156 ENVIRONMENTAL CONDITIONS. 



frost " (in the agricultural sense) practically marks the end of the active 

 season for the vast majority of plants. The average dates of occur- 

 rence of the last killing frost in spring and of the first in autumn should 

 furnish us with a valuable index to the approximate length of the 

 temperature season of general plant activity at any given station. It 

 is of course well known that the growth activities of many plant- 

 forms are checked and that death of all but dormant phases frequently 

 ensues with temperatures far above those requisite for the recurrence 

 of a killing frost. It is just as clear, however, that many forms make 

 a considerable growth before and after the frostless season, which we 

 consider as limited by the average dates of occurrence of the last and 

 first killing frosts respectively. 



It seems therefore safe, in default of any better means for improving 

 our knowledge, to resort to the average length of the frostless season 

 as the basis for the duration aspect of the temperature conditions in 

 the United States.^ A somewhat detailed consideration of the frost- 

 .less season in the United States will comprise the next following sub- 

 section. 



It will have been remarked that no attention has here been directed 

 toward an approximation of the upper temperature limit for plant 

 growth. This matter will receive some consideration in the sequel, 

 but it may be remarked here that it is not nearly so easy to attain 

 to an approximation of this general maximum as of the general mini- 

 mum for plant growth. This is partly due to the fact that the effect 

 of freezing is quickly manifested upon many living plants, while in- 

 jurious effects of high temperature are generally but slowly exhibited 

 and therefore less readily observed. Furthermore, the response to 

 frost is almost always a direct and unequivocal effect of the surround- 

 ing temperature upon the organism, while with high temperatures, 

 alterations in the transpiration-rate, and in the rate of possible water- 

 supply to plant roots — alterations in the water-relation, in short — 

 become quite hopelessly confused with the temperature effects. It 

 seems probable that relatively few plants will be found that are directly 

 prevented by high temperature from thriving anywhere in the United 

 States. AMierever this appears to be the case, a more thorough ex- 

 amination of the facts has usually resulted in the conclusion that the 

 high temperature is at least primarily effective only as the more remote 

 climatic cause of an alteration in the moisture-relation. 



* On the employment of the average length of the frostless season in this sort of studies, see the 

 following: Livingston, B. E., Climatic areas of the United States as related to plant growth; 

 invitation paper read before the American Philosophical Society, Philadelphia, April 1913, 

 Proc. Amer. Phil. Soc. 52: 257-275, 1913. — Livingston and Livingston, 1913. — Fassig, O. L., 

 The period of safe plant growth in Maryland and Delaware, Monthly Weather Rev., 

 42: 152-158, 1914. Fassig calls attention to the fact that the occurrence of the last and first 

 minimum of 32° F. furnishes as good a criterion for the determination of the average length of the 

 growing season, and perhaps a better one, than does the occurrence of actual killing frost. 



