402 Plant Physiology 



maturity unless protected during the first season in New 

 England. The cotton and the cowpea disappear entirely 

 in a little more than half the range of corn, while timothy 

 and barley, almost unknown southward, approach their 

 prime near the northern limit of this area. 



In any scheme of continental plant zones, temperature is 

 recognized as most important. In general, such zones are, 

 therefore, constructed with special reference to the annual 

 or seasonal isotherms. No scheme of regions based largely 

 upon a single factor is entirely satisfactory. It is better, 

 however, than no attempt at classification. Koeppen, 

 Schimper, and others have indicated, on a broad basis, the 

 plant zones of the earth, and Merriam has arranged for 

 North America a suggestive scheme of life and crop zones 

 (Fig. 2). 



240. Cardinal temperatures. Certain cardinal temper- 

 atures are recognized. " Maximum " and " minimum " 

 are terms referring respectively to the highest and lowest 

 temperatures at which the development of a particular 

 organism may occur. It is apparent, however, that there 

 may be separate maxima and minima for every process or 

 activity of the plant. The maximum temperature for 

 germination may be below that which will support con- 

 tinued growth in the developing plant. It is difficult, or 

 at least inconvenient, to determine the most favorable 

 temperature for any process or function ; yet, within cer- 

 tain limits, such determinations are possible. The most 

 favorable temperature is designated the optimum. It is 

 also customary to employ the terms " ultra-maximum " and 

 " ultra-minimum," denoting respectively the death point at 

 high and at low temperature. The following tables from 



