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; 7. ¢., 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 
