XEROPHYTES 839 
many cases they are continually changing their form and 
are frequently moving 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 
Fic. 309.—Dunes of Lake Michigan encroaching landward, in this case diverting 
Calumet 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. 
