90 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



fungi. Coprinus nycthemerus only produces a mycelium in darkness, whereas 

 C. stercorarius , C. plicatilis, and C. ephemerus form sporocarps with or without 

 a complete pileus. Above i5C., however, C. stercorarius is able to form 

 a complete pileus on fruits formed from the mycelium or from sclerotia 1 . In 

 this and other species of Coprinus the stipe elongates considerably in darkness, 

 whether the pileus is formed or not, but in Coprinus ephemerus only a minimal 

 elongation occurs. 



Similarly, the formation of sporangia is suppressed in Pilobolus microsporus 

 by darkness, but not in other species of this genus nor in many other Mucorineae \ 

 Even here light may act favourably, for, according to A. Lendner 3 , on certain 

 nutrient media Mucor flavidus only produces sporangia when illuminated. Under 

 similar conditions sporangia are formed by Mucor racemosus in darkness, but 

 the spores differentiate only when exposed to light. As before, the sporophore 

 of many but not of all Mucorineae becomes longer in darkness, even when the 

 sporangium is formed. This over-elongation is, however, but slight in Pilobolus 

 microsporus, if a formation of sporangia has been induced by previous illumination, 

 so that the power of apical growth has been lost. In the absence of sporangia, 

 however, the apical growth continues, and Pilobolus produces extremely long 

 sporophores in continuous darkness. A comparatively short exposure to light, 

 however, suffices to induce the production of sporangia in Pilobolus microsporus 

 (^ to 5 hours), or the full development of the pileus in Coprinus (3 to 20 hours, 

 Grantz, 1. c., pp. 38, 49). 



Similar peculiarities are exhibited by other fungi 4 . Thus in many Hymeno- 

 mycetes light is necessary for the production or growth of the sporophore, whereas 

 those of Agaricus campestris develop normally in darkness. Sphaerobolus stellatus 

 again is sterile in darkness (Brefeld), whereas light is not essential to the common 

 mould-fungi (PeniciUtum glaucum, Aspergillus mger, Mucor stolonifer, &c.), and 

 indeed Elfving (1. c., p. 50) found that light of moderate intensity produces a slightly 

 lessened increase of dry weight. Klein 5 observed that the formation of conidia 

 was retarded by light, which probably indicates that the optimal intensity of 

 light is very low for this function. 



Algae and Bryophyta*. Batrachospennum grows actively in weak light, 



1 Brefeld, Unters. a. d. Gesammtgeb. d. Mykologie, 1889, Heft 8, p. 275; 1877, Heft 3, 

 pp. 87, 114; Bot. Ztg., 1877, p. 402 ; Grantz, Einfluss d. Lichtes a. d. Entwickelung einiger Pilze, 

 Leipziger Dissert., 1898, p. 20. 



3 Brefeld, Unters. a. d. Gesammtgeb. d. Mykologie, 1881, Heft 4, p. 76, and 1889, Heft 8, p. 275 ; 

 Grantz, 1. c., p. 6. 



3 Lendner, Ann. d. sci. nat., 1897, 8 e ser., T. Ill, p. 60. 



4 Scbmlzer v. Miiggenburg, Flora, 1878, p. 122; Schroter, Jahresb. d. Schles. Ges. f. vaterl. 

 Cultur, 1884, p. 290; Hartig, Der achte Hausschwamm, 1885, p. 18; Elfving, Einwirkung d. 

 Lichtes auf Pilze, 1890; Bachmann, Bot. Ztg., 1895, p. 130; Klebs, Bedingungen d. Fortpflanzung, 

 1896, p. 475 ; Holtermann, Mykol. Unters. a. d. Tropen, 1898, pp. 92, 114. Cf. also Zopf, Pilze, 

 1890, p. 199; Goebel, Organography, 1900, I, p. 227; Klebs, Jahrb. f. wiss. Bot., 1900, Bd. xxxv, 

 p. 140 ; Ternetz, ibid., p. 309. 



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



6 Cf. Berthold, Jahrb. f. wiss. Bot., 1882, Bd. xni, p. 672 ; Klemm, Flora, 1893, p. 469 ; 

 Goebel, Organography, 1900, I, p. 227. On the etiolation of a lichen {Baeomyces} see Krabbe, 

 Bot. Ztg., 1882, p. 93. 



