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many cases they are continually changing their form and 

 are frequently movinr landward (Fig. 309). In the case of 

 these moving dunes a peculiar type of vegetation is de- 

 manded. Very few plants are able to live in such severe 

 conditions, and these plants have developed at least two pe- 

 culiar characteristics. One is that they are sand-binders; 

 that is, the underground structures are extremely far- 

 reaching, giving the plants a firm anchorage in the shifting 

 sand. As soon as enough of the sand-binders have es- 

 tablished themselves, a shifting dune becomes a fixed one. 

 Another characteristic of such plants is that they are able 



FIG. 309. Dunes of Lake Michigan encroaching landward, in this case diverting 

 Calutaet River. 



to grow up through the sand after they have been engulfed. 

 Along certain coasts where moving dunes encroach upon 

 farms and villages and threaten to engulf them, great at- 

 tention has been given to checking them by means of sand- 

 binding plants. 



