212 OXYLOPHYTES SECT, vi 



Many of the shrubs, including Empetrum, Vaccinium, Arctostaphylos, 

 have fleshy fruits that are eaten by birds. 



Beneath and between the dwarf -shrubs grow mosses and lichens, 

 which permeate the soil with their rhizoids ; among the lichens are 

 Cladonia rangiferina, Cetraria islandica, and Sphaerophoron corallioides ; 

 and among the mosses, species of Polytrichum, Rhacomitrium, Hypnum, 

 and Hylocomium. In addition, there occur numerous grasses and herbs 

 which are mainly perennial : annual and biennial species, such as Aira 

 praecox and A. caryophyllea, maintain themselves with difficulty amidst 

 the dense growth, and at most occur in well-lighted bare spots (to this 

 rule parasitic Rhinantheae provide an exception). The herbs and 

 grasses, for example, Arnica montana, Solidago Virgaurea and Campanula 

 rotundifolia, associated with this habitat are caespitose, and thus are 

 better fitted to grow in dense soil than are species possessing shoots 

 that travel underground. 



The evergreen dwarf-shrubs are distinctly xerophytic in structure, 

 but so likewise are many of the herbs. In regard to the latter we merely 

 note here that broad, thin, smooth leaf-blades scarcely occur, and that 

 the grasses mostly have setaceous or filiform leaves with stomata in 

 furrows that open or shut according to circumstances, as is exemplified 

 by Weingaertneria canescens, Nardus stricta (a tunic-grass), and Festuca 

 ovina. 1 Many species such as Rumex Acetosella, Campanula rotundi- 

 folia, Scleranthus, and Artemisia campestris, which are interspersed here 

 and there, have small and narrow leaves when compared with their 

 congeners of other habitats ; other species, such as Antennaria dioica, 

 and Gnaphalium arenarium, are densely hairy ; while Sedum acre repre- 

 sents succulent plants. 



ASSOCIATIONS AND DISTRIBUTION 



Dwarf-shrub heath occurs in a number of countries within the tem- 

 perate and cold zones of the Northern Hemisphere, and develops typi- 

 cally over extensive areas for example, in West Denmark (Jutland) 

 and in North- West Germany. Several kinds of associations of it are 

 met with. 



In northern Europe in most cases there is callunetum with Calluna 

 vulgaris as the dominant species ; on wet soil ericetum Ericae Tetralicis 

 prevails ; and in other places with moist soil is myricetum composed 

 mainly of Myrica Gale, Erica, and Calluna. Among other associations 

 may be named myrtilletum in which Vaccinium Myrtillus prepon- 

 derates. 2 



Calluna vulgaris, the ling, which is the species that forms associations 

 and gives the tone to heath, is a remarkable plant. It is accommodating 

 and tenacious of life ; it demands a not inconsiderable degree of atmo- 

 spheric humidity, yet is not nice in its demands as regards soil. It can 

 grow equally upon most sterile sands which may be at least temporarily 

 somewhat dry, and upon very wet, boggy soil, which is periodically dry, 



1 See pp. 107, 1 10. 



* In regard to heath in North- Western Europe, see Grabner, 1901 ; Rob. Smith, 

 1900 ; W. G. Smith, 1902, 1905 ; W. G. Smith, with Moss and Rankin, 1903 ; 

 Pethybridge and Praeger, 1905 ; Mentz, 19006, 1902 ; Borgesen and C. Jensen, 1904. 



