64 THE FACTORS [Part I 



6. DAY AND NIGHT. 



The processes of plant-life that depend on light are for the most part 

 strictly confined to daytime ; yet in certain cases more or less prolonged 

 after-effects intervene and exercise a disturbing influence. Apart from this 

 the life of a plant at night differs altogether from its life by day. This is 

 clear to the most superficial observer. The leaves of many plants assume 

 a position at night that is frequently, though not always, similar to the 

 profile position resulting from intense illumination. Many flowers gradually 

 close on the approach of darkness, whilst others, less numerous however, 

 then begin to open ; many flowers exhale their scent only at night. 

 Careful observation shows that as light decreases in the evening assimila- 

 tion becomes gradually weaker and eventually ceases, only to recommence 

 at daybreak. The retarding influence of light on growth 1 , however, 

 exhibits no such immediate dependence on the intensity of the illumination, 

 but attains its highest effect only in the afternoon or evening, whilst the 

 maximum growth usually occurs not at night but in the early hours of the 

 morning. 



The increasing duration of sunlight from the Equator to the Pole certainly 

 acts in a modifying manner on the daily oscillations of plant-life. These 

 would indeed come to an end within the polar circle, if they did not in part, 

 like other periodic phenomena, depend on internal causes, and if they were 

 merely regulated by external influences whenever the latter occur. Apart 

 from this, the more prolonged but less intense illumination in the polar 

 regions is a factor in geographical botany, the importance of which was 

 already recognized by Schuebeler, and more accurately investigated by 

 Bonnier, Flahault, Kjellman, and Curtel. 



The works of the above-named investigators will be again referred to in 

 the sections of this work dealing with the polar regions. At present notice 

 will be taken, on account of their general importance, only of the experi- 

 ments which Bonnier carried out regarding the effects of continuous electric 

 light on the development of plants. In order to render the electric light 

 as much as possible like sunlight, the ultra-violet rays were weakened by 

 means of thick glass plates. The electric light used was, at any rate as 

 regards quantity, considerably weaker than daylight, a circumstance which, 

 while indeed influencing the results, did not, as experiments with inter- 

 mittent electric illumination (twelve hours light, twelve hours darkness) 

 proved, absolutely determine them. The plants experimented on were very 

 varied in character, some woody, some herbaceous, and the cultures lasted 

 for several months. Plants that were continuously illuminated differed 

 most remarkably from those that were cultivated normally, as well as from 

 those grown with intermittent electric light, by possessing a much greater 

 1 Cf. works of Baranetzki and Godlewski. 



