II2 BOTANICAL GAZETTE [februaby 



the 



Gulf of Finland, where almost the whole shore line consists of a 

 steep wall of rock, leaving only a narrow strip of beach along the 

 water edge. In places where this rocky barrier was broken and the 

 winds are allowed free play over a wider stretch of land, dunes ap- 

 peared at once. The vegetation on the beach of the former type 

 presents a somewhat different aspect from that on open shores with 

 the background of a dune-complex. The best evidence of the influ- 

 ence of surroundings on the composition of the vegetation can be seen 

 if we compare that on a sand field and on a dune-complex with its 

 diversified topography. Also on the slopes of an unbroken dune, the 

 vegetation is usually quite different from that on a train of dunes fre- 

 • quently cut through by furrows and valleys. 



On beaches a similar difference can be noticed, and the cause 

 underlying this effect can only be attributed to the topography. Where 

 . we have a long continuous beach the plants associate according to 

 rules different from those which have determined the composition of 

 the communities on cuspate forelands. This was especially evident 

 on the shore stretches of sandy beaches that are so common on 

 shores of the islands in the Baltic. 



Elevation.— This factor is of minor importance in regard to the 

 sand-strand vegetation. The sand formations do not rise to any 

 great height, but it seems that certain plants choose their place of 

 growth with reference to altitude, even on these formations. Without 

 taking into account the fact that humus naturally accumulates more 

 rapidly in the depressions, we find that some plants prefer the foot of 

 a dune, while others are found only on the middle of the front slope 

 and others again do not thrive except on the top of the dune, W 

 they are constantly being covered with drifting sand. 



On the beach a corresponding selection of habitat takes place- 

 Some plants never occur on a low beach although the conditions 

 otherwise seem to be favorable, but only a short distance away, where 

 the beach rises more abruptly, they appear again. We have pre- 

 sumably two different causes for this. While on the dunes the selec- 

 tion of a place of growth is determined apparently by the plant' 5 

 greater or less power of resistance against the drifting sand, on the 

 beach the dominant cause must be the sensitiveness of the plant to 

 inundations of salt water. 



.■ 



here 



