CHAP. LXVI ADAPTATION OF SPECIES 255 



is less differentiated and includes rounded cells ; the arctic leaf also has 

 a thinner cuticle (in evergreen species this is scarcely the case). 1 This 

 difference in structure is due to the circumstances, first, that the atmo- 

 spheric humidity 2 in Polar countries increases with the latitude, whereas 

 on high mountains it decreases above a certain altitude ; secondly, 

 that alpine plants live in an atmosphere usually free from mist and are 

 exposed to frequently alternating light, which is very intense during the 

 day time, whereas Polar plants live almost continuously in mist or in 

 light of low intensity. This explanation harmonizes with the researches 

 made by Lothelier, 3 and Bonnier, on plants growing in dry and moist 

 air respectively, and with Bonnier's investigations on plants grown 

 in continuous (electric) light. The low intensity of the light seems to 

 be of greater import than the mist, which at a sufficient distance from 

 the coast is scarcely more frequent in Polar countries than in the Alps. 

 These results obtained by Bonnier agree with older ones obtained by 

 Theo Holm 4 and more recent ones by Borgesen. 5 



11. Aromatic, also bitter and resinous, substances are scantily formed 

 in Polar lands, but are more frequent on high mountains. For instance, 

 on the Andes 6 small Compositae containing these substances are more 

 numerous than in the allied lowland-flora. This is presumably due to 

 the more intense light. The flowers on high mountains are throughout 

 more fragrant than those of Polar countries. 



12. Pigments, foliage-leaves, according to Bonnier, 7 often become 

 deeper green in colour with increasing altitude (and latitude ?) ; they 

 produce more chlorophyll, whereby they acquire greater assimilatory 

 powers and atone for their small size. Bonnier remarks that there is an 

 optimum altitude at which leaves attain their deepest shade of green. 

 Red cell-sap, anthocyan, frequently occurs in Polar lands, 8 also on high 

 mountains. Anthocyan apparently is capable of converting the incident 

 rays of light into heat-rays 9 ; and according to Tischler 10 red races of 

 plants endure cold better than do green ones. 



The colours of flowers become deeper and purer with increasing altitude 

 and latitude. The full, pure colours of gentians, hare-bells, and poten- 

 tillas on the Alps, and Mimulus, Lupinus, and Sida on the Andes are 

 well known. White-flowered species show more marked reddish tinges 

 in subglacial situations than elsewhere ; according to Blytt the flowers 

 of Achillea Millefolium, Trientalis, and of Carum Carvi, and the bracts of 

 Cornus suecica, on Norwegian mountains are more red in tint than in the 

 lowland. Subjective impressions, nevertheless, play some part in this ; 

 floral hues seem to be more intense on humble plants, which often grow 

 within a sterile environment, but Bonnier and Flahault, 11 by the use of 

 graded colours, found that the colours in reality are deeper. This must 

 be attributed on mountains to intensity of sunlight, and in Polar lands 

 to its long duration. 



Borgesen, 1894; H. E. Petersen, 1908. ' See p. 249. 



Lothelier, 1890, 1893. * T. Holm, 1887. ' Borgesen, 1894. 



Meyen, 1836. ' Bonnier, 18946, 1895. 



According to Wulff, 1902. * See p. 20. 10 Tischler, 1905. 



Bonnier et Flahault, 1878 ; see also Bonnier, 1890, 1894, 1895. 



