CLIMATIC CONDITIONS OF THE UNITED STATES. 197 



(C) LENGTH OF PERIOD OF AVERAGE FROST SEASON. 



By the frost season is here indicated the period of the year during 

 which frost is apt to occur. In this season such plant-forms as are 

 killed or thrown into the dormant state by the occurrence of freezing 

 temperatures should not be active. While actual growth of such 

 plants often occurs within this season of any year, in frostless periods 

 of a few days, yet this growth is soon checked, and foliage, etc., thereby 

 produced is usually destroyed by the recurrence of frost, so that the 

 result of such short growing-periods is seldom to be considered as 

 advancing the organism very much toward maturity or reproduction. 

 It may thus be generally assumed that the average frost season for any 

 region represents the average period of dormancy for a large number 

 of plant forms. 



It is obvious that the average length of the frost season is the com- 

 plement of the average length of the frostless period. Thus, from 

 table 2, the mean length of the frost season may be obtained for any 

 station by subtracting the number of days given for the frostless season 

 from 365, the total number of days in the year. It is also obvious that 

 the chart of the mean duration of the frostless season is simultaneously 

 a chart of the mean length of the frost season. Thus, on plate 34, the 

 area represented as having a mean length of frostless season of less 

 than 120 days is characterized by an average period of general plant 

 dormancy of over 245 days, etc. 



It seems highly probable, though there is at hand no direct informa- 

 tion in this connection, that many plant-forms are excluded from cer- 

 tain areas in the United States, not by the lack of an adequately 

 long growing-season nor by killing temperatures, but by too great a 

 duration of the dormant period. It may thus be possible that, for a 

 given plant, a certain locality might possess a growing-season quite 

 adequate in every way for maturation and reproduction, and yet 

 the length of the enforced period of dormancy might be so great that 

 death from autolysis, respiration, and the like might ensue before the 

 return of the conditions requisite for full activity. The question thus 

 raised can not be answered until after the accumulation of a much 

 more thorough knowledge of the limiting conditions of plant-life than 

 is now available. Indeed, the first prerequisite for an attack upon such 

 questions is some such laboratory for the study of environmental 

 relations as has attracted our attention earlier in the present publication. 



(D) LENGTH OF PERIOD OF HIGH NORMAL DAILY MEAN TEMPERATURES. 



(TABLE 3, PLATE 35.) 



On the supposition that high temperature may, directly or indirectly, 

 prevent the appearance of certain plants in certain areas, or that this 

 may be the critical requisite for the complete development of certain 



