THE BRYOPHYTA OF ICELAND 591 



flexicaule, Lophozia lycopodioides , L. Knnzeana and Plagiochila as- 

 plenioides. 



The Moss Heath. On dry stony tracts there often occurs a 

 continuous growth of mosses, in which higher plants grow scat- 

 tered without being able to give character to the vegetation. In 

 heather-moors mosses often form a continuous carpet, and then all 

 transitions between typical heather-moor and moss-heaths are found. 

 Bv the decav of the mosses a humus-laver is gradually formed, 



, * / * 



and the moss-heath is thus the pioneer of other associations such 

 as heather-moors, coppices and grassland (H. Jonsson, 1905, p. 41). 



The moss heath occurs most frequently as a Rhacomitrium- 

 hcath (Griminia-healh, H. J., 1895, p. 70; 1900, pp. 68 and 85; 1905, 

 p. 40). It is found in the low land - where it is typically developed, 

 especially in the lava fields, which are often entirely covered 

 by it as well as on mountain slopes, and in the Alpine region 

 as far upwards as 600 700 metres above sea-level (Fig. 27). It also 

 occurs in patches in other formations, for instance in grassland, 

 where it is usually developed on the top of the knolls. Rhacomi- 

 trium hijpnoides is the dominant species, and is often the only one 

 found over considerable tracts; it forms a uniformly grey carpet, 

 as much as a foot deep. There is only a very small intermixture of 

 other Bryophyta. The most frequent are Rhacomitrinm canescens, 

 Hylocomium prolifernm , H. parietinum , Dicranum scoparinm , Ptili- 

 dinm ciliare, Frnllania Tamarisci, rarer are Lophozia lycopodioides, 

 L. barbata, and other species. The Rhacomitrium heath is extensively 

 distributed in many places on the taluses of fallen blocks and de- 

 bris (Urd), and there in case conditions are favourable by 

 the mosses decaying and forming humus, it quickly passes into other 

 associations, especially into grass-slopes and herb slopes. 



The Rhacomitrinm canescens heath is developed on a somewhat 

 damper substratum than is the R. hijpnoides heath, and is met with, 

 for instance, in patches in depressions in the latter, where it is 

 immediately distinguished by its light greyish-green colour, which 

 differs distinctly from the whitish-grey colour of its surroundings. 

 Here it must frequently share the locality with Hypnnm iincihalum 

 and is often greatly intermixed with other Bryophyta, for instance 

 Polytrichnm alpinnm, Hijlocominm spp., Lophozia lycopodioides and 

 L. qiiinquedentata, and several other species. As an example of 

 such a Rhacomitrinm canescens heath may be mentioned a slightly 

 inclined gravelly flat near Holt in South Iceland, about 400 me- 



