CHAPTER XXV. 



THE UMIT ZONE OF THE SEMPERFLORENS CARNATION— 

 ISOTHERMAI. LINES-CLIMATIC CONDITIONS-EVERY TYPE 

 OF SPECIES CONFINED TO ITS ISOTHERM -THE CAR- 

 NATION'S IS FIFTY DEGREES MEAN HEAT. 



AN isotherm is a band or belt of the earth's surface, across a 

 continent, along which a definite number of heat units are 

 evolved during the year, vi^hich gives the belt a certain an- 

 nual average temperature. Isotherms are not strait, regular lines. 

 Their courses are changed by large bodies of water, ranges of 

 mountains, and elevations of land above sea levels. On the volcano 

 of Teneriffe there are five successive different zones of heat, each 

 producing a different class of vegetation. An elevation of i,ooo 

 feet above sea level equals one degree north latitude. Every iso- 

 therm on earth has its own Flora and Fauna, its modified men and 

 plants. Soil, sunlight, heat and moisture, are the prime features 

 in climate and factors of life. An isotherm is the composite of 

 these elements. Xo law in geographical botany is better deter- 

 mined than that a mean annual heat is required by each species of 

 plants for their full development, and that they will tolerate but a 

 slight variation in the number of yearl}^ heat units without modi- 

 fying their nature to conform to conditions which accompany a 

 less or greater number. 



It is surprising to how narrow a belt of specific annual heat 

 some species of plants confine themselves. The cotton plant will 

 submit to but 3 degrees of a variation. The sugar cane requires 

 83 degrees of heat to mature and will submit to but 5 degrees of a 

 change. It is not temporary fluctuations of temperature that af- 

 fect the fate of plants, but it is the number of heat units in the 

 definite time that rounds their lives into fruition. Some species 

 are more cosmopolitan in their nature, and their lives are normal 

 in broader zones. 



