92 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



seems necessary for the production of flowers, entirely apart from its importance 

 for photosynthesis. Many plants do not flower in weak light, and Vochting 

 (I.e., 1898, p. 47) was able to cultivate Mimulus luteus in weak light for seven 

 years without it flowering. 



General. It is evident that the formative action of light may be 

 confined to the embryonic or post-embryonic development, or may affect 

 both of these. The formation of the primordia appears usually to be 

 independent of light, but in many Cryptogams and Phanerogams light is 

 essential for the formation of reproductive organs. The buds which give 

 rise to the aerial shoots of rhizomes develop independently of light, but the 

 latter is essential if the subsequent growth of the shoot is to take place 

 normally. On the other hand, even those fungus-spores which are only 

 formed in light are able to germinate in darkness. Buds expand and 

 seeds germinate in darkness or in light of insufficient intensity for continued 

 development, whereas the spores of certain chlorophyllous Cryptogams 

 only germinate when illuminated. The stimulus of light is also necessary 

 to bring those leaves which remain small in darkness to their full develop- 

 ment, and if the stimulus is sufficiently intense it may persist long enough 

 to enable the rest of the development to be completed in darkness. 



Light is necessary for the germination of the spores of ferns 1 , and for that 

 of the spores and gemmae of Bryophyta 2 , whereas the spores of Equisetitm, 

 Marsilia, and Pilularia will germinate in darkness 3 . At 32 C, however, the 

 spores of ferns will germinate in darkness 4 , as will also those of mosses, when 

 placed in sugar solution 5 . Exposure to light is essential for the germination of the 

 seeds of Viscum album, but not for those of tropical species and of Loranthus 

 europaeus*. Illumination apparently favours the germination of many seeds, and 

 the partly contradictory results of various authors are probably due to the unequal 

 ripeness of the seeds and the dissimilar intensity of the light, and to variations 



1 Borodin, Bull. d. 1'Acacl. d. St.-Petersbourg, iS6S, T. xm, p. 432; Heald, Gametophytic 

 Regeneration, Leipziger Dissert., 1897, p. 44. The other literature is quoted there. On the 

 etiolation of fern prothalli see Prantl, Bot. Ztg., 1879, p. 701. 



Mosses, Borodin, 1. c., p. 438 ; Heald, 1. c. Hepaticae, Leitgeb, Die Keimung d. Leber- 

 moossporen in ihrer Beziehung zum Licht, 1876, p. 3 (repr. from Sitzungsb. d. Wien. Akad., 1876, 

 Bd. i.xxiv, i.Abth.). Gemmae, Borodin, I.e.; Pfeffer, Arbeit, d. Wurzbnrg. Instituts, 1871, Bd. I, 

 p. 80; Correns, Unters. iiber Vermehrung der Laubmoose, 1899, p. 424. 



Sadebeck, Bot. Ztg., 1877. p. 44 ; Stahl, Ber. d. Bot. Ges., 1885, p. 334 ; Heald. 1. c., p. 63. 

 The spores of most fungi germinate equally well in darkness and in diffuse light. Cf. Hoffmann, 

 Jahrb. f. wiss. Bot., 1860, Bd. II, p. 321, and the works of cle Bary and Brefeld ; also Zopf, Pilze, 

 1890, p. 199. According to de Bary (Ann. d. sci. nat., 1863, 4 ser., T. xx, p. 37) the spores of 

 Peronospora macrospora germinate best in darkness. 



1 Heald, I.e., p. 62. 5 Goebel, Flora, 1896, p. 75 ; Heald, 1. c., p. 54. 



6 Wiesner, Ber. d. Bot. Ges., 1897, p. 512 ; Sitzungsb. d. \Yien. Akad., 1894, Bd. cm, i. Abth., 

 p. 401; 1^93, Bd. cil, i. Abth., p. 323. According to Raciborski (Extrait d. Bull. d. ITnst. 

 Botan. d. Buitenzorg, 1900, 6) the stimulus of light is necessary for the germination of the seeds 

 of Nicotiatia. [This does not apply to the seeds of Nicotiana rustica if properly sown, but the 

 seedlings rapidly die in darkness.] 



