1909] OLSSON-SEFFER—VEGETA TION ON SEA SHORES 113 



Depressions between the dunes offer to many plants a refuge from 

 the sand-laden winds, and the richer soil in the troughs induces other 

 plants to settle. 



Grade of slope. — We have elsewhere (n) referred to this 

 factor as being of great moment in the growth of dunes and in the 

 development of sand formations generally. Its influence on the 

 moisture conditions is of no less importance. The higher up on the 

 slope the drier is the soil, the greater the evaporation, and the more 

 intense the influence of the wind. 



The exposure of the slope is another matter of the greatest con- 

 sequence to the vegetation on the sand formations. The various 

 degrees of slope ought always to be considered when a description of 

 a habitat is given in order to arrive at a correct understanding of the 

 conditions that have determined the composition of the plant com- 

 munity. Southern slopes in the northern hemisphere and northerly 

 -lopes in the southern are drier than those facing other directions, and 

 the vegetation has a corresponding aspect. On sea coasts the expo- 

 sure to the prevailing winds has to be noticed. 



Light relations 



gree determi 



direction of the slope, and this is of special importance in northern 



latitudes, where light even during the period of growth is not too 

 abundant. 



Historical factors 



tnder this heading we include all those factors whose influence 

 °n plant life is determined by time. This must not be understood 

 as if time was not involved in the action of other factors, but that the 

 Penod of influence and the time required for attaining any results is of 



a long duration, that it cannot be ascertained within a few 

 generations of plants. Physiographical changes of land and sea, 



co riH Within a centur y or within lon S geological periods, have to be 

 considered in this connection. One of the most important historical 

 ors to be looked upon in explaining the present conditions on 

 strands is the oscillation of the coast line. The erosion of the 



fact 

 sand 



shore by 



w 



aves and the deposition of sand or other material are 



rapiV- ! mp0rtant Sand deposits are in many places formed so 



influ * V the effeCt Can be noticed witnin a vei T short time - The 

 Uence of animals, principally through grazing, and the interven- 



