178 BOTANICAL GAZETTE [September 



of all stations for each area averaged, excluding figures for 1893) 

 seems to express the main fact of the table, that the rainfall reaches 

 successively lower minima downward and eastward from the 

 mountain zone through the foothills, mountain-front, and adjacent 

 plains to the dry plains belt, beyond which the minima rise gradually 

 with the gradual increase of rainfall eastward into the prairie- 

 grass region. It appears also that annual rainfall values falling 

 considerably below the average (as much as 4 inches below) occur 

 more frequently in the plains than in the mountain-front and foot- 

 hills areas. The well known uncertainty of farming without irri- 

 gation in much of eastern Colorado, due to frequency of very dry 

 years, indicates further that it is not the average rainfall so much 

 as the constantly recurring minimum which determines whether 

 or not an area can support a cultivated or natural vegetation which 

 is other than decidedly xerophytic. 



Seasonal distribution of rainfall. — On the whole, precipi- 

 tation during the cooler months is quite low ; this is not so true of the 

 montane area just to the west of and above the foothills.. The 

 summer rainfall is greater, but in most places distributed rather 

 unevenh'. June is thus drier than either May or July over prac- 

 tically the entire region. The northern area near the mountain- 

 front receives more rain in the spring and early summer months, 

 while the southern area receives more of its rain during late summer. 

 This difference between north and south is of far-reaching influence 

 upon the character and distribution of vegetation. The details 

 of seasonal distribution of rainfall are shown in the table of averages 

 of rainfall data, and in figs. 13-16. 



The northern and southern parts of the zones at and near the 

 mountain-front are so different as to rainfall that they cannot be 

 incorporated in single graphs. The northern parts of the zones 

 are selected, therefore, as the more typical. The graph for the 

 mountain-front is omitted to avoid overcrowding, but it can be 

 seen in fig. 14. Excluding the eastern plains, the zones have the 

 same type of rainfall, with greatest abundance in May and July, 

 and a decline in June. The zones are successively drier with 

 decrease of elevation, and this is almost as true for particular 

 months as it is for the entire year. The eastern plains have higher 



