678 ZONES AND REGIONS [Pt. Ill, Sect. Ill 



to absorption of light by the atmosphere. Plants reared in continuous 

 daylight were, indeed, considerably larger than those exposed to the 

 alternation of day and night by the use of artificial darkness. The more 

 vigorous growth in the former case is naturally not a direct effect of light, 

 but the consequence of better nutrition owing to continuous assimilation. 

 No experiments are available regarding the effects of the direct sunlight. 



Some of Kjellman's experimental plants 1 remained uncovered, others were 

 placed in darkness for 12 hours daily (8 p.m. to 8 a.m.). Lepidium sativum was 

 sown as the first object. After two months, during which the little plant had deve- 

 loped quite normally, the weight of the largest 15 of the continuously illuminated 

 plants was 378 g., that of the periodically darkened ones was 3-53 g. ; in the first 

 group the maximum length (measured from the point of attachment of the cotyledon 

 to the tip of the longest leaf) was no mm., and the mean length 95-2 mm., whilst the 

 corresponding figures for the second group were 94 mm. and 75 mm. 



Experiments with true polar plants, Cochlearia fenestrata and Catabrosa algida, of 



which specimens that had lived through the winter 

 were used, led to much more marked differences in 

 favour of uninterrupted illumination. Of the Coch- 

 learia after 24 days (on the 20th of July) the largest five 

 of the periodically darkened individuals weighed 5-80 g., 

 thus averaging 1-16 g. each, whilst the corresponding 

 figures in constantly illuminated plants were 10-5 1 g., or 

 2- 10 g. each. Catabrosa showed similar differences. 



Finally eight seedlings of Cochlearia fenestrata 

 casually picked up were grown, one half of them in 

 continuous, the other half in interrupted illumination, 

 from the 12th of May to the 18th of June. The four 

 specimens with the full supply of light had, at the close 



of the experiment, 4-6 fully formed leaves, the others 

 Fig. 403. Salix polaiis. Natural , , . . . _ . . 



S J2 C- only 2-4 ; the total weight of the former group was 



13-5 eg., that of the latter only 6 eg. (Fig. 404). 

 The great difference between the two polar species, Cochlearia fenestrata and 

 Catabrosa algida, and the Lepidium sativum derived from seed brought from Scan- 

 dinavia, as regards the favourable influence of continuous illumination, appears to 

 be connected with adaptations. 



Assimilation during continual summer sunlight was measured by Curtel 2 , 

 not however in polar countries, but in Norway. Naturally the results are 

 valid for still higher latitudes. They show that assimilation proceeds unin- 

 terruptedly, but with a minimum at midnight corresponding to the minimum 

 of illumination. The experiments were carried out in the night from 

 the 31st of July to the 1st of August. 



As Bonnier has proved, and as this book has already stated :1 , plants under 

 continuous illumination develop differently from those subject to the alter- 



1 See Kjellman, op. cit., p. 503. 2 Curtel, op. cit. 3 p. 64. 



