96 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



even in very sensitive organs, until after the lapse of 10 to 30 minutes. 

 Indeed, we shall see later, in discussing the daily periodicity of growth, 

 that the adjustment to the new conditions of illumination may not be 

 completed until after 4 to 12 hours. Apart from the gradual alteration no 

 transitory disturbance seems to be produced as the result of a sudden 

 change of illumination, but in certain photonastic movements the reaction 

 to light may take place rapidly, and the same applies to the move- 

 ments of such motile organisms as Bacterium pkotometricum and Monas 

 Okenii. 



The influence of light on growth was investigated by Sachs and Baranetzky 

 on shoots, by Prantl and Stebler on leaves, and by Strehl on roots, in connexion 

 with the daily periodicity 1 . These authors also in part determined to what 

 degree light and darkness produce a rapid effect on growth, and fuller investiga- 

 tions were made subsequently by Reinke, Vines, Godlewski, and Stammeroff 2 . 

 The fact that negatively geotropic organs exhibit similar responses in their rate 



of growth to positively heliotropic ones, 



f m 9 ff> 1t 42 , when exposed to changes of illumina- 



tion, was shown by Miiller's and Wies- 

 ner's observations on negatively helio- 

 tropic aerial roots, by F. Darwin's on the 

 root of Sinapis alba, and by Stammeroff' s 

 (I.e., p. 149) on the rhizoids of Mar- 

 chantia*. In an experiment by Darwin 

 on the root of Sinapis alba, in which 

 the growth was measured microscopi- 

 cally, the average hourly increments 

 of growth in light were 0-514 mm. 

 during the morning, 0-992 mm. in 

 darkness during the afternoon, and 

 0-583 mm. while exposed to light dur- 



FlG. 21. Growth curve of Phycomyces in light and 



darkness. The fainter line shows the variations of tern- ing the evening, 

 perature, and the white bands show the periods of expo- T , i -*T /-r-.- 



sure to light. In an experiment by Vines (Fig. 



21) on Phycomyces nitens grown on 



bread moistened with sugar solution, the increments of growth were also measured 

 by means of a microscope. The numbers 5, 10, 15 give the increments of 



1 Sachs, Arb. d. Bot. Inst. in Wiirzburg, 1872, Bd. I, p. 99; Baranetzsky, Die tagliche 

 Periodicitat im Langenwachsthum, 1879 (Me"m. d. 1'Acad. d. St.-Petersbourg, T. xxvn) ; Prantl, 

 Arb. d. Bot. Inst. in Wiirzburg, 1873, Bd. I, p. 371; Stebler, Jahrb. f. wiss. Bot., 1878, Bd. xi, 

 p. 47 ; Strehl, Unters. ii. d. Langenwachsthum d. Wurzel u. Hypocotyl, 1874, p. 19. 



2 Reinke, Bot. Ztg., 1876, p. 139; Vines, Arb. d. Bot. Inst. in Wiirzburg, 1878, Bd. II, p. 137; 

 Godlewski, Anzeig. d. Akad. d. Wiss. zu Krakau, 1890, p. 169; Stammeroff, Flora, 1897, p. 149 

 (fungi, pollen-tubes, and rhizoids). On pollen-tubes see also Mangin, Bot. Centralbl., 1887, 

 Bd. xxxn, p. 68 ; Kny, Sitzungsb. d. Bot. Vereins d. Mark Brandenburg, 12. Juni 1881 ; Strasburger, 

 Befruchtung u. Zelltheilung, 1877, P- 2 35 Ewart, Trans. Liverpool Biol. Soc., 1894, Vol. ix, p. 196. 



3 H. Mtiller, Flora, 1876, p. 95; Wiesner, Die heliotrop. Erscheinungen, 1880, II, p. 17; 

 Fr. Darwin, Arb. d. Bot. Inst. in Wiirzburg, 1880, Bd. I, p. 521. 



