THE PRAIRIES. 23 



weekly humidity of the air during this same period. Not infrequently 

 during the long summer afternoons the humidity falls to 20 per cent 

 or less, while the temperature may reach 95° to 100° F. 



During the growing season the prevailing winds are from the south 

 or southeast. A standard anemometer placed in the prairie on a gentle 

 southwest slope and at a height of 0.5 meter recorded 4,905 miles of 

 wind from July 13 to September 19, 1916, an average of 72 miles per 

 day. A similar instrument placed at the same height above the soil 

 surface, but on a hill top in the same prairie area, showed an average 

 daily wind movement of 122 miles per day. 



The factors of high temperature, low humidity, and wind movement 

 combine to increase the evaporating power of the air. Livingston's 

 porous-cup atmometers, fitted with non-absorbing mountings, were 

 operated in the usual manner in duplicate and at a height of about 17 

 cm. above the soil surface during 1916 and 1917. The combined 

 evaporation records from two prairie stations, reduced to the readings 

 of the standard cup, are shown in figure 5. During the last week in 

 July 1916, when the mean daily temperature was 81.4° F. and the 

 mean relative humidity 57.4 per cent, the daily evaporation was 38 

 c.c. During this period no soil moisture was available in the first foot 

 of soil. The average daily evaporation throughout this whole summer 

 was 21.6 c.c. Atmometers exposed at a height of 0.5 meter during the 

 last 10 days in July gave losses over 50 per cent greater than similar 

 instruments placed at the usual height of 17 to 23 cm. In the prairies 

 near Peru, Nebraska, the average daily evaporation from May 15 to 

 September 22, 1917, was 20.9 per cent. 



Summarizing briefly, we find that prairie plants of this region grow 

 under semi-arid climatic conditions in w^hich the supply of water is the 

 chief limiting factor of plant growth. During certain portions of the 

 growing season extremely xerophytic conditions are brought to bear 

 upon the vegetation. It has been shown that the water-content of the 

 soil is reduced to the non-available point to a depth of 4 or 5 feet at 

 least during certain years and often at times when the evaporating 

 power of the air is very high (38 c.c. daily). 



In response to these environmental conditions many species have 

 developed surprisingly extensive root systems. In fact, all of the 

 dicotyledons examined, as well as many of the grasses, extend their 

 root systems to depths of from 3 to 7 feet or more. For just as the 

 evaporating power of the air and the nature of the transpiring organs 

 determine the water-loss of plants, likewise the soil water and the nature 

 of the root systems determine the supply. These findings of great root 

 depths, which are correlated with deep soil moisture, bear out Can- 

 non's suggestion of the probability that the longest or most deeply 

 penetrating roots are found, not in deserts, but where there is consider- 

 able rainfall, and where the penetration of rain is considerable and the 

 water-table relatively deep. 



