SUMMARY. 123 



feet is available for plant growth. The wind, prevailingly from the 

 south or southeast during the growing season, averages about 70 

 miles per day over the vegetation. The mean summer temperatures 

 are high (70° to 75° F.), while the mean humidity is often low (58 to 70 

 per cent). The average daily evaporation throughout the growing 

 season is about 21 c.c. 



The plants of the prairie community, in response to these environ- 

 mental conditions, have developed very efficient widely spreading and 

 deeply penetrating root systems. The prairie community root habit 

 emphasizes depth of penetration and widely spreading, deep laterals 

 much more than the desert community (Cannon, 1911, 1913; Markle, 

 1917) . The roots of prairie plants are grouped into more or less definite 

 absorbing layers, many of the deeper-rooted species having few or 

 no absorbing roots in the first few feet of soil. The layering of the 

 roots reduces competition and permits the growth of a larger number 

 of species. 



The root systems and mechanism of invasion of several dominant 

 shrubs of the chaparral community, which occupies the tension zone 

 between forest and prairie, were examined. All are supplied with 

 splendid absorbing systems which are somewhat variable as to depth 

 (5.5 to 21 feet), but all of which are deep-seated. In addition, they 

 all have excellent methods of vegetative propagation. Their presence 

 indicates a soil richer in humus and higher in water-content than 

 that of adjacent grassland areas. These differences, as well as a lower 

 evaporating power of the air, are brought about by the reactions of 

 the shrubs. These consist of the accumulation of wind-blown snow and 

 plant debris, a lowering of the light values and a consequent disap- 

 pearance of most or all of the prairie species, obstruction to wind 

 movement, lower temperatures and higher humidity, in part due to 

 shade, and a less rapid oxidation of the humus materials. 



The root systems of 18 dominant and subdominant species were 

 examined in the prairies of southeastern Washington. These prairies 

 represent an extreme westward extension of the great grassland 

 formation lying east of the Rocky Mountains. They are characterized 

 by the absence of late-maturing grasses and in general by an earlier 

 seasonal development. Three of the four dominant grasses have root 

 systems confined to the surface 18 inches of soil. This is correlated 

 with early seasonal growth and maturity. The fourth, hke the other 

 herbaceous species, has a deep, widely spreading root system. All of 

 these reached depths from 4 to 6 feet, while some penetrated even 

 deeper. However, the plants of the prairie community are not so 

 deeply rooted as those of the prairies of eastern Nebraska. This may 

 be correlated with the environment. 



These prairie species grow under a mean annual precipitation of 

 21 inches, only about 7 inches of which fall during the growing season. 

 But the silt-loam soil is extremely retentive of water, so that the 



