SCANDINAVIA. 



249 



Warming (1906) has characterized in detail the commiuiities of the Danish 

 strand: 



Strand rocks show mostly only halophilous lichens (Ferrttcano, Placodium), 

 while the steep shore develops a community of Ttissilago, Eguisetum, etc., 

 when undisturbed. The sand-strand consists of four formations, the outer of 

 sand algae, then halophjrtes (Chenopods, Glaux, etc.), followed by a maritime 

 zone, and then by sand-field. The "geest," marsh, dunes, and "rade" 

 receive especially thorough treatment. The marine aquatic formation con- 

 sists of Zostera, ZannichelUa, Ruppia, Potamogeton, Naias, Myriophyllum, 

 Batrachium, and Chara. Sand-meadows contain in particular Triticum 

 junceum, Glyceria maritima, and Agrostis alba stolonifera. The muck-marshes 

 consist of Glyceria maritima, together with Suaeda, Aster, Spergvlaria, Plan- 

 tago, Glaux, Triglochin, and Juncus gerardii, with Armeria, Festuca, Odontie, 

 Artemisia, Statice, etc. The reed-swamps consist of Sdrpus taberbaemontani, 

 S. mariiimus, and Phragmites, with Juncus maritimus, Agrostis alba, Aster 

 tripolium, Triglochin, etc. 



Andersson and Hesselmann (1907) have made a structural study of a primi- 

 tive forest in Sweden, with especial reference to the influence of man, and with 

 some measurement of physical factors. Since the advent of private holdings 

 the number of species has increased from 175 to 260, but the plant communities 

 are in general unchanged. Four types of spruce forest are recognized, indi- 

 cating, it would seem, as many different stages of development: 



VaUey moors have developed chiefly from the accumulation of run-off, 

 rarely from the filling of lakes; the slope moors arise in spring-water. The 

 wetter areas are characterized by sedges, e. g., Carex chordorrhiza and C. 

 ampullacea. In drier places, Sphagnum appears abimdantly with the sedges, 

 while the driest parts consist of high moss hummocks, upon which Betula nana 

 and Rvbus chamaemorus are characteristic. 



Hesselmann (1907) has given an interesting accoimt of the causes which 

 prevent or limit ecesis and hence succession in the pools of the moors of 

 Norrland: 



Not only is the pool itself practically without vegetation, but the drained 

 hollow may also remain imcolonized for decades. This is due to the dried 

 felt of diatoms, which keeps the seeds from reaching the soil below, and also 

 to freezing, which forces the turf bottom upward, tearing out such plants as 

 may have established themselves. In other pools, colonization is prevented 

 by the application of salts preliminary to cultivation. 



Hesselmann and Schotte (1907) have considered in some detail the invasion 

 of the fir, and have examined the conditions which make it the successful 

 competitor in the majority of the forests. Of the three kinds of Calluna heath, 

 two terminate in fir forest, though it seems that this must also be the ultimate 

 fate of the heath with Kchen groimd cover. 



Skottsberg (1907) has described the vegetation on three parallel strand- 

 ridges buDt up by sea-weeds in Finland: 



The uppermost, 3 m. from the shore and 0.5 m. above it, was not reached 

 by waves during the growing-season. It contained 28 species and permitted 

 the development of perermials. The middle ridge, probably formed the 

 previous autumn, was 0.5 m. lower. It exhibited 45 species, but was destroyed 

 in large measure at the end of the season, as was also true of the lower ridge 

 with 19 species. ' 



