H4 BOTANICAL GAZETTE [February 



tion of man belong strictly to the group of biotic factors, but have here 

 been considered in connection with the historical factors as a matter 

 of convenience. 



Oscillation of the coast line. — Many coasts are slowly rising, 

 while in other instances the coasts are sinking. We have excellent 

 examples of both kinds of movements on the Baltic. While the 

 whole coast of Sweden north of Stockholm, and the coasts of Finland 

 ire in a state of elevation, the southern shore of the Baltic is in a cor- 

 n-ponding state of depression. Besides having a great influence on 

 the development of dunes, this oscillation of the coast line has had a 

 marked bearing on the evolution of the flora on the coastal sands. 



On the shores mentioned which are rising, one may see in some 



instances how long stretches of land are slowly raised above water 



and in a few years carry a cover of vegetation that gives an instructive 



demonstration of the successive stages of development of the plant 



associations. Again, on the sinking shores of the southern Baltic 



may be frequently found examples of plant communities being 



destroyed in the course of a few years through the submersion of the 

 shore. 



On the coasts bordering upon the oceans oscillations also take 

 place, but they are usually neither so regular nor so rapid as tWp 

 ckangi s on the Baltic. 



In postglacial times considerable changes of the coast line of the 

 Scandinavian countries have taken place, and as we are able to follow 



4- L* _ _ _ 1 • .* . m . 1 L 



these 



beaches 



remains 



i, we 



hich 



-„,^v.^. iii^ii CkkJKJXK. Lilt. pUJSCllL 1CVU ^J- ****~ 



can to some extent interpret the various stages of development i 

 have been passed before the flora arrived at its present state. This 

 question will be discussed in another paper. 



Eolian deposits.— The influence of the wind on formation 



communities 



the 



peculiar 



* x / — 



aggregations of plants, which f^iwiallv in the case of trees 



shrub 



ttacked by the 



strong winds laden with spray or sand. Shrub associations on open 

 strands are usually lower toward the shore, gradually increasing i* 

 height inland under the shelter of the more exposed specimens. 



