ECADS. 97 



rains, aside from the increased water-content, especially in heavy soils, 

 is the increased oxygen-supply to plant roots, rain-water being a solu- 

 tion highly charged with oxygen and having a markedly stimulating 

 effect upon growth. (Cf. Russell and Appleyard, 1915; Richards, 1917.) 

 However, the importance of aeration in the soils of the semiarid regions 

 under discussion would undoubtedly be much less than in more humid 

 areas. 



Aside from water-content, the chemical composition of soil, espe- 

 cially in studying root development in sandhills species, should be 

 given careful consideration. Since the early work of Nobbe (1862), 

 Stohmann (1862), and Hoveler (1892), Benecke (1903) and Tottingham 

 (1914) have done perhaps the most comprehensive work with solutes. 

 As pointed out by Waterman (1919), such work on the whole has been 

 done chiefly on the roots of seedlings, and it seems doubtful whether the 

 results would have been the same with mature plants. Benecke's 

 "hunger etiolation" theory, where scarcity of nutrients had a tendency 

 to increase root-length, is supported by the findings of Hoveler, wh© 

 grew plants in alternating layers of sand and humus. However, 

 Seelhorst (1902) concludes, after counting the number of roots found in 

 fertilized and unfertilized patches of soil, that plants strongly fertilized 

 not only produce stronger roots, but also roots penetrating to lower 

 levels. These and other experiments with soil nutrients show dif- 

 ferences in length and weight of roots, but offer no definite evidence as 

 to the causes of root extension. Waterman (1919), working with seed- 

 lings in the sand dunes about Lake Michigan, concludes that, after 

 "giving due weight to the possibility of moisture, oxygen-content, and 

 penetrability of the sand as influencing factors, the evidence seems to 

 point conclusively to nutrients or at least chemical influences as the 

 cause of variability in symmetry in the extension of roots under dune 

 conditions." Among all the roots examined by the writer, in various 

 sandhill areas, not a single instance of marked variability in symmetry 

 of a mature root system was encountered. Nor can the widely spread- 

 ing more superficial part of the root system, characteristic of most sand- 

 hills species, be accounted for in this manner, for the habit is almost 

 equally developed among species on the hard lands. However, as 

 Waterman suggests, there is a strong contrast between the pure dune- 

 sands with which he worked, in which, except for calcium carbonate, 

 mineral salts are practically absent, and organic matter so rare and 

 scattered that as a general factor it is practically negligible, and the 

 Nebraska-Colorado sandhills, where large quantities of desirable 

 mineral nutrients are present, needing only the addition of water to 

 make them available for plant use. The luxuriant growth of the 

 natural vegetation of sandhills regions, as well as of ruderal species 

 where favorable soil-moisture is found, emphasizes the paramount 

 importance of the latter factor. 



