70 PLANT HABITS AND HABITATS IN THE 



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are shown in plate SB. At the climax of its development a typical 

 Nitraria hillock measures about 3 by 13 meters in horizontal plan. 

 The height rarely exceeds 1.5 meters and is usually somewhat less. 



The origin and growth of a hillock, mound, or dune seems to be about 

 as follows: In its earlier condition the seedling Nitraria has a well- 

 marked main stem with branches fairly well raised from the surface 

 of the ground. In this stage it does not interrupt the movement of 

 the drifting soil. Early in its development, however, the lower 

 branches come to lie on the surface of the ground and thus provide 

 an obstruction to free movement of the sand. The result is that such 

 horizontally disposed branches become covered by soil; they develop 

 roots and send up shoots which in growing maintain their position 

 above the accumulating sand drift. Thus the plant adds to its size 

 on every side ; the central portion grows actively and keeps above the 

 surface of the accumulating soil, and the characteristic hillock, ever 

 increasing in diameter and height, results. 



The long diameter of the mounds seems to be usually at right angles 

 to the direction of the prevailing wind, southeast and northwest, al- 

 though there may be exceptions to this. The " runners" which extend 

 the hillock colony, as well as characteristic features of such a colony, 

 are shown in plate 8c. 



When the maximum size has been attained the hillock begins to 

 break up in the following way : Portions of the plant which are about 

 centrally located die out, for various reasons, and the soil about them 

 begins to be removed, or possibly the converse is the initial step. 

 However this may be, the soil placed in the midst of the dune is gradu- 

 ally removed by wind action until the original level has been attained 

 a process of "base-leveling" and the two ends are completely sepa- 

 rated. Thus two daughter mounds ultimately result in the natural 

 course of disintegration of any Nitraria dune. 



From observations on the character of the flora which occurs on and 

 around the base of a Nitraria mound, it seems probable that the accu- 

 mulated soil of the hillock does not carry the large amount of salts 

 found in the soil characteristic of the plain proper. Thus a greater 

 proportion of nonhalophytic annual growth is found on the hillocks 

 than on the lower contiguous ground. 



ROOT-HABITS OF PLANTS OF THE PLAINS. 



The roots of typical annuals and of a few perennials (halophytes) 

 which were growing on the lowlands in the vicinity of Copley were 

 examined. The depth of soil in all cases was greater than the depth 

 attained by the roots, so that the maximum penetration took place. 

 The roots of the perennials were for the most part studied along washes 

 where they had been exposed, although in a few instances they were 

 examined remote from drainage channels of whatever sort. The roots 

 of annuals were seen on the open plain, but mostly where diminutive 



