PRAIRIE PLANTS OF SOUTHEASTERN WASHINGTON 



239 



The three days from July 27 to 29 were clear, and, judging 

 from records of preceding and following days, quite typical. 

 The daily maximum air temperature was reached somewhat 

 earlier on the north than on the south slope, and was from 

 2°F. to 3°F. lower. 



A comparison of a large number of temperature readings at 

 1 foot on the two slopes brings out the facts that at this depth the 

 daily range of temperature is seldom over 1°F.; that the south- 

 west-side soils are from 3°F. to 5°F. warmer in early spring than 

 those of the north slopes, and that these differences may increase 

 by late summer to 7°-10°F. Table 5 indicates these temperature 

 differences of the soils of the two slopes to a depth of 5 feet, in 



TABLE 5 



Soil temperatures at 1-5 feet on April 18 and August 15, 1914, on a northeast and 



southwest slope, respectively 



early spring and in late summer. Undoubtedly the higher soil 

 temperatures prevailing on the south side have much to do with 

 the earlier seasonal activities of many plants. 4 



OTHER FACTORS 



Just as the possible growth of the aerial parts of plants is 

 affected by the extent of the development of the root system, 

 conversely the environmental conditions to which the aerial 

 parts are subject, especially as concerns their water relations, 

 must reflect themselves in the root development. Therefore, 

 it will be instructive before passing on to a consideration of the 



4 Several species of plants were observed to blossom from 10 to 17 days later 

 on north than on south slopes. 



