PHOTOMORPHIC ACTIONS 93 



in the other external conditions 1 . Trees form more foliage buds in light than in 

 darkness, and in weak light this fact, together with the death of certain of the 

 branches, causes the plant to assume a less compact character than when under 

 stronger illumination 2 . 



Light of sufficient intensity retards growth, and in many cases growth 

 is accelerated when the plant is removed from diffuse daylight to darkness. 

 Darkness favours the formation of roots upon the shoots of Cactaceae, and 

 upon the stems of etiolated plants 3 . Nevertheless, ordinary illumination 

 does not suppress the formation of roots upon stems submerged in water 

 and exposed to light, and in water-cultures exposure of the main root 

 to light only slightly retards the development of lateral roots. Feeble 

 illumination, however, suffices to inhibit the formation of tubers upon 

 single subterranean shoots of the potato, but not if all of them are 

 exposed to light 4 . Similarly, the growth of the buds on the potato is 

 retarded by light, and other cases may be found in which darkness favours 

 the development and growth of organs which are not normally exposed to 

 light. The apical growth of the stem of Phyllocactns which has ceased in 

 light recommences in darkness 5 , and fully grown but still young internodes 

 of Myriophylhim, Elodca, and Ceratophyllum undergo further elongation 

 by stretching-growth when placed in darkness 6 . On the other hand, 

 the runners of Adoxa cease to grow in darkness, and instead form their 

 tuberous bulbs 7 . 



Unilateral illumination may induce various changes in the growth or 

 production of organs on the illuminated or shaded sides. Thus roots are 

 formed mainly or entirely on the shaded sides of stems of Hedcra lielix* 

 and Lcpismiiim radicans*. Similarly, rhizoids appear mainly on the 

 shaded side of a gemma of Marcliantia^, and only on that surface in 



1 Heinricher, Ber. d. Bot. Ges., 1899, p. 308; Wiesner, Sitzungsb. d. \Yien. Akad., 1894, 

 Bd. CHI, i. Abth., p. 427; Jonsson, Bot. Jahresb., 1893, Bd. i, p. 39; Cieslar, Forsch. a. d. Gebiete 

 d. Agriculturphysik, 1893, Bd. vi ; Nobbe, Samenkunde, 1876, p. 239. [Most small seeds, such as 

 those of many Solanaceae, Orchidaceae, Scrophulariaceae, and Gramineae, rapidly perish after 

 germinating in darkness, partly owing to the small store of nutriment they contain. Careful control 

 is needed to distinguish this effect from a true stimulating action of light.] 



2 Vochting, Organbildung, 1884, II, p. 66 ; N. J. C. Miiller, Bot. Unters., 1877, Bd. I, p. 500; 

 Hartig, Lehrb. d. Anat. u. Physiol., 1891, p. 256; Jost, Ber. d. Bot. Ges., 1894, p. 194; \Viesner, 

 Sitzungsb. d. \Vien. Akad., 1895, Bd. CIV, I, p. 669; Biisgen, Waldbaume, 1897. p. 22. 



3 De Candolle, Ann. d. sci. nat., 1826, T. vil, p. 12 ; Physiologic, 1835, T - n > P- 34 1 J Sachs, 

 Bot. Ztg., 1863, Beilage, p. i ; Arb. d. \Viirzburg. Inst., 1880, Bd. II, p. 486; Vochting, Organ- 

 bildung im Pflanzenreich, 1878, pp. 148, 152. Darkness favours the development of rhizoids on 

 Chara (Richter, Flora, 1894, p. 407), but does not cause them to appear on the upper nodes. 



4 Vochting, Bibl. Bot., 1887, Heft 4 ; Jahrb. f. wiss. Bot., 1899, Bd. xxxiv, p. i. Other cases 

 are also given. 5 Id., Jahrb. f. wiss. Bot., 1894, Bd. xxvi, pp. 450, 465. 



6 Mobius, Biol. Centralbl., 1894, Bd. XV, p. 33. 7 Stahl, Ber. d. Bot. Ges., 1894, p. 389. 



8 Sachs, Vorlesungen, 1887, 2. AufL, p. 529. Czapek (Flora, 1898, p. 425) states that the 

 hypocotyl behaves similarly. 9 Vochting, Organbildung, 1878, I, p. 148. 



10 Zimmermann, Arb. d. Bot. Inst. in Wiirzburg, 1882, Bd. II, p. 666 ; Pfeffer, Unters. a. d. Bot, 

 Inst. zu Tubingen, 1885, Bd. I, p. 530. 



