180 



grass on the sand-hills of Nebraska (Pound and Clements, '00, 

 p. 353). 



Next in importance as sand-binders are certain species 

 which grow in close hemispherical or flattened bunches, and 

 hold the sand in the same manner as the bunch-grasses. Rhus 

 aromatica (PI. XIX.) grows in dense thickets, sometimes twen- 

 ty feet in diameter, its long branching roots descending to a 

 depth of six feet or more, and effectually resisting the move- 

 ment of the sand. Cracca virginica, Lithospermum linearifolium, 

 Salix tristis, Chrysopsis camporum, and especially Amorpha 

 canescens, have the same habit as Rhus, but on account of their 

 smaller size are less effectual as sand-binders. 



Sporobolus er yptandr 'us, and ( ■ommelina virginica have stems 

 prostrate and rooting at the nodes, and both are quite effectual 

 sand-binders, although the latter species is not frequent, and 

 neither reaches a large size. Sporobolus is quite abundant on 

 level blow-sand, where it builds up mounds from four to eight 

 inches high (PI. XVIII., Fig. 2). It is one of the principal 

 species concerned in fixing blow-sand and preparing it for the 

 re-establishment of the bunch-grass association. Commelina 

 also builds up small mounds and ridges of sand. 



A third type of sand-binders is composed of mat plants, 

 which have prostrate stems radiating from a central root. 

 Euphorbia Geyeri is common on blow-sand, where it grows in 

 circular mats from six inches to two feet in diameter. It thus 

 effectually holds the sand beneath it, and is frequently seen on 

 a flat mound an inch of two high, closely similar in shape to 

 the plant itself. Mollugo verticillata, frequently naturalized on 

 blow-sand and in the bunch-grass, has the same habit. Opuntia 

 humifusa may be classed in the same group. It forms dense 

 mats two to five feet in diameter, and effectually holds the 

 sand. The center of the mats, however, are frequently buried. 

 Acerates viridiflora, usually erect in a richer soil, is prostrate 

 when growing on blow-sand, and builds up small mounds. 



Intermediate between prostrate forms like the last and 

 bunch-forming species like Chrysopsis are such plants as Soli- 

 dago missquriensis, and ( 1 <(Mrhoe triangulata. These have sev- 



