i9'3.] OF THE UNITED STATES. 261 



very common to-day, especially among agricultural institutions, and 

 considerable practical information is no doubt resulting therefrom. 

 In this agricultural work, however, as also in the observational 

 studies upon natural vegetation, with which plant ecologists are so 

 generally engaged, the physiological characters of plants are deter- 

 mined almost solely with reference to the locations at which they 

 grow. Thus, seedsmen, to describe the physiological properties of 

 the plants with which they deal, must name the regions in which 

 these plants succeed. " A greatly approved variety among the truck 

 gardeners of Long Island," "one of the most successful earlies 

 throughout the South." — so run such trade descriptions. 



Measurement and summation of environmental conditions. 



When we describe the physiological capabilities of a given strain 

 or species by stating the geographical region in which it thrives, we 

 are of course employing the environmental conditions as a unit for 

 measuring and defining the internal ones. \"aluable as this sort o£ 

 definition undoubtedly is, it falls far short of perfection, even for 

 practical purposes. The climatic conditions of any locality vary 

 from day to day throughout the year and their annual march is never 

 the same for clifi'erent \ears. An agricultural plant that proves very 

 successful for one season in a certain place may be a complete failure 

 for the following year. It is clear, therefore, that we must seek 

 methods for describing climatic conditions, other than their simple 

 reference to certain geographical regions. If such methods can be 

 devised, even though we may have no better ways of characterizing 

 our plants than are already at hand, it should become possible to 

 compare the environmental conditions of dififerent regions, and plant 

 geography, as well as scientific agriculture, should be greatly 

 advanced. 



Methods and data. 



Turning now to the consideration of the methods which are at 

 hand for comparing climates, we are struck with an amusing fact ; 

 the most intelligible and most widely used way to do this is to char- 

 acterize the climatic conditions of anv region in terms of the kinds 



