124 THE ECOLOGICAL RELATIONS OF ROOTS. 



precipitation, which occurs so gently that there is httle run-off, is held 

 as in a great reservoir that is rather thoroughly emptied during the 

 following summer. Soil-moisture determinations for two seasons 

 show that the water-content is sometimes reduced to the non-available 

 point for plant growth, especially on exposed slopes, even to a depth 

 of 3 or 4 feet. The wind, prevailingly from some southerly direction, 

 averages about 96 miles a day just above the vegetation on south 

 slopes. The air temperatures, which may reach 90° to 100° F., show 

 a mean daily range varying from 25° F. in April and May to 38° F. 

 in July and August. The mean daily soil temperatures at a depth of 

 3 inches vary from 40° to 50° F. in May to from 66° to 92° F. in August, 

 depending upon the slope. The humidity, normally low, frequently 

 falls to 10 or 15 per cent during afternoons. The average daily evapora- 

 tion throughout the growing season is about 29 c.c. on southerly slopes 

 and 20 c.c. on northerly ones. 



Root penetration and the amount of branching were found in both 

 the prairies of Washington and those of Nebraska to be profoundly 

 affected by hard soil. Both the root length and the amount of branch- 

 ing of a prairie species growing under the two conditions of soil texture 

 were found to be more pronounced in the less compact soil. Plants 

 growing in the mellow loess soil of eastern Nebraska are deeper-rooted 

 than those growing in glacial drift soils. Aeration may be an impor- 

 tant factor in effecting these modifications. Roots show a marked 

 increase in their output of branches upon leaving compact soil and 

 entering earthworm burrows or small crevices. 



The root systems of 28 important plains species were examined near 

 Colorado Springs, Colorado. They were found to have deeply pene- 

 trating and widely spreading roots. The lateral spreading of roots in 

 the surface soil at depths of from 2 to 12 inches is much more pro- 

 nounced than in either of the preceding prairie communities. Only 

 four plants, including two cacti, had their root systems confined almost 

 entirely to the surface 2 feet of soil. One-third of the plains species 

 penetrated to depths greater than 2 feet but seldom beyond 5 feet. 

 This group includes such dominant grasses as Bouteloua gracilis and 

 Aristida purpurea. Nearly 60 per cent of the plains species had roots 

 extending to depths greater than 5 feet. The roots of several plants 

 reached depths of 10 to 12 feet. Although as a conamunity plains 

 plants are deeper-rooted than those of the prairies, no roots were found 

 at a depth greater than 13 feet, as was the case with several prairie 

 species in Nebraska. It is probable that w^ater never penetrates more 

 deeply than 13 feet in the plains. 



Like the plants of prairies and sandhills, the roots of plains species 

 show a wide range in type from those with the tap as the principal 

 feature or with the laterals placed near the surface and especially well 

 branched, to roots with both tap and laterals well developed. How- 



