230 CLIMAX FORMATIONS OF NORTH AMERICA. 



ject to erosion. Here are found also scattered plants of Atriplex and Prosopis. 

 During the time of flood in early summer the river rises and covers nearly the 

 whole delta. After it subsides in July, the characteristic growth of annual 

 herbs takes place. 



MacDougal (1908: 11, 13) has indicated the successional relations of the 

 vegetation of the sand-dunes of Chihuahua, Mexico, and of the gypsum dunes, 

 or "white sands" of the Otero Basin, New Mexico: 



"The Chihuahua dunes are about 40 feet high, with scant winter vegetation 

 consisting of a few woody plants, principally a labiate bush {Poliomintha 

 nicana), an Artemisia, a Chrysothamnus, Yucca radiosa, and a suffrutescent 

 Senedo. Two perennial grasses, an Andropogon and a Sporobolus with spike- 

 like panicle {Sporobolus cryptaridrus) are of frequent occurrence, as are the 

 remnants of many annual plants. The Yucca takes an important part in 

 binding the sands; roots were seen extending in a horizontal direction nearly 

 40 feet from the plant. From the dunes toward Samalayuca, the valley bot- 

 tom has vegetation of mesquite mixed with Zizyphus, Koeberlinia spinosa, and 

 Atriplex canescens." 



In the Otero Basin, the 'white sands' constitute an area of about 300 square 

 miles covered with dunes of gypsum sand rising to a maximum height of 60 feet: 



"The surface of the dunes is sparkUng white, due to the dry condition of 

 the gypsum powder, but a few inches beneath it is of a yellowish or buff color 

 and is distinctly moist and cool to the touch, even when the air is extremely 

 hot. The smallest particles may be crumbled in the fingers, and as a conse- 

 quence the dunes are solidly packed except on newly forming steep slopes. 



"The most characteristic plant of the dunes is the three-leaf sumac (Rhv,s 

 trilobata), which occurs in the form of single hemispherical bushes 4 to 8 feet 

 high, the lower branches hugging the sand. The plant grows vigorously, the 

 trunk at or beneath the surface often reaching a diameter of 3 inches. The 

 binding and protecting effect of this bush is often shown in a striking manner 

 when in the cutting down of an older dime by the wind a column of sand may 

 be left protected above from the sun by the close covering of the branches and 

 leaves, and the sand in the column itself bound together by the long penetrating 

 roots. One of these columns was about 15 feet high from its base to the sum- 

 mit of the protecting bush and about 8 feet in diameter at the base. 



"Other characteristic woody plants of the dunes are Atriplex canescens, 

 two species of Chrysothamnus, and Yucca radiosa. The imderground trunks 

 of the Atriplex often attain a diameter of 4 inches, those of the Yu£ca 6 inches. 

 A marked pecuharity of the white sands is that a cottonwood is occasionally 

 found in the lower dunes, reaching a foot in diameter, but seldom more than 

 15 feet in height; yet at the same time not a mesquite was seen. The mesquite 

 is a tree requiring less moisture than the cottonwood. Apparently the pres- 

 ence of an excess of gypsum is prejudicial to the growth of the mesquite. 



"The bottoms among the dunes have a dense vegetation as compared with 

 that of the dimes themselves. It is characterized especially by the presence 

 of a grama grass {Boutelouu), forming almost a turf, and by frequent clumps 

 of Ephedra of a grayish purple color at this season and with 3-scaled nodes. 

 These bottoms usually show no signs of moisture, but in two places we found 

 water-holes, the water so alkaUne that the horses would not drmk it at the end 

 of then- first day's drive. About both holes occurred the salt-grass {Distichlis 

 spicata) and wire-grass {J uncus balticus), both of them characteristic of moist 

 alkaline soils. 



