ioo INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



Naturally a deficiency of food retards or prevents growth. Hence the formation 

 of flowers in Phanerogams 1 , and of sexual organs in Vaucheria-, is partially or 

 entirely suppressed when the plants are exposed to light in air deprived of carbon 

 dioxide. Nor is it surprising that in many cases the adult leaves die after 

 being kept for a time in continuous darkness, or when the absence of carbon 

 dioxide renders impossible the performance of the normal functional activity 3 . 



Both the supply of water and the temperature influence the phenomena 

 of etiolation, and in some cases to a considerable extent 4 . Certain green 

 algae, moreover, are able to develop in darkness if provided with suitable 

 food, and although the green and red bacteria which contain chlorophyll 

 and assimilate carbon dioxide by photosynthesis perish in darkness, 

 they may be able to develop on special nutrient media in the absence 

 of light. 



Under similar illumination the length of the internodes of Taraxacum is but 

 little influenced by rich or scanty supplies of water, whereas the internodes of both 

 Sempervimim and Plantago become considerably longer in moist air 5 . It is, 

 however, uncertain whether the increased growth is dependent solely upon the 

 higher turgidity, or is also due to a stimulating action exercised by the decreased 

 transpiration. Palladin 6 is, however, incorrect in ascribing the action of light 

 solely to its influence upon transpiration, for this is only one of the factors 

 influencing growth, and the formative effect of light is shown even when the 

 transpiration is kept constant. 



A low temperature decreases the elongation of the internodes, and since 

 in Alpine regions every evening, and also on cloudy days, the temperature falls 

 considerably, and often as low as zero centigrade, growth takes place mainly 

 in the daytime, and therefore under strong illumination. Hence plants under such 

 conditions acquire a more compact and condensed habit, which peculiarity is 

 also aided by the fact that transpiration is active during the growth periods. 

 In the extreme North a similar effect is produced by the length of the summer 

 days during the season of vegetative activity. Alpine plants when grown at low 

 levels assume their normal habit if exposed to low temperatures during the 

 night, or if cultivated under continuous illumination 7 . For example, Edelweiss 



1 Vochting, Jahrb. f. wiss. Bot., 1893, Bd. xxv, p. 178. 



2 Klebs, Bedingungen d. Fortpflanzung, 1896, pp. 19, 103. 



3 Vochting, Bot. Ztg., 1891, p. 140; Ewart, Journ. of Linn. Soc., Vol. xxxi, 1896, 

 pp. 559-73; Jost, Jahrb. f. wiss. Bot, 1897, Bd. XXVII, pp. 450, 478; MacDougal, Bot. Ztg., 

 1897, p. 162 ; Journ. of Linn. Soc., Vol. xxxi, 1896, p. 526; Teodoresco, Rev. ge"n. de Bot., 1899, 

 T. xi, p. 463. 



* Further instances are given by Klebs, Bedingungen der Fortpflanzung, 1896, pp. 103, 131. 



5 Wiesner, Sitzungsb. d. Wien. Akad., 1893, Bd. en, I, p. 327 ; Ber. d. Bot. Ges., 1891, p. 46; 

 Godlewski, Anzeig. d. Akad. d. Wiss. zu Krakau, 1890, p. 170; W. Brenner, Flora, 1900, p. 387. 



6 Palladin, Rev. ge"n. d. Bot., 1893, T. ir, p. 170. 



7 Bonnier, Rev. ge"n., 1890, T. n, p. 513; 1895, T. vil, p. 412; Ann. d. sci. nat, 1894, 

 7" ser., T. xx, p. 217; Compt. rend., 1898, T. cxxn, p. 307; Pfeffer, Physiol., I. Aufl.. Bd. II, 

 p. 106 ; Kerner, Pflanzenleben, 1891, Bd. II, p. 494; Schimper, Pflanzengeographie, 1898, pp. 739, 

 7535 Curtel, Rev. g<n., 1890, T. n, p. 16 ; Stenstrom, Flora, 1895, pp. 145, 155. On the 



