stretching in the cell-walls of 

 the illuminated sides, but neither of these hypotheses has any value as 

 an explanation of heliotropism 4 . Similarly, historical interest alone 

 attaches to de Candolle's 5 view that the curvature towards light is due 

 to the partial etiolation of the shaded side. Organs which are not etiolated 

 in darkness are, however, capable of heliotropic reaction, while negatively 

 heliotropic organs may grow more rapidly in darkness ; and in this case 

 it is the exposed side which grows more rapidly during heliotropic 

 curvature. Further, when the zones of perception and action are some 

 distance apart the curvature may take place when the active zone is not 

 illuminated at all. Wolkoff 6 assumed that negative heliotropism was 

 produced by the refraction and concentration of the light-rays in the tissues 

 upon the shaded side, so that this side was the more strongly illuminated 

 one; but this quaint idea is totally incorrect. In any case phototropism 

 and phototaxis are simply general terms for orienting movements produced 

 by light, and it does not follow that precisely the same irritability and 

 mpde of response are involved in all cases. Yerkes 7 has suggested the 

 term ' photopathy ' for orienting movements due to differences of illumina- 



1 Darwin, The Power of Movement in Plants, 1881, p. 398. 



2 Loeb, Einleitung in d. vergleichende Gehirnphysiologie, 1899, p. 128 : cf. Nagel, Bot. Ztg., 

 1901, p. 294. 



8 Quincke, Annal. d. Physik, 1902, Folge iv, Bd. vn, p. 742. 



* Vines, Arb. d. bot. Inst. in Wurzburg, 1878, Bd. II, p. 145 ; Wiesner, Heliotropische Er- 

 scheimmgen im Pflanzenreiche, 1880, Bd. II, p. 21 : cf. also Godlewski, Bot. Ztg., 1879, ? JI 3' 



5 A. P. de Candolle, Physiologic ve"getale, 1832, T. ill, p. 1083. 



6 See Hofmeister, Pflanzenzelle, 1867, p. 293 ; Sachs, Lehrbuch d. Botanik, 1874, 4. Aufl., 

 p. 810. 



7 Cf. Nagel, Bot. Ztg., 1901, Abth. ii, pp. 291, 298. 



