UNSTABLE SOILS 



153 



99. Unstable Soils.— In order tliat soil may be used by plants it 

 must remain reasonably quiet. The most notable examples of 

 unstable soils are found in sand dune areas alon^ sea coasts and in 

 other regions of drifting sand hills. Very few i)lauts can grow on 

 shifting sand dunes. In the case of such dunes the sand is constantly 

 being blown up from behind and dumi)ed in front so that the dune 

 gradually moves along in the direction toward which the wind is 

 blowing. Plants that may be growing on the sand near the rear of 

 the dune, therefore, are likely to be torn out by the roots or at least 

 to have their roots uncovered and exposed to the drying air. On 



-J^'i:^ * 



*!%' 



Fig. 66. — Atmnophila arenaria (beach grass) and Hiidsonia tomentosa, two sand- 

 binding plants. (Photograph by A. G. Eldredge.) 



the other hand, plants growing in the path of the dune may be 

 buried by the sand. A large dune may even overwhelm a forest 

 and, passing on, leave it a veritable cemetery of dead trees. 



There are, however, certain grasses and a few other plants, known 

 as "sand binders," which are capable of growing on shifting sand 

 and often help to stop its motion (Fig. 66). Their stems elongate 

 as the sand piles up around them so that their tops are kept above 

 the surface and at the same time adventitious roots are developed 

 on the buried portions so that the distance from the top of the plant 

 to its nearest roots is never excessive. Some trees, as for example the 



