350 STRUGGLE BETWEEN PLANT-COMMUNITIES SECT, xvn 



particularly where cultivated land is left to itself. In the last cases the 

 soil is not new to the same extent as in the first cases ; it is not barren, 

 but includes a greater or smaller number of seeds and the like. The 

 following examples will serve to illustrate the development of various types 

 of vegetation. 



Vegetation on Sand. 



Psammophytic vegetation on the coasts of northern Europe 1 first 

 arises on the flat foreshore, which is sometimes several hundred feet in 

 width and receives from the sea deposits of sand : this assemblage of 

 psammophilous halophytes constitutes the vegetation of shore-sand. 

 Thereafter the wind raises up in such places dunes (shifting dunes), which 

 are colonized by true dune-plants, such as sea-marram. These plants, 

 if they emerge successfully from their struggles with the wind, prepare 

 the place for a fresh kind of vegetation ; between them and under their 

 shelter new species can now flourish. When these latter grow up and 

 constantly produce a denser vegetation, the spaces become too confined 

 for dune-plants, which gradually perish and give way to the vegetation 

 of grey (fixed) dunes, or to sand-fields, or, in many cases, to dwarf-shrub 

 heath. 2 



G. Beck 3 has described the kinds of vegetation that succeed one 

 another on sand-banks cast up by high water on the Danube. First, on 

 the bare moist sand are found some herbs, including species of Polygonum 

 and Chenopodium, among which seeds of Salix, Populus, Alnus, and 

 Myricaria germanica then germinate. The next colonists are a number 

 of other herbs, particularly belonging to species with travelling rhizomes ; 

 some settle upon moister spots, others upon drier, and produce a ' shifting- 

 sand vegetation '. The willows, poplars, alders, and other trees in the 

 meanwhile grow up and produce bush-forest ' willow-meadow ' which 

 suppresses the herbs by means of its shade. But where humus can form 

 and is not carried away by high water the willows and alders are van- 

 quished, and there arises another type of forest ' poplar-meadow ' com- 

 posed of Populus and Ulmus. Similar phenomena are witnessed in the 

 willow-flora skirting the Vistula. 



All the world over one sees on like habitats like struggles. And in 

 this connexion we may allude to Stefansson's 4 account of the development 

 of the vegetation in Vatn Valley in Iceland, where islets of mud and sand 

 are formed in the river, and become gradually colonized by Eriophorum, 

 Carex, and grasses. These plants vanquish each other in a definite order. 



The origin of heath-moor upon sand has been described by Grabner.* 

 The first plants to appear are Cyanophyceae, whose threads permeate the 

 sand to a depth of three millimetres ; then Polytrichum juniperum, 

 Radiola millegrana, Juncus capitatus, and other annual and perennial 

 herbs occur ; finally, Sphagnum, Ledum, Calluna, and others appear. 



1 See Sect. X, also Sect. VII. 



2 Warming, 1891; Grabner, 1895, 1901; Gerhardt, 1900; Cowles, 1899; 

 Adamovicz, 1904. 



3 G. Beck, 1890. 4 Stefansson, 1894. 5 Grabner, 1901. 



