98 ROOT DEVELOPMENT IN THE GRASSLAND FORMATION. 



As regards soil-texture, it seems clear that its chief influence on root 

 development is through water-content and aeration (cf. Hunter, 1912), 

 although mechanical resistance exerts some effect upon root structure. 

 The effects of compact soil upon penetration is shown not only by the 

 tortuous courses pursued and the distortions of the roots themselves, 

 but also by their modified branching. Roots of many species show a 

 marked increase in their output of branches upon leaving the compact 

 soil and entering earthworm burrows. This difference is probably due 

 largely to increased aeration. In soils with a subsoil of alternating 

 layers of sand and clay, a striking distribution of laterals was observed. 

 They often occurred abundantly near the bottom of the sandy layers 

 and in the clay strata when the latter, acting as a rather impervious 

 layer, had retained much soil moisture (cf. Cannon, 1913a). 



In "Ecological Relations of Roots" fairly close correlation was 

 established between the water-content of soils and the root distribution 

 of plants on gravel-slide, half gravel-slide, and in forest communities, 

 as well as in the chaparral of southeastern Nebraska and the grassland 

 association in the Pacific Northwest. Further studies in the grassland 

 formation confirm the conclusion that water-content is the dominant 

 factor. That different species respond differently to similar environ- 

 mental conditions, and that the reactions in certain cases are specific 

 and apparently controlled by heredity, have likewise been pointed out. 

 Of 10 polydemic species, each growing in at least 2 different habitats, 7 

 showed very striking changes in their root habits, adapting their ab- 

 sorbing organs to conform with the stratum of moisture-supply, 2 made 

 practically no change, while 1 exhibited only moderate differences of 

 root development (Weaver, 1919:121). Summarizing the present 

 study of plant ecads, we find that practically all of the species examined 

 under distinctly different conditions showed marked changes in root 

 development, either in surface lateral spread, depth of penetration, or 

 output of branches. Bulbilis dactyloides and Bouteloua gracilis, while 

 showing no consistent differences in depth of penetration, have a much 

 more marked surface lateral spread in the drier grassland communities. 

 Andropogon furcatus, Agropyrum glaucum, Andropogon scoparius, and 

 Aristida purpurea showed considerable difference in both depth of 

 penetration and, the last two especially, in surface lateral spread. In 

 hard lands this is quite clearly correlated with water penetration, while 

 in sand the remarkable lateral spread of Andropogon scoparius and 

 Aristida purpurea may also be considerably influenced by soil nutrients. 

 Gutierrezia sarothrce and Artemisia frigida showed a somewhat poorer 

 root development under the more unfavorable growth conditions of 

 short-grass plains than in mixed prairie. Psoralea tenuiflora retained a 

 fixed habit under widely vaiying conditions of soil-moisture and soil- 

 texture. Petalostemon purpureus, Lygodesmia juncea, Artemisia frigida, 

 Chrysopsis villosa, and Car ex pennsijlvanica all showed a somewhat 



