154 ENVIRONMENTAL CONDITIONS. 



out its course. This possible difference in the interpretations of two 

 students is the more emphasized, the more complex are the climatic 

 characters of the regions dealt with, and the less adequate are the data 

 available. We have therefore become convinced that, for the sort 

 of subject that is here involved, it is quite wide of the mark for a 

 WTiter to transform his series of data into charted lines and to publish 

 merely the chart. It is quite essential that the data themselves, on 

 which the chart is based, be placed in the hands of those who are 

 interested. In the climatic studies which follow, we have been careful 

 to point out just how each series of data have been derived and have 

 uniformly presented these data by means of tables. The positions 

 of the stations for which data were employed are generally shown upon 

 the charts by small circles. 



In the following discussions, the different conditions or climatic 

 features will be treated serially, under the three sectional headings. 

 "Temperature," Moisture," and "Light," each one of which will be 

 subdivided. 



II. TEMPERATURE CONDITIONS. 



1. DURATION OF TEMPERATURE CONDITIONS. 

 (A) PRELIMINARY CONSIDERATIONS. 



Probably the most important environmental condition in the de- 

 termination of plant distribution is the length of season or seasons, in 

 each year, during which growth may occur. These are seasons, or 

 time periods, during which every one of the factors of the surround- 

 ings exists in an intensity or quality such that growth activities can 

 go forward. If a single factor were effective beyond the limits for 

 growth — in its quality or intensity — then this must suffice to throw 

 the plant into a dormant phase, in spite of the fact that other factors 

 might still be favorable to growth activities. 



Directly or indirectly, the ebb and flow of the environmental condi- 

 tions affecting organic life are dependent upon astronomical causes, 

 and the annual rhythm so commonly manifest in plant activities, as 

 far as this is due to alterations in the surroundings, may be traced 

 finally to the movement of the obliquely placed earth along its orbit 

 and to the resulting procession of the equinoxes. This rhythm is 

 always, then, either itself a temperature rhythm or else it is more or 

 less directly connected with a temperature rhythm. 



This fact that the seasonal temperature fluctuations stand in a 

 casual relation to the fluctuations in the other environmental condi- 

 tions has given us logical reason for basing many of our climatic 

 studies upon the duration aspect of the temperature factor. To this 

 must be added the practical reason that temperature fluctuations, in the 

 United States as well as elsewhere, have been much more thoroughly 



