THE ACTION OF RAYS OF DIFFERENT WAVE-LENGTH 101 



retains its characteristic appearance when cultivated on the plains, if placed in 

 an ice-chest every evening, and exposed to bright light during the day. 



The precise character of the stimulatory action of light has yet to 

 be determined, but it is certainly not the result of any direct influence 

 upon turgidity, for the latter does not increase in darkness, and hence 

 is not responsible for the over-elongation of an etiolated stem l . Nor 

 are the cell-walls stretched beyond their elastic limit in growing etiolated 

 organs, as is shown by the immediate cessation of growth in the absence 

 of oxygen. Similarly in etiolated leaves which have fallen into dark- 

 rigor, growth is reawakened by exposure to light without the turgor being 

 increased or the elastic properties of the cell-walls being modified 2 . Nor 

 are the thickening, lignification, and other metamorphoses of the cell-wall 

 of decisive importance in regulating its growth in surface-extent, but rather 

 serve to adapt the adult cell to special functions. All such changes are 

 vital phenomena, and the stimulatory action of light is undoubtedly of 

 similar character, and is not merely due to the direct mechanical influence 

 of the illumination upon the cell-walls. We are indeed unable even to 

 say whether the action of light is chemical in nature, or whether in some 

 other way it effects a change in the internal conditions regulating growth 3 . 

 The hypothesis that light retards the motility of the protoplasmic micellae 4 , 

 and hence decreases the activity of growth, is incapable of experimental 

 proof, and it is a mere self-deception to suppose that the action of light 

 upon growing cells can be explained by this assumption. 



SECTION 27. The Action of Rays of Different Wave-length. 



Although the less refrangible rays are most active in photosynthesis, it 

 is the more refrangible ones (blue to ultra-violet) which exercise the greatest 



anatomical peculiarities of Alpine plants cf. Bonnier, 1. c., and the literature quoted by Haberlandt, 

 Physiol. Anat, 1896, 2. Aufl., p. 260. 



1 This explanation was propounded by de Vries, Bot. Ztg., 1879, p. 852, but Weng (Pfeffer, 

 Pflanzenphysiologie, 1881, I. Aufl., Bd. II, p. 145) showed that no rise of turgidity occurs in darkness, 

 and this was confirmed by de Vries, Jahrb. f. wiss. Bot., 1884, Bd. Xiv, p. 561 ; Wortmann, Bot. 

 Ztg., 1889, p. 296; Stange, Bot. Ztg., 1892, p. 412. Cf. also Copeland, Einfluss von Licht und 

 Temperatur auf den Turgor, Haller Diss., 1896, p. 53. The photonastic movements of pulvini 

 involve in part decreases of turgidity in darkness, and the pileus of Coprinus droops in darkness and 

 becomes turgid again on exposure to light. Cf. Brefeld, Bot. Unters. u'ber Schimmelpilze, 1877, 

 Heft 3, p. 114; Grantz, Einfluss des Lichtes auf einige Pilze, 1898, p. 34. 



2 Godlewski (Anzeig. d. Akad. d. Wiss. zu Krakau, 1890, p. 287) found the same stretching of 

 the cell-wall by turgor to exist in etiolated as in non-etiolated organs. 



3 No sure conclusions can be drawn from the action of light on plasmodia. 



4 Vines, Arb. d. Bot. Inst. zu Wurzburg, 1878, Bd. I, p. 144. [The fact that strong light 

 retards protoplasmic streaming affords no support to this hypothesis, for moderate illumination 

 either exercises no effect on streaming or accelerates it. Moreover no direct connexion exists 

 between growth and streaming. Cf. Ewart, On the Physics and Physiology of Protoplasmic 

 Streaming, 1903, Clar. Press, pp. 55 and 69.] 



