196 ENVIRONMENTAL CONDITIONS. 



ver}^ strenuous smoothing process. Since a smoothed chart is thus 

 already in existence, we have thought it best to let plate 34 represent 

 as nearly as possible the present status of our climatic and physio- 

 graphic information, and, as has been pointed out, we have made no 

 attempt to smooth our equiseasonal lines. Professor Day informs us 

 that some scattered data other than those available for our use were 

 included in his study of the frostless season, and it seems probable 

 that a few discrepancies between his chart and ours may be related to 

 this fact. At any rate, for all practical purposes, and until such time 

 as much more complete and reliable observational data may have been 

 obtained, we may say that these two charts are in very good agreement. 

 So far as we are aware, no attempt, other than Day's and our own, to 

 prepare a chart of the average length of frostless period for the United 

 States has yet been made. 



The main generalization to be derived from plate 34 is that the areas 

 of equal frostless seasons traverse the country, roughly, in a west-east 

 direction, being displaced, however, to the northward in the vicinity 

 of the Pacific and Atlantic coasts and to the soutlTward in the regions 

 of the western and eastern mountain systems. The coastal displace- 

 ment is especially pronounced on the Pacific, where the 300-day 

 season reaches as far north as Washington. Here the season of 180 

 days seems to extend even into British Columbia. On the eastern 

 coast the last-named season extends as far northward as Massachusetts. 

 The northward extension of the same season is seen to be limited in the 

 central portion of the country, approximately by the southern boundary 

 of Iowa, while the western mountains displace its northern limit, in 

 southern New Mexico, to about latitude 33° north, and the Appalach- 

 ians displace it, in northern Georgia, to about latitude 35° north. 

 Again, the 120-day season is not represented at all on either coast, but 

 extends as far southward as latitude 35° north, in Arizona and New 

 Mexico. 



The Great Lakes exhibit a tendency, as far as our data extend, to 

 lengthen the average frostless season in their vicinity. The chart 

 shows also a frequent tendency toward an upstream extension of any 

 given length of average frostless season in the vicinity of the larger 

 rivers, even where this has no obvious relation to altitude. 



As has already been stated, the data for the average length of the 

 frostless season have been made the basis for many of our studies of 

 other climatic features, on the assumption that this time period may 

 be taken as a rough approximation of the length of the season of active 

 growth for a large number of plant-forms. It seems probable that it 

 is proportional to the average growing season for most plants, at any 

 rate. 



