Plant Communities of the World 287 



growth. Cold deserts are the most polar communities, hori- 

 zontally the token of the high arctic zone, vertically of the niveal 

 and subniveal belt. The alpine group has strong sun radiation 

 in the short season; the arctic, less radiation but very long days; 

 other factors are about the same. 



In the High Alps such desert communities on fixed rubble are 

 a wonderful symphony of colors. In the siliceous Androsacetum 

 alpinae gleams the deep blue Gentiana bavarica subacaulis, 

 changing with the delicate rose of Androsace alpina cushions 

 and the sky-blue heavens' herald, Eritrichium nanum. Other 

 communities are the Oxyrietum digynae, the Allosoretum crispi, 

 and, on calcareous soil, Arabidetum coeruleae. The New Zea- 

 land niveal belt has the "vegetable sheep" Haastia pulvinaris. 

 To the arctic cold deserts belong open tundra, the "fell field" of 

 Warming, and the "rocky flat" of Ostenfeld. 



22. Litorideserta: Strand steppes. — These are deserts developed 

 under the influence of the neighboring sea where the atmos- 

 phere is full of salty spray. Rock-cliff strand steppe is the Sene- 

 cionetum cinerariae, which is well developed on Mediterranean 

 shores. As dominant, glitters the silk-hairy Senecio cineraria. 

 Slighdy darker is the also hairy, abundant, Helichrysum angus- 

 ti folium. Corsica's lies Sanguinaires have in this association their 

 endemics, Nananthea perpusilla and Evax rotundata. Southern 

 France has on its calcareous shore a similar association, the Arte- 

 misietum gallicae. The surge-sprayed lava rocks of the Canary 

 Islands bear a most unusual type of this class, the Limonietum 

 (Statice). A large number of purely endemic Limoniums spread 

 their flower-veils in the most delicate gradations from lilac to 

 blue to violet to red, and color for a short springtime the brown 

 rocks. Flat shore-strand steppe is dominated in northern Europe 

 by Salicornia herbacea with Festtica thalassica, or^on more sandy 



