CLIMATIC CONDITIONS OF THE UNITED STATES. 157 



But the occurrence or non-occurrence of a given plant-form in a 

 given region depends upon other features than length of the season 

 of active growth. As was mentioned earlier, it frequently occurs that 

 the limiting condition preventing the occurrence of a certain plant in 

 any area is to be sought in the nature of the surroundings during a 

 dormant period. The length of the season or seasons during which 

 growth can not occur is perhaps frequently as important in plant 

 distribution as is the duration of the growing-period itself. Also, 

 if environmental conditions are adverse enough they may result in the 

 destruction of plant protoplasm even in its dormant phases, and it 

 thus becomes necessary to study the duration of extremely low tem- 

 perature, a thing which it seems quite possible to do and to which we 

 shall devote some space in the present section. 



We turn our attention now to the variations in the length of the 

 average frostless season throughout the United States. 



(B) THE LENGTH OF THE PERIOD OF THE AVERAGE FROSTLESS 

 SEASON. (TABLE 2, PLATE 34.) 



Although data for the determination of this exceedingly and very 

 obviously fundamental condition of plant growth, whether it be 

 agriculturally or ecologically considered,^ have been in existence and 

 have been increasing in volume and in value for a long time, it is 

 apparently not until very recently that the subject has received even 

 cursory mention in the literature. Abbe (1909), in his excellent re- 

 view of the literature upon the relation of climates to crops, already 

 cited, makes no reference to the length of the frostless season as a 

 climatological feature. As has been mentioned, this work should be 

 connected with the date of its preface (1891) rather than with that of 

 its long-delayed publication. It is thus highly improbable that this 

 feature of climate had entered very seriously into the considerations 

 of workers in climatology up to about the year 1891. 



The publication in 1906 of Henry's elaborate presentation of the 

 climatological data available for the United States- put the informa- 

 tion upon spring and autumn frosts, that had been collected up to 

 that time in this country, into a form so that it could be made use of. 

 In the early stages of the studies reported in the present publication, 

 in the winter of 1909-10, Mrs. Grace J. Livingston undertook to derive 

 an approximation of the average length of frostless season for each 

 station for w^hich Henry gives the requisite data. This was done by 



1 "Probably no factor in the study of climate from the standpoint of the agriculturalist should 

 be given more consideration than the average length of the grow-ing-season. This is the key to 

 an actual knowledge as to the possibilities of success or failure in the production of crops, since 

 in practically all portions of the United States agricultural products are menaced by frost at some 

 period of their growth." (Day, F. C., Frost data of the United States, and length of the crop 

 growing season, as determined from the average of the latest and earliest dates of killing frost, 

 U. S. Dept. Agric, Weather Bu. Bull. V, 1911.) 



•Henry, A. J., Climatology of the United States, U. S. Dept. Agric, Weather Bu. Bull. Q. 1906. 



