SUMMARY. 125 



ever, the latter or generalized type is the most common. Notwith- 

 standing these individual differences, each community, viewed as a 

 whole, has as its own root habit the one best fitted to the particular 

 environment. 



These plains species grow under a mean annual precipitation of 

 about 15 inches, 12 inches of which fall during the growing season. 

 The soil is a sandy loam so compacted that ordinarily it can be removed 

 only with a pick. Because of the great degree of compactness, run-off 

 after rains is high. Moisture determinations during 1918 show that 

 the soil to a depth of 7 feet is rather uniformly dry and that at certain 

 periods during the summer no water is available for plant growth to 

 a depth of 5 feet. The wind movement over the plants averages about 

 120 miles per day. The daily fluctuation of temperature among the 

 plants is usually about 35° to 40° F., the air reaching a maximum of 

 90° to 95° F. Soil temperatures among the roots at a depth of 4 

 inches ranged daily from 60° or 70° F. to 90° or 95° F. The average 

 daily evaporation throughout the growing-season is about 48 c.c. 



The well-developed system of shallow, widely spreading laterals is 

 undoubtedly a response to the moisture in the surface soils resulting 

 from frequent Hght summer showers. These surface roots are especially 

 well developed in cacti. Those of Yucca are remarkable for their great 

 lateral extent, often reaching a distance of more than 30 feet on every 

 side of the plant. Bisects show that root competition among plains 

 plants, especially in the first 2 feet of soil, is very severe. 



The root habits of 19 important sandhill species were studied in 

 eastern Colorado, about 40 miles southeast of Colorado Springs; 8 of 

 these have roots which are entirely or nearly confined to the surface 2 

 feet of soil. The deeper-rooted species, none of which were traced 

 below 11 feet because of the caving sand, practically all show a striking 

 profusion of long, widely spreading laterals in this surface-soil stratum. 

 The lower parts of deeply penetrating roots are often much more 

 poorly branched. Several deep-rooted plains species under sandhill 

 conditions develop only shallow roots. 



General climatic conditions are almost identical with those described 

 for the plains; hence differences in root habit must be attributed to 

 edaphic causes. Precipitated moisture is readily absorbed and there 

 is practically no run-off, even in the heaviest showers. During dry 

 weather the surface sand forms a very efficient means of retarding 

 evaporation. Moisture determinations and observations show that 

 the surface 2 or 3 feet of soil often is moister than the deeper strata, 

 and thus offers a logical explanation for the typically shallow but 

 widely spreading sandhill root habit. Several plants have in addition 

 deeply penetrating roots; hence the deeper soils must receive moisture, 

 perhaps during wet phases of the climatic cycle. Ipomoea leptophylla 

 has the most extensive root development of any species examined in 

 the eight communities studied. 



