254 PSYCHROPHYTES SECT, ix 



Occurring in their typical form (e. g. Azorella and others) on high moun- 

 tains in South America, on Kerguelen Island, and in New Zealand, these 

 dense cushions may be protected against desiccation by their old, densely 

 packed parts greedily absorbing and retaining water ; and when the 

 surroundings become cold they possibly can remain warm for a consider- 

 able time because of the high specific heat of the water. 1 



8. Lianes. Climbing plants are lacking because tall plants are 

 absent. 



9. Thorns and prickles are almost unrepresented in subglacial plants ; 

 the species of Rosa and Rubus concerned usually have few or no prickles. 

 This must be attributed to the intense humidity prevailing during the 

 spring season. 



10. Leaf -structure. Xerophytic structure, evoked by the factors men- 

 tioned in Chapter LXV, reveals itself in the perennial, evergreen, foliaged 

 shoots. For the leaves are coriaceous, rigid, and very glossy (cutinized), 

 as in Loiseleuria procumbens and Globularia cordif olia ; or are thick and 

 juicy, as in species of Saxifraga and Sempervivum ; or show a more or 

 less dense coat of hairs, as in species of Rhododendron, Draba, Cerastium 

 alpinum, Espeletia, and Culcitium. Many possess the pinoid, juncoid, 

 or cupressoid or rolled type of leaf. 2 



Stomata are concealed in furrows, or under the revolute leaf-margins, 

 or under hairs, as in Cassiope tetragona, Ledum palustre var. decumbens 

 and other Ericaceae, also in Empetrum, Dryas, and others. Deciduous 

 foliage shows this xerophytic structure to little or no extent. 



The leaf-structure of alpine plants has been investigated by Lazniewski, 

 Leist, Wagner, and Bonnier. 3 The two last-named agree in their general 

 conclusions, which relate to alpine leaves when compared with the leaves 

 of corresponding lowland plants, and are as follow : Alpine leaves are 

 designed for probably more vigorous assimilation by means of a better 

 developed palisade, and are consequently thicker (to the extent of 

 or even ^) in proportion to the surface or often absolutely, than those 

 of lowland plants. They are always dorsi ventral and of loose texture, 

 thanks to large intercellular spaces. They possess many stomata on 

 both faces, but especially on the upper face, where these are often more 

 numerous than on the lower face. The guard-cells lie at the level of the 

 general epidermal surface, excepting in the case of the evergreen leaves 

 already discussed. Wagner is of opinion that alpine plants require 

 greater powers of assimilation, because the absolute amount of carbon 

 dioxide in the air is less, and the vegetative season may be shorter 4 ; added 

 to this the intensity of light to which alpine plants are exposed may be 

 greater, also richer in strongly refractive rays. 



Bonnier 5 compared the leaves of nineteen species growing respec- 

 tively in Spitzbergen or Jan May en Island and the Alps ; he came to 

 the following conclusions, which certainly are too sweeping : the arctic 

 leaf is thicker and more fleshy, has looser mesophyll, in which the palisade 



1 Gobel, 1889; Reiche, 1893; Meigen, 1894; Diels, 1896; G. Andersson and 

 Hesselman, 1900. * Gobel, 1891 ; Lazniewski, 1896; Diels, 1905. 



3 Lazniewski, loc. cit. ; Leist, 1889; Wagner, 1892 ; Bonnier, 18940. 



4 In this connexion reference should be made to Schroter ( 1 904-8 ) and to pp. 

 14-15, also to the remarks in the sequel dealing with construction of arctic leaves. 



* Bonnier, 18940. 



