251 



CHAPTER LXVI. ADAPTATION OF SPECIES IN SUBGLACIAL 



FELL-FIELDS 



The climatic conditions already recounted stamp the vegetation with 

 their impress. It is obvious that a series of extreme xerophytic types 

 must result from the different factors that promote transpiration, and 

 from coldness of soil, which checks the absorption of water.^ 



The adaptations of the vegetation are discussed in the succeeding 

 jxiragraphs. 



A. Duration and Development of the Plant. 



I. Perennation. The vast majority of the plants concerned are 

 perennial herbs or dwarf-shrubs. Trees and tall shrubs arc absent 

 (wcluded by drought and wind. In arctic countries annuals are repre- 

 ^(, iited by the polygonaceous Koenigia islandica, probably also by Gentiana 

 nivalis, G. serrata and other species, and the gentianaceous Pleurogyna ; 

 a few others, including Draba crassifolia, are probably biennials. In 

 the Alps there are several species of Gentiana that flower only once ; 

 (as reputed annuals may be mentioned species of Euphrasia ; but these 

 and similar hemi-parasites may be left out of consideration, as their 

 conditions of life are so different). Bonnier and Flahault - give the 

 following statistics in reference to the western Alps: the number of 

 annual species at altitudes 200-600 metres above the sea-level is sixty 

 per cent., at 600-1,800 metres is thirty-three per cent., and above 1,800 

 metres is only six per cent. And at this last altitude Kerner ^ gives the 

 estimate at four per cent, in the Tyrol, but judges the annual and perennial 

 species to be about equally numerous in the valleys below. In regard 

 to different latitudes Vahl ** gives the following figures concerning annual 

 species : Portugal, 34 per cent. ; Denmark, 20 per cent. ; Iceland, 11 per 

 cent. ; Greenland, 8 per cent. According to Warming,^ in Greenland 

 north of 73 N. there are no annual species, leaving out of consideration 

 those probably introduced by man ; between 73 N. and 71 N. there 

 is I per cent. ; between 71 N. and 69 N., 2 per cent. ; between 69 N. 

 and 67 N., 3 per cent. ; between 67 N. and 62 N., 4 per cent. ; and 

 between 62 N. and 60 N., 5 per cent. Some species are annual in the 

 lowlands, but perennial on mountains : such is the case with Arenaria 

 serpylhfolia and Poa annua ; "^ or annual lowland species are replaced 

 on mountains by perennial species for instance, in the Alps, Draba verna 

 by D. laevigata, Viola tricolor by V. lutea, and so forth. In northern 

 Europe some species, which south of the limit of forest are annuals, 

 become perennial north of this limit.' 



The cause for these phenomena is to be sought in brevity of the vegeta- 

 tive season and lowness of temperature.'' Annual species blossom when 

 the temperature is highest ; as the climate becomes colder their seeds 

 must ripen under unfavourable circumstances and therefore are easily 

 rendered sterile. Possibly some annual species become changed into 



' Hence the term ' Psychrophytes ', that is, plants belonging to cold soil. 



' Bonnier ct Flahault, 187c;. ' Kerner, 1869. * Vahl, 1904 i*. 



' Warming, 1887. " Kerner, loc. cit. ; Bonnier, 1884. 



' Kjellman, 1884. " Sec p. 25. 



