106 PROCEEDINGS OF THE ACADEMY OF [Feb.* 



its advance across the United States, is illustrated in the accompany- 

 ing map which was constructed from the recorded mean temperatures 

 for 15 years from 1870-1885. 7 This line may be regarded as the edge 

 of spring. In the Gulf States there is no true advent of spring. On 

 February 1 the isotherm in question crosses the United States from 

 the vicinity of Cape Hatteras on the east to the north of El Paso, then 

 turns northwestward and reaches the Pacific coast some distance north 

 of San Francisco. The advance on the first of successive months is 

 illustrated by the map. 



Spring may be said to begin in our most northern locality, Lansing, 

 Michigan, about May 1. Auburn, Alabama, lies below the isotherm of 

 February 1. We therefore have a difference of three months in the 

 awakenment of vegetation under the influence of the advent of spring 

 conditions. Yet in our experimental cultures no such great discrep- 

 ancy existed, because the heat of the greenhouse is more uniform 

 (not so fluctuating), and yet, notwithstanding the fact that the interval 

 between the ■ successive events is shortened, nevertheless the same 

 phenologic sequence is maintained as described above. The difference 

 of three months in the advent of spring between the most northern 

 and the most southern of our localities indicates the real amount of 

 acclimatization of the same species and race growing in the different 

 places from which the experimental material was obtained. That is, 

 the growing season of trees and shrubs is very much abbreviated com- 

 pared with the growing season in more southern situations. Three 

 months difference is a very appreciable amount between the northern 

 and southern States, and the fact that such fruit trees as the Maiden 

 Blush apple and the Kicffer pear can be grown in Michigan and Ala- 

 bama is noteworthy as an instance of acclimatization of the most pro- 

 nounced type. In Alabama the growing season is about nine months 

 long, from February 1 to November 1. In Michigan the growing season 

 begins May 1 and is completed by the beginning of October, a length 

 of about five and a half months. The fact that many of the fruit 

 trees mentioned above did not take kindly to the American climate, 

 and that they afterwards became adapted to its wide range of climatic 

 conditions is proof of their acclimatization. Yet some doubt that 

 acclimatization occurs. A writer in Forest Leaves (XI : 108) says 

 that "Trees are fixed, almost inflexible, in their habits. For centuries, 

 indeed as long as we have record, each species has kept in its beaten 

 ways, insisting on the same average of temperature and refusing to 



7 Harrington, Mark W.: The Advent of Spring. Harper's Magazine, May, 

 1S94. 



