214 PLANT RELATIONS. 



has two remarkably developed characters. The plants are 

 what are known as " sand-binders/' that is, the underground 

 structures become extremely developed, reaching to great 

 distances horizontally and vertically, so that one is always 

 surprised at the extent of the underground system. This 

 wide searching for water results in giving the plants a deep 

 anchorage in the shifting soil, and at the same time helps 

 to prevent the shifting. As soon as enough of the sand- 

 binders have established themselves, a shifting dune becomes 

 a fixed one. Another characteristic that must be strongly 

 developed by these plants is the ability to grow up through 

 the sand after they have been engulfed. The plants of the 

 shifting dunes are often buried as the dune shifts, and 

 unless the burial has been too deep, they are able to continue 

 their development until leaves may be exposed to the air. 

 In this way plants have often developed a length of stem 

 which is far beyond anything they attain when growing in 

 ordinary conditions. 



The sandy field associations are represented by a much 

 more abundant flora than the beach or the dune associa- 

 tions, the general character being tufted grasses and low 

 shrubby growths (see Fig. 188). 



158. Shrubby heaths. — The shrubby heaths are very 

 characteristic of the more northern regions, and are closely 

 related to the sandy field associations. The heath soil is 

 apt to be a mixture of coarse sand, or gravel and rock, with 

 an occasional deposit of humus, and would be regarded 

 in general as a sterile soil. The flora of the shrubby 

 heaths shows well-marked strata, the upper one being the 

 low shrubby plants of the heath family, most prominent 

 among which are huckleberries and bearberries (see Fig. 

 172). The lower stratum is made up of mosses and 

 lichens. A branching lichen, usually spoken of as the 

 "reindeer moss," often occurs in immense patches on 

 such heaths. While these shrubby heaths occur most 

 extensively towards the north, small areas showing the 



