CLIMATIC FACTORS : RADIANT ENERGY 



127 



be five degrees higher under forest litter in the fall of the year 

 than in bare ground and the daily maximum to be seven degrees 

 lower. The average diurnal range was eighteen degrees in bare soil 

 and only six degrees under litter. In North Carolina, 165 litter re- 

 duced the depth of frost penetration 40 percent, and, whereas the 



TABLE 7. The average day and night temperatures (°F) in three upland 

 forest communities in central Iowa. Air temperatures in contiguous prairie 

 are higher than those in shrub by about 10° (day) and 4° (night). From( 4 ). 



bare soil was frozen solidly, the soil under litter remained porous 

 and loose, permitting deeper percolation during winter rains and 

 thaws and causing very little runoff. The effects of snow as an 

 insulator are much the same as are those of litter. 



Temperature and Physiological Processes.— There is probably 

 for every species an average optimum temperature at which it 

 grows most successfully, other factors being equal. Likewise there 

 must be a maximum and a minimum temperature that it can with- 

 stand. These limits may result from the temperature tolerances of 

 the protoplasm peculiar to the species, but they may likewise 

 result from responses of one or more physiological processes, 

 which vary from species to species. 



The temperatures affecting germination might alone limit the 

 range of a species. Among our cultivated crops, the minimum- 

 maximum range of temperature for germination is 35°-82° F. for 

 flax and 49°-115° F. for corn. The optimum for each, respectively, 

 is 70° and 93°. That the center of production for flax is consid- 



