28 GEOLOGY OF SOIL. 



Observations in these regions, and experiments performed 

 by profound vegetable physiologists, confirm this statement. 

 They have proved that the seeds of the food-bearing plants, 

 even after germination has begun, can support greater de- 

 grees of drought and heat, than ever occur in the hottest 

 climates. The grains all germinate in the soil of a temper- 

 ature from 104 to 105 degrees, and require at least from 

 116 to 120 degrees to arrest this process. Barley ceases to 

 germinate at the lowest temperature. After barley, follows 

 wheat, then rye. Indian corn endures the highest heat, viz., 

 120 degrees, before its germination is arrested. The grains 

 flourish under a mean annual temperature of from 77 to 80J 

 degrees. Defining their equatorial limits, they are bounded 

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

 ture; 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 har- 

 vested in October. It is this line of equal winter tempera- 

 ture, or rather that of the coolest months, which allows the 

 grains to be cultivated in many places within the torrid zone, 

 and the line of 68 to 70 degrees Fah., which constitutes the 

 tropical limit of wheat culture, varies between 20 and 23 

 degrees of latitude. The other grains enduring from 5 to 7 

 degrees lower temperature, are found in higher latitudes. 



27. The wide belt of our globe, comprised within these 

 limits, extending from 20 to 70 degrees north latitude, pre- 

 sents every variety of geological structure ; yet nowhere, 

 in all this space, is the quantity or quality of crops affected 

 by the chemical nature of the underlying rocks. 



28. A similar principle governs the growth and cultiva- 

 tion of the grain-bearing plants on mountains. Their limits 

 are found at heights which correspond to the latitude which 



