266 PSAMMOPHYTES sect, x 



The soil is very sterile ; only in dunes lying nearest to the sea is there 

 a certain amount of calcic carbonate, which is derived from the shells 

 of marine animals ; but in dunes more remote from the sea this salt is 

 dissolved. Of nitrogen and humus extremely little is present ; the 

 humus bodies are rapidly oxidized to carbon dioxide and water, and thus 

 disappear. 



Heat. The dune is strongly and rapidly heated by direct rays of the 

 sun ; the temperature at the surface may rise to 5o-6o C. at midday 

 in July 1 ; warm currents of air stream out of the soil and play upon the 

 plants. The sun's heat dries the upper layers of sand so completely 

 that the grains of sand lie loose, but at a slight depth the sand is cold and 

 moist ; for the upper loose stratum checks evaporation. The changes 

 of temperature within twenty-four hours may be very great. 



Light is reflected from the surface of the sand and impinges upon the 

 lower faces of the leaves. The illumination as a whole is intense. 



Strong winds are wont to prevail where sandy soil and dunes occur ; 

 and wind has a double action : it desiccates ^ and acts mechanically, in 

 the latter case partly through the agency of the particles of sand that 

 it transports. These blown particles by their action may even polish 

 stones and perforate the thin broad leaves of plants that are not fitted 

 to live in this community : such for instance is the case with poplars, 

 which are often planted out on tracts of shifting sand.^ 



ADAPTATIONS * 



These accord with the differences in kind of the soil and vary with 

 them. The more shifting is the sand the more there come to the front 

 species which possess far-reaching subterranean organs (rhizomes and 

 roots) and exhibit a luxuriant activity in the production of shoots and 

 adventitious roots ; for these species can endure being buried beneath 

 a covering of sand, through which they can force their way upward. 

 The more coherent and stationary is the soil the more prominent do other 

 growth-forms become. 



In grey dunes of northern Europe we note growth-forms showing the 

 following characters : 



{a) Long creeping rhizomes, or roots capable of producing adventitious 

 shoots, are possessed by Carex arenaria, Galium verum, Sonchus arvensis, 

 Helichrysum arenarium, Festuca rubra, Rumex Acetosella. Under this 

 category we may also place mosses and the shrubs Hippophae Salix 

 repens, and Rosa pimpinellaefolia. 



{h) A tufted habit is exhibited among grasses by Weingaertneria cane- 

 scens, Festuca ovina, and Nardus stricta ; and among Dicotyledones by 

 Ononis repens, Anthyllis Vulneraria, Eryngium maritimum, Dianthus 

 deltoides, Artemisia campestris, and Armeria vulgaris, nearly all of which ^ 

 are very deep-rooted. Here, too, may be included such dwarf-shrubs as I 

 Calluna and Empetrum, and the undershrub Thymus Serpyllum. Many 

 species have shoots that lie prostrate on the sand and closely applied to 

 it, but do not strike root ; and they show leaves radiating from a common 



^ Giltay, 1886. * See p. 38. * See Massart, 1893, 1908. 



* See Giltay, 1886; Buchenau, 1889; Warming, 1891, 1907; Kearney, 1900; 

 Abromeit, 1900; Massart, loc. cit. ; Chodat, 1902; Coville, 1893; Cowles, 1899. 



