22 



THE ECOLOGICAL RELATIONS OF ROOTS. 



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

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

 environmental conditions to which the aerial parts are subjected, 

 especially as concerns their water relations, must reflect themselves 

 in the root development. Therefore, it will be instructive to consider 

 briefly the above-ground environment. 



Table 5. — Total water-content of soil during 1917. Table 6. 



The mean monthly temperature at Lincoln, Nebraska, for a period of 

 more than 30 years is shown in table 6. 



The season without kilUng frosts usually extends from the first day 

 of May into the first week of October, but frosts have occurred as late 

 as the last week of May and as early as the second week of September. 

 Figure 4 shows the average daily temperatures obtained during the 

 growing season of 1916, as recorded by a hygrothermograph placed 

 in an appropriate shelter in the prairie at a height of 17 cm. The 

 weekly means were obtained by drawing a horizontal line through the 

 weekly record sheet in such a manner that the total area above this 

 line included by the graph was equal to the total area below the line. 



Fig. 4. — Graphs showing the average daily 

 temperature (heavy line) and humid- 

 ity (light line) in the prairie during 

 1916. 



Fig. 5. — Graphs showing the aver- 

 age daily evaporation rates in 

 the prairie during 1916 (heavy 

 hne) and 1917 (light line). 



The areas were determined by the aid of a planimeter. In this inter- 

 pretation both temperature (or humidity) and the time factor are 

 taken into consideration. This figure also gives a record of the mean 



