98 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



phenomenon will, however, take place under continued illumination, which 

 also does not retard the cell-division of Spirogyra l y although, as in the 

 case of most algae, this takes place usually at night. It is not yet certain 

 whether this periodicity in autotrophic plants is due to the direct stimu- 

 lating action of the presence or absence of light, or whether in correlation 

 with the activity of photosynthetic assimilation during the daytime, 

 growth and its accompanying formative changes are somewhat retarded. 

 There is, however, no general tendency towards the performance of growth 

 mainly at night-time, or to withdraw the embryonic cells from exposure 

 to light 2 . As a matter of fact, in nature growth is often more active 

 in the daytime than at night, and the protection of the apical meristem 

 by enfolding leaves, and of the cambium by bark, is mainly for the purpose 

 of avoiding desiccation and mechanical injury. Moreover, respiration and 

 metabolism in general are but little influenced by the normal exposure 

 to light 3 . 



Plants are able to grow when continuously illuminated both in the 

 polar regions and under artificial conditions 4 , but the future must show 

 whether all plants grow normally under light of constant intensity. For 

 various reasons the same total quantity of light will not produce the same 

 physiological effects when spread over the entire twenty-four hours, as when 

 restricted to twelve hours of the day, or an even shorter period. Thus in 

 the first case the intensity of light required to induce the commencement 

 of growth might never be reached, while on the other hand periods of 

 darkness might be required for the production of primordia able to continue 

 their development in light. 



SECTION 26". The Mode in which Light acts. 



Of the stimulating actions of light, its direct retarding action on 

 growth seems to be of general occurrence, whereas its phototonic action 

 is restricted to special plants and plant organs. Since both these reactions 

 may take place at the same time, and may be combined with effects 

 due to correlation, the final result may have a very complex origin. In 

 non-chlorophyllous plants the photosynthetic influence of light does 

 not come into consideration, and the same applies to all autotrophic 

 plants which are abundantly supplied with nutriment. Thus, leaves which 



1 Famintzin, Jahrb. f. wiss. Bot., 1867-8, Bd. VI, p. 40. Strasburger (Zellbildung 

 Zelltheilung, 1880, 3. Aufl., p. 171) was able to produce cell-division in the daytime by cooling 

 Spirogyra filaments below 5 C. during the night. On the continuous illumination of other algae 

 cf. Klebs, Bedingungen d. Fortpflanzung, 1896, p. 27. 



2 Cf. Sachs, Bot. Ztg., 1863, Beilage, p. i ; Experimentalphysiol., 1865, p. 30. 



3 Kolkwitz, Jahrb. f. wiss. Bot., 1899, Bd. xxxm, p. 128. 



* Bonnier, Rev. gen. d. Bot., 1895, T. vil, pp. 242, 412 (flowering plants). 



