THE BUYOPHYTA Ol ICELAND 591 



flexicaiile, Lophozia lycopodioides , L. Kunzeana and Plu(jiochila as- 

 plenioides. 



The Moss Heath. On dry slony tracls Ihere often occurs a 

 continuous growlh of niosses, in which liigher phints 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. 

 By the decay of the mosses a humus-layer 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 frequenlly as a Rhacomitrium- 

 hcath (G/7/J7/7?/a-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 fieids, 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 knoils. Rhacomi- 

 triiim hypnoides is the dominant species, and is oflen 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, 

 Hijlocominin proliferum , H. parietiniini , Dicraiiiim scopdriiim , Ptili- 

 diiim ciliare, Friillania Tamarisci, rarer are Lophozia hjcopodioides, 

 L. barbata, and other si)ecies. The Rhacomitrinm heath is extensively 

 dislributed in many piaces on the taluses of fallen blocks and dé- 

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

 the mosses decaying and forming humus, it ([uickly 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. hypnoides heath, and is met with, 

 for instance, in patches in dci)ressions in the lutter, where it is 

 iuimediately distinguished by its light greyish-green colour, which 

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

 Here it must fre([uently share the localily with Hypnnm nncinatnm 

 and is often greally intermixed wilh other Bryophyta, for instance 

 Polytrichnm alpinnm, Hylocominm sp[)., Lophozia hjcopodioides and 

 /.. ({ninqnedent(da, 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- 



