SCANDINAVIA. 247 



land is invaded. The first colonists on the young lava-streams are crusta- 

 ceous lichens. These are followed later by a pure carpet of Grimmia hyp- 

 noides. Just as soon as this moss has gathered the necessary humus, isolated 

 rock and heath plants enter and the lava-field is gradually turned into a scanty 

 heath. Wherever hollows occur in which a thicker humus layer can arise, a 

 grass vegetation develops. 



Hayren (1902) has followed in detail the succession in new land formed by 

 streams in a small bay on the coast of Finland: 



In the outer part mud is gathered and peat formed by Phragmites, Scirpus 

 lacustris, and Typha angustifolia. Between these communities are foimd 

 pools with Myriophyllum and other water-plants which accumulate mud. 

 The first terrestrial plants, Calla palustris, Lystmachia thyrsiflora, and Menyan- 

 thes, then enter, and the reed-grasses and rushes disappear, Typha first, Phrag- 

 mites last. The sedges, Carex aquatica, C. pseudohelvola, etc., then appear, 

 and wet meadows or meadow moors are formed. Sphagnum subsecundum 

 prepares the way for other species of Sphagnum, and a moor is produced. 



The line between the loose and the firm soils is constantly pushed outward. 

 Agrostis alba is the pioneer, followed quickly by Carex vulgaris, the two forming 

 a closed community. Sphagnum then often spreads over the meadow, Agros- 

 tis and Carex decrease, and moor results. In drier places, Deschampsia 

 replaces the meadow grasses, and finally Carex vulgaris, and the herbs are 

 replaced by Rumex acetosa. Ranunculus acris, etc. The most important of the 

 later changes are due to the influence of man, exerted in drainage, and seen 

 especially in the extension of land at the rate of 7.6 meters a year for the last 

 half centvuy. 



Resvoll (1903) has described the vegetation developed in consequence of a 

 land-slide near Trondhjem in Norway in 1893: 



The vegetation on the primary area uncovered by the slide varied with the 

 nature of the soil. On gravel, 32 species had appeared by 1898. Of trees 

 and shrubs, only isolated individuals of spruce, juniper, birch, aspen, and 

 willow were found. This was also true of the majority of the herbs, only 

 Tussilago, Vacdnium vitis-ida^a, Empetrum, and Ldnnaea forming dense fami- 

 lies. The vegetation of the mud flats showed 43 species, chief among them 

 Triglochin pahistre. Polygonum aviculare, Cerastium mdgatum, Rumex acetosella, 

 Alopecurus geniculaius, and Tussilago, the latter alone forming families. The 

 water and swamp vegetation showed the richest development. In the pools 

 were Potamogeton, Sparganium, Callitriche verna, and NiteUa opaca, and around 

 the margins communities of Equisetum jlwoiatile, Carex, Glyceria fluitans, Jun- 

 cu^, etc. A pool filled in 1898 with little but Sparganium minimum showed in 

 1902 a dense mass of Equisetum fluviatile, with Sparganium still present, 

 Hippuris vulgaris, Epilobium palustre, and isolated firs, birches, and willows 

 at the margin. 



The vegetation of the secondary mixed soil of the land-slide was in many 

 respects like the primary vegetation, due to the fact that the latter was strewn 

 with islets of the original vegetation. Tussilago and Equisetum arvense 

 covered most of the surface. 



Cajander (1904) has described the mountain vegetation of northern Finland: 



In the alpine region, the commimities are the lichen on rock plateaus; heath 

 of Empetrum, Arctostaphylus alpina, and Phyllodoce; heath-moor, from which 

 grass moors of Sdrpus caespitosus or Eriophorum angustifolium arise by so- 



