102 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



influence upon growth and upon all irritable movements or curvatures. 

 Hence, the shape and growth of a well-nourished plant remain the same 

 behind a solution of cupric oxide in ammonia as in somewhat weakened 

 white light, whereas behind a solution of potassium bichromate, which allows 

 the red and yellow rays to pass, but cuts off the blue and ultra-violet ones, 

 flowering plants turn green, but otherwise grow as though in darkness or 

 in very feeble light l . The same applies to most plants which are etiolated 

 by darkness. Thus Pilobolus microsporus and Coprinus, as regards the 

 formation of sporangia, and of the pileus and stipe respectively, behave 

 behind potassium bichromate solution as though in darkness, and behind 

 cuprammonia as though in daylight 2 . In this case the formative processes 

 are induced by light, whereas such as are retarded by ordinary light 

 will be favoured by reddish-yellow light, and retarded by blue light. 

 Thus according to J. Klein 3 the formation of spores in Botrytis cinerea, 

 and according to Klebs 4 that of zoospores in Vaucheria, is induced by 

 darkness or by cutting off the blue and violet rays. The retarding 

 and ultimately fatal effect exercised by ordinary light upon bacteria, 

 and by concentrated light upon all plants, is due mainly to the more 

 refrangible rays. 



There are, however, certain exceptions. Thus according to G. Kraus 5 

 the stalk of the pyrenocarpous fructification of Claviceps microcephala 

 becomes equally long in red and in blue light, but shorter than in darkness. 

 Similarly the red and yellow rays excite the germination of fern and 

 moss spores, but not the blue and violet ones 6 . The germination takes 

 place equally well in the absence of carbon dioxide, and hence the red and 

 yellow rays act not by producing organic food, but as an exciting stimulus. 



In general the formative influence of the less refrangible rays appears 

 to be somewhat greater than its heliotropic action, for the latter is often 

 absent in reddish-yellow light, whereas a distinct difference in shape can 

 usually be seen between green plants grown in red light and in darkness. 



1 Sachs, Bot. Ztg., 1864, p. 371; G. Kraus, ibid., 1876, p. 505; Vines, Arb. d. Bot.-Inst. in 

 Wiirzburg, 1878, Bd. I, pp. 120, 139; Wiesner, Sitzgsb. d. Wien. Akad., 1893, Bd. en, I, p. 322 ; 

 Teodoresco, Ann. d. sci. nat., 1899, 8 e se"r., T. X, p. 140. Anatomical peculiarities have 

 been studied by Teodoresco, I.e., and also by Rauwenhoff, Ann. d. sci. nat., 1878, 6 e ser., T. v, 

 p. 282. The first observations which were in the main correct were made by Senebier, Phys.-chem. 

 Abhandl., 1785, Bd. n, p. 29 ; Physiol. ve'getale, 1800, T. iv, p. 273. 



' 2 Brefeld, Bot. Unters. iiber Schimmelpilze, 1877, Heft 3, p. 96; 1889, VIII, p. 290; Grantz, 

 Ueber d. Einwirkung d. Lichts auf Pilze, 1898, pp. 18, 29; Lendner, Ann. d. sci. nat., 1897, 

 8 e ser., T. ill, p. 63; Elfving, Einwirkung d. Lichtes auf Pilze, 1890, p. 43. On the retarding 

 action of the blue rays on growth cf. Vines, Arb. d. Bot. Inst. in Wiirzburg, 1878, Bd. II, p. 139. 



3 Klein, Bot. Ztg., 1885, p. 6. 



4 Klebs, Bedingungen d. Fortpflanzung, 1896, pp. 25, 35. 



5 G. Kraus, Bot. Ztg., 1876, p. 505. According to Sorokin (Bot. Jahresb., 1874, p. 216) a few 

 fungi grow less in blue light than in darkness. 



6 Borodin, Bull. d. 1'Acad. d. St.-Petersbourg, 1868, T. xni, p. 436 ; Heald, Gametophytic 

 Regeneration, 1897, pp. 47, 61. 



