18 GEOLOGY OF SOIL. 



has a distinct isotheral line parallel to that of wheat 

 and harley. Indian corn and the potato have each 

 its isotheral line. Turning to the equatorial limits 

 of the grains it will he found, that extreme heat ar- 

 rests their cultivation. Observations in these re- 

 gions, and experiments performed by profound veg- 

 etable physiologists, confirm this statement. They 

 have proved that the seeds of the food-bearing 

 plants, even after germination has began, can sup- 

 port greater degrees of drought and heat, than ever 

 occur in the hottest climates. The grains all ger- 

 minate in a soil of a temperature from 104 to 105 

 degrees, and require at least from 116 to 120 de- 

 grees to arrest this process. Barley ceases to ger- 

 minate at the lowest temperature. After barley, 

 follows wheat, then rye. Indian corn endures the 

 highest heat, viz : 120 degrees, before its germina- 

 tion is arrested. The grains flourish under a mean 

 annual temperature of from 77 to 80 1-2 degrees. 

 Defining their equatorial limits, they are bounded 

 not by lines of equal summer, but equal winter tem- 

 perature ; the reverse of their polar limits. Hence, 

 climate, always determines the sowing season. In 

 Bengal, wheat, barley, oats, are sown in October 

 and harvested in March and April, while rice and 

 maize are sown in May, to be harvested as with us 



