HISTORICAL 103 



Arabis, Solidago and Chrysanthemum. In the protection of the 

 individuals of these species develop colonies of lichens, 

 Gladonia, Cetraria islandwa, Baeomyces roseus, etc. and mosses, 

 Bryum argenteum, Dicranum scoparium, Ceratodon purpureus, 

 «tc., while here and there spring up little plantlets of 

 Galluna and Empetrum. During rainy seasons, a tenacious 

 crust of algae, largely Gyanophyceae, covers the sand every- 

 where; this crust serves to hold the sand particles together, 

 and upon dying produces the first humus. The mosses also 

 play an important part in fixing the sand, in as much as 

 many species, when more or less covered by the blowing 

 sand particles, send up new shoots, forming a tuft. The 

 lichens especially are humus builders. Finally, in a locality 

 modified in the way above, the seedlings of flowering plants 

 are able to maintain themselves in large quantity. Galluna 

 and Empetrum enter more and more abundantly, Jasione, 

 LeucanthmauiH and Solidago decrease in number, while 

 Hypnum schreberi spreads steadily and covers the moist 

 places with a thick turf. In a similarly detailed fashion, 

 Graebner has traced the development of the heath moor and, 

 more generally, the modification of forest, and heath moor 

 into heath, as well as the changes exhibited by the latter, 

 especially under culture. GrevilHus ( J895) investigated the 

 vegetation of the islets of the Hjelmar sea, which Oallm^ had 

 studied in 1886. He found 215 species in place of the 115 

 observed by the latter. These were more uniformly dis- 

 tributed over the island, and the vegetation had become 

 much more closed. The typical structure of the vegetation 

 was as follows: (1) a strand-zone of grasses and sedges, (2) 

 a zone of shrubs, mostly Salix, below which grew small 

 strand plants, (3) a central nucleus of dense young forest, 

 mostly of Betula verrucosa, Populus tremula, and Alnus gluti- 

 nosa. Pines were found sparingly on a few of the islands. 

 Meigen (1896:212) found that, in the vineyards of Saxony, 

 which had been destroyed to exterminate the Phylloxera, 

 73% of the invaders of the first year were annual, 13^ 

 biennial, ruderal plants, 13% were perennials, while none 

 were woody plants. The changes of the second year con- 



