36 Development and Activities of Roots of Crop Plants. 



influence on the water-content relations. The potato leaves had begun to 

 wither and die. However, it seems probable that the rapid development of 

 the aerial parts of the corn was the major cause of decreased water-content. 



Table 5. — Water-content in excess of hygroscopic coefficient in the corn 

 and potato plats. 



An examination of table 6 shows how efficient is the massive, deeply pene- 

 trating root system of corn. During the dry month of August the available 

 moisture had been reduced to only 4 or 5 per cent, even to a depth of 5 feet. 



Table 6.— Water-content in excess of the hygroscopic coefficient in the corn plat. 



The enormous transpiring surface presented by a field of corn should be 

 emphasized. Kiesselbach (1916) has calculated that an acre of corn in east- 

 ern Nebraska planted in hills 3.5 feet apart with 3 plants per hill has a tran- 

 spiring surface of approximately 4 acres. Closely correlated with the devel- 

 opment of the transpiring area is the growth of the absorbing system. Fig- 

 ures 8 and 9 well illustrate the enormous absorbing area of a single stalk, 

 while table 6 indicates the degree to which this cereal exhausts the available 

 moisture even to a depth of 5 feet. 



Soil temperature is an important ecological factor affecting root growth. 

 Like soil aeration, it affects the development of the root system not only 

 directly, but also plays a part in the Hfe activities of soil micro-orgamsms. 

 These may affect the plants directly or alter the chemical composition of the 

 soil and thus influence the root environment, which in turn may modify 

 its development. The importance of temperature in the development of 

 root systems has only recently been emphasized (Cannon, 1918), and coni- 

 paratively few investigations have been made on the evaluation of this 

 factor in root growth. An examination of figure 15, which gives thermometer 

 readings at various depths, shows at once that the temperature of the wet 



