34 INTRODUCTION 



55. Following the method used by some investigators of similar 

 problems, who have held that the average maximum temperatures 

 were the controlling factors, these were taken. But here again 

 the comparative equality could offer no satisfactory solution, as 

 in both places the maximum is about 90 . Then, too, the 

 maximum temperatures in a region such as ours are so much 

 below the physiological optimum, that it is difficult to conceive 

 of their being operative on a sufficiently large scale to affect the 

 distribution of the flora. 



56. Reversing the process, and taking the average minimum 

 temperatures, a procedure followed by still others, netted more 

 suggestive results. The differences here are considerable, as the 

 average minimum at Windham in the Catskills is — 12 , while at 

 Cape May in southern New Jersey it is 8°, a discrepancy of about 

 20°. This, however, is vitiated by the protective nature of the 

 snow blanket which covers the colder region for the greater part 

 of the winter; an advantage lacking in lower New Jersey, where, 

 however, the increased temperatures during winter about equalize 

 matters. Then, too, it has been shown that seeds can stand 

 artificial temperatures enormously lower than are ever found in 

 nature, so that plants which rely on their seeds for perpetuation 

 must be indifferent to any natural minima. Against this average 

 minimum temperature as a delimiting factor in the distribution of 

 our local plants, also, is the protective dormancy of all the woody 

 plants in the region, during the cold weather. 



57. Merriam's "life zones," an attempt to plot out the more 

 prominent belts of animal and vegetable life in North America 

 upon the basis of temperature, was found to come more nearly to 

 the known facts of the distribution of our local plants, than any 

 of the above hypotheses. But while its general principles were 

 found to hold good, the difficulty of using a scheme of continental 

 scope upon a limited area was such that accuracy seemed unlikely. 



58. Many investigators have thought that some method of 

 reckoning the accumulated temperatures of a part of a season, or 

 of all of it, would throw light on the problem, but the dangers 

 here are many. Such a scheme, particularly when there is a large 

 percentage of woody plants in the flora under consideration, leaves 

 out of the calculation the stored up effect of heat units, generated 

 during the previous season, when the very important operation 



