1909] OLSSON-SEFFER— VEGETATION ON SEA SHORES 11 1 



Development of humus. — There is no other soil which so little 

 favors development of humus as the loose shifting sand. The organic 

 -ubstances that happen to be deposited on the sand are very rapidly 

 decomposed by the admission of air, and the physical structure of the 

 sand allows rain water to percolate and thus to carry the fine humus 

 panicles deep into the soil and out of reach of the roots. The earth- 

 worms, which are the most active agents in mold formation in forests, 

 as Darwin and Mueller have shown, are entirely absent in sand 

 and the mycorhizal fungi seem not to thrive on the beach, where they 

 are likely to be exposed to occasional contact with sodium chlorid. 



When a shrubby vegetation has got a foothold on the sand, the 

 humus is developed to the best advantage. In the shade of the 

 bushes remains of plants do not decompose so easily as on the open 

 ground, they are more sheltered from the rain, and an accumulation 

 of humus can take place, so that grasses and herbs are able to get a 

 footing. When this has happened the sand is usually made perma- 

 nently stable. The few animal remains that are thrown up on the 

 front or middle beach enrich the soil on these formations only tempo- 

 rarily, and do not play any important r61e in the formation of the humus 



on the sand. 



Topographic factors 



\Y 



The topography as a factor influencing the development of vegeta- 

 ! 10n ls verv often overlooked by writers on plant geography. Its 

 importance, however, is so considerable that it cannot be omitted in a 



iscussion of the agents which exert their influence on plant life. 



n the vegetation topography acts principally indirectly, by determin- 

 J° g t0 a S r eat extent the moisture content of the soil, by influencing 

 w tem Perature, the exposure to winds, and also the light relations. 

 to C lntend her e to mention briefly only the principal features of 

 ^Pography as f ar as they influence the conditions on coastal sand 



opme RR ° UNDINGS '~" Fr0m OUr P revious discussion (n) of the devel- 



roundf ° f the Various sand formations it is apparent that the sur- 



lngs are of the greatest consequence to the evolution of dunes. 



h ave ? any of the coasts investigated the topographical conditions 



dune eC Q ° f SUCh cnaracter as to prevent any greater development of 



Such was the case, for instance, on the southern shore of the 



