4 BOTANY. 



Compare this with 39.87 inches, which is the mean precipitation for a 

 series of years in West Virginia. This State is selected because it has some 

 points of similarity to Central Colorado, and because its precipitation is far 

 from excessive. The difference is so great as to suggest that this is an 

 essential feature in the difference of the floras in Colorado, where we prob- 

 ably have a difference as great between the meteorological conditions of its 

 plains and its mountains, as between West Virginia and Colorado. 



Another meteorological element will probably be sufficient to explain 

 the problem in part. Where we have so small a mean precipitation, it is 

 safe to infer that the atmosphere comparatively seldom reaches the point of 

 saturation; *. e., that. there is less than the ordinary amount of aqueous 

 vapor in it. Then it follows that however much of the sun's heat be 

 absorbed by the soil during the day, it will be most freely radiated back 

 into space at night. I cannot better illustrate the full import of this fact 

 than by a quotation at second hand from Tyndall : '.Aqueous vapor is a 

 blanket more necessary to the vegetable life of England than clothing is to 

 man. Remove for a single summer night the aqueous vapor from the air 

 which overspreads this country, and every plant capable of being destroyed 

 by a freezing temperature would perish. The warmth of our fields and 

 gardens would pour itself unrequited into space, and the sun would rise 

 upon an island held fast in the iron grip of frost.' 



These, then, being the climatic conditions, somewhat, of the plains 

 during the growing period of the year, it does not seem strange that the 

 ensemble of the flora should be as peculiar. The diurnal range of tempera- 

 ture during the summer months is at times immense. In South Park, I 

 have seen the temperature as high as 90° Fahr. at 2 p. m., and on rising 

 the next morning found a film of ice coating the little accumulations of water 

 around camp. Our familiar forms of plant-life would almost all be destroyed 

 under such an alternation of heat and frost for year after year. The plants, 

 then, that we do find surviving are, as a rule, more dwarfed, more villous, 

 and with denser tissues than those of more genial regions. Nature would 

 appear to have especially guarded them against excessive evaporation of 

 their fluids on the one hand and freezing on the other, and meeting both 

 contingencies by a small supply of water in their tissues, retaining that 



