CLIMATIC CONDITIONS OF THE UNITED STATES. 155 



studied than have the seasonal changes occurring in any other 

 chmatic factor. Still another consideration to be mentioned in this 

 connection is this, that the most advanced modern civilizations of the 

 world have developed in humid temperate regions, where temperature 

 changes seem to be actually the immediate causes of the annual rhythm 

 in plant activities, and hence the temperature-relation in its general 

 aspect is more familiar and more useful to most of us than might be 

 any other. Practically only in arid regions do moisture conditions 

 play an important and direct role in determining the march of the 

 natural vegetational seasons, and the light-relation as such is perhaps 

 never important in this connection with outdoor vegetation. By 

 greenhouse culture in temperate regions the summer temperature 

 season is prolonged throughout the winter, and here alone is it notice- 

 able that plants often suffer, during the early winter months, appar- 

 ently for lack of light. In the open, however, whenever temperatures 

 favorable to growth prevail, the light intensity is much greater than it 

 is in Vvdnter in our greenhouses. 



Since natural fluctuations in temperature are usually characterized 

 by being gradual and continuous, it is quite impossible, even for a 

 single year and for a single plant, to determine sharply what are the 

 time limits of the growing-season. It is much more difficult to fix 

 seasonal limits in general; the only method at all possible here is that 

 of averages. Thanks to the elaborate routine observations continued 

 through many years by the United States Signal Service and its 

 successor, the United States Weather Bureau, a vast accumulation of 

 temperature data are at hand for a large number of stations in the 

 United States. From these it is possible to determine averages and 

 means, of various types and by various procedures, which may be 

 taken as fairly representative of the average conditions of the country 

 throughout a series of years. It is these temperature data, and the 

 results of various mathematical treatments of these, upon which we 

 base our temperature considerations. 



Data bearing upon plant activities, sufficiently detailed to be at 

 all comparable to the temperature data just mentioned, are totally 

 lacking, and must remain so long after the much-needed laboratories 

 for the study of environmental relations shall have become available. 

 While the discouraging character of this state of affairs is pronounced 

 enough, nevertheless it need not cause us to refrain altogether from 

 attempts to relate growth activities of plants to temperature conditions. 

 Considerable preliminary information, and perhaps some that will 

 later prove to be of a deeper-going sort, may be obtained if we 

 merely approximate the temperature limits of general plant growth by 

 a simple inspection of the knowledge which is available at present. 

 Everyone who has dealt with plants at all, from the standpoint of the 

 temperature relation, is convinced that the occurrence of a ''kilUng 



