VEGETATION OF THE STRAND. 593 



or active, or stationary ones may be further divided into embry- 

 onic dunes and mature dunes. A stationary dune is one that 

 may continue to increase in size, but does not actively move 

 from place to place. An active or wandering dune is one that 

 is constantly moving from the action of the wind blowing the 

 sand from the landward to the leeward side. Embryonic dunes 

 are, of course, young ones, and there is no distinct line to be 

 drawn between an embryonic and an old or mature dune. Then 



Fig. 519- 

 Embryonic dune, shore of Lake Michigan. (After Cowles.) 



one might speak of fossil dunes and rejuvenated dunes. Fos- 

 sil dunes are those which have become covered for years by 

 forest and where dune action has ceased by the disappearance 

 or retreat of shores where wave action increases the amount 

 of sand. Rejuvenated dunes are old dunes which have sprung 

 into life again. 



1087. Dune-forming plants. Dune-forming plants must pos- 

 sess certain biological characteristics which enable them to not 

 only live under these very severe conditions, but they must also 

 possess the habit of growing in mats or tufts, or with the stems 

 and branches so related or interlaced as to check the force of 

 wind and cause the precipitation of the sand particles and afford 

 the sand shelter. Some of the biological characters best adapted 



