70 CARNEGIE INSTITUTION OF WASHINGTON. 



stations, ranging in their conditions from the expectancy of frost on 

 any day in the year to the invariable absence of it. The annual means 

 have been secured for each of the major climatic factors and the annual 

 extremes for some of them, but in the great majority of cases the cli- 

 matic conditions have been determined for the growing season only. 

 This results in bringing into comparison the conditions of a very short 

 period in the northern parts of the United States and the conditions 

 of a long period in the southern States. Inasmuch as the activities 

 of vegetation are chiefly controlled by the conditions of the growing 

 season, the distribution of the climatic conditions of the year as a 

 whole are, therefore, of relatively small significance in an investigation 

 of this character. 



Considerable attention has been given to the elaboration of moisture 

 ratios, owing to the significance of the relation which exists in all parts 

 of the country between the precipitation and the rate of evaporation. 

 The importance of this ratio has not only been demonstrated in experi- 

 mental work by both of the authors, but has been shown by Transeau 

 to be of importance in connection with distributional work. Moisture 

 ratios have been worked out for all of the evaporation data which 

 it is now possible to secure for the United States, and for the year as 

 well as the growing season. 



Summations of temperature have been made by four methods, the 

 best of which is based upon Lehenbauer's work on the relation of tem- 

 perature to the growth of corn. Tliis "physiological summation" is 

 one in which the different degrees of temperature are given weight in 

 proportion to their influence upon growth. The older methods of 

 summation provide for the addition of all temperatures to form a single 

 total, without regard to the very different physiological effects of the 

 lower and higher temperatures which are added. 



The temperature and moistm-e factors have, in general, been given 

 independent treatment, but an effort has been made in the moisture- 

 temperature indices to obtain a composite expression of the moisture 

 and temperature conditions of the various sections of the United States. 



Three series of vegetational data have been secured for correlation 

 with the climatic conditions: (a) the distribution of distinctive types 

 of vegetation, or fonnations; (6) the cmnulative occurrence of selected 

 growth-forms, or anatomically sunilar plants; (c) the distribution of 

 selected individual species. Under the first heading nine leading types 

 of vegetation have been used: four areas of evergreen needle-leaved 

 (coniferous) forest, deciduous forest, grassland, the transition region 

 between the last two, and two types of desert. Under the second head- 

 ing several maps have been made which show the cumulative occur- 

 rence of all species of a given growth-form, or of the commonest ones. 

 These maps show, for example, in what part of the eastern United 

 States there may be found the largest number of species of deciduous 



