FLORA OF VICINITY OF NEW YORK 



552 



one-sided, wholly erroneous conception. Our present climate, particu- 

 larly temperature, seems quite certainly to be the controlling factor in 

 the present distribution of many of our native species. As to rainfall 

 and the winds, their variation seems almost negligible in so small a 

 region, but temperature is a much more serious matter. There seems 

 to be a rather well-defined temperature barrier through which some 

 plants have never been known to go. 



For a variety of reasons that need not detain us here, the particular 

 criterion of temperature response that has been studied in connection 

 with our native flora is that of the length of the growing season. This 

 is determined by figuring the number of frostless days in different parts 

 of the area. The accompanying map illustrates the method better than 

 a page of explanation could do. The arbitrarily drawn black line 

 through the map indicates the dividing line between colder and warmer 

 regions of our area. It marks, with occasional exceptions, the southerly 

 limit in our area of many cold-country plants. North of it occur most 

 of our higher elevations where the mountain species are found. The 

 difference of three months in the growing season as between the 

 Catskills and Cape May is very nearly as impressive as the conspicu- 

 ously different vegetation of these widely separated localities. 



The mental convenience of considering separately the effects of 



Fig. 1. Map Illustrating the Length of the Growing Season within 100 

 Miles of New York, The figures represent the number of days between the last 

 killing frost in spring and the first of autumn. The dark line separates the warmer 

 from the colder parts of the area, and Indicates generally speaking a climatic barrier 

 through which certain of our native plants have never been known to go. 



