Chap. I] CHARACTERS OF THE ARCTIC CLIMATE 



679 



nation of day and night. On the authority of the same observer certain dis- 

 tinctions in the histological structure of identical species, according as the 

 latter have developed in a mountain climate in lower latitudes or in a lowland 

 climate in higher latitudes, are to be attributed to the long duration of the 

 light during the vegetative season ; and this refers in particular to the 

 greater thickness and smaller differentiation of the leaves. This question, 

 however, can be decided only in polar countries, by means of comparative 

 cultures with interrupted and continuous illumination respectively. 



iv. XEROPHILOUS STRUCTURE OF THE VEGETATIVE ORGANS. 



The character of the polar vegetation is decidedly xerophilous (Figs. 5, 

 6, 12). In this respect Warming was justified in comparing the vegetation 

 of Greenland with that of the Sahara ; yet it is not, as he assumed, an 

 instance of the same climatic cause, namely too little moisture, operating 

 in the two cases ; for the protective measures 

 against transpiration are not less marked on 

 constantly moist soil than on dry soil. Here 

 rather, as in so many other cases, different 

 external factors are physiologically equi- 

 valent, and have accordingly evoked similar 

 adaptations. 



A short time after I had demonstrated the 

 connexion between xerophilous structure and 

 the coldness of the soil in reference to our 

 evergreen woody plants, and had suggested 

 that the xerophilous character of polar vegeta- 

 tion might depend on the same cause, Kihlman, 

 quite independently, after extensive observa- 

 tions in Russian Lapland, showed that, owing 

 to the permanent presence of ice in the ground, 



polar plants do suffer from impeded water-supply, even at a slight depth 

 below the surface, and have consequently developed devices for reducing 

 transpiration just as in other physiologically dry stations. 



Kihlman gives 1 the following summary of the xerophilous characteristics 

 of arctic vegetation : — 



' The leaves are leathery, stiff and hard, strongly cuticularized, with a 

 reduced surface cataphyllary or needle-like (Lycopodium, Diapensia, Andro- 

 meda hypnoides), or they have a distinct tendency to succulence (Saxifraga 

 oppositifolia and other species of Saxifraga, Eutrema, Rhodiola). The 

 stomata also are concealed, either in more or less closed cavities (Andromeda 



Fig. 404. Seedlings of Cochlearia 

 fenestrata. b had been deprived 

 of light for twelve hours daily. 

 a had been exposed to continu- 

 ous light. After Kjellman. 



1 Kihlman, op. cit., p. 105. 



