. Carnegie Institution of Washington publication. ROOT SYSTEMS OF TRUE-PRAIRIE SPECIES. 35 effect is a limitation in the development of both root and shoot of winter annuals (Cannon, 1911:88). With such superficially rooted species as Opuntia polyacantha and 0. camanchica, abundant both in the short- grass plains and mixed prairie, as indeed with all persistent portions of perennial roots in the surface soil, the seasonal changes in soil tempera- ture must be extreme. Soil temperature, like soil aeration, affects the development of root systems not only directly, but also plays a part in the l
. Carnegie Institution of Washington publication. ROOT SYSTEMS OF TRUE-PRAIRIE SPECIES. 35 effect is a limitation in the development of both root and shoot of winter annuals (Cannon, 1911:88). With such superficially rooted species as Opuntia polyacantha and 0. camanchica, abundant both in the short- grass plains and mixed prairie, as indeed with all persistent portions of perennial roots in the surface soil, the seasonal changes in soil tempera- ture must be extreme. Soil temperature, like soil aeration, affects the development of root systems not only directly, but also plays a part in the life activities of soil microorganisms. These, as conditioned by soil temperature, may affect the plants directly (Jones, 1917) or alter the chemical composition of the soil and thus influence the root environment, which in turn may modify its development. Thus, soil temperature as an ecological factor is clearly of much importance and warrants careful Fig. 11. A meter quadrat in the true- prairie community at the upland station at Lincoln, Nebraska. A, Andropo- gon scoparius; Af, Andro- pogonfurcatus; Al, Allium mutabile; Am, Amorpha canescens; An, Andropo- gon nutans; At, Aster multiflorus; B, Bouteloua gracilis; Be, Bouteloua curtipendula; C, Carex pennsylvanica; E, Eu- phorbia serpens; F, Fes- tuca octoflora; K, Kceleria cristata; L, Linum sul- catum; Le, Lepidium virignicum; Li, Liatris punctata; O, Oxalis stricta; P, Poa pratensis; Pa, Pa- nicum scribnerianum; S, Stipa spar tea; So, Soli- dago missouriensis; V, Viola pedatifida. Summarizing, these combined differences of lower rainfall, drier soil, greater evaporating power of the air, and less favorable temperatures for growth at the mixed-prairie station are reflected both in the vegeta- tional cover and in the root habit. Figure 10 shows a meter quadrat of the vegetation at the station in the mixed prairie (plate 4, a). In figure 11 is shown a similar area from the upland-prairie station at Lincoln (plate 4, b).
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