PLANT MIGRATION. 5OI 



dently these belts vary for different plants, but there are many 

 plants whose temperature requirements are the same. These 

 come into competition over similar areas. The temperature 

 belts favorable for the growth and reproduction of plants and 

 animals are called "life zones." In general "life zones" are 

 transcontinental, but they do not correspond to belts limited by 

 lines of latitude because of the variations in altitude, and because 

 of warm air-currents from bodies of water which affect areas that 

 do not coincide with latitudinal ones. It was once thought that 

 these life zones coincided with isothermal lines or belts, i.e., 

 those lines or belts formed by connecting numerous points hav- 

 ing the same mean annual or seasonal temperature. While this 

 is approximately true in tropical regions where the fluctuations 

 in temperature are slight throughout the year, it does not apply 

 to other regions because of the great fluctuations of tempera- 

 ture, and because isotherms show the mean temperature for 

 arbitrary periods (year or month) which do not correspond with 

 the period of growth and reproduction. The life zones are 

 determined by the physiological constant of a species, and may 

 be called biothermal lines, or biotherms. 



968. Biothermal lines, or biotherms. The physiological con- 

 stant of a species is determined by the total effective heat that 

 is, the total heat above 6 C., during the season of growth and re- 

 production. It is the sum of the mean daily temperatures (above 

 6 C.) during the season from the time when growth begins in 

 the spring (about 6 C. or 43 Fahr.) to the time when growth 

 ceases in the fall (6 C. or 43 Fahr.). By connecting numerous 

 points having the same total heat during the growing season the 

 biothermal lines are known. These lines, which in general are 

 transcontinental, mark the limits beyond which species do not 

 migrate (except in rare instances) in their northward distribution. 



969. Life regions. Three transcontinental life regions are 

 recognized in the northern hemisphere: the Boreal, or Northern; 

 the Austral, or Southern; and the Tropical. These regions were 

 first established by Alexander von Humboldt when he divided 

 the globe into the great life belts. The regions were separated 



