ON THE CAUSE OF THE ABSENCE OF TREES. 699 



dominant types: ist. Those having thick succulent stems for storing up moisture, 

 of which the cactuses are the best examples. 2d. Those having large and deep 

 roots, such as Ipomcea leptophylla, the bulk of which is really subterranean; and 

 3d. Those having either glandular, hairy or woolly surfaces, belonging to numer- 

 ous families, but chiefly to the Compositce., the Borraginacece, and the LeguminoscB. 

 Large interspaces of bare ground occur between the more and more sparse occu- 

 pants of the soil; grass, when seen at all, appears in small, disconnected bunches; 

 the face of the country assumes a dull, leaden hue, and the transition at length 

 becomes complete. 



A sojourn upon the plains for a short period reveals other marked peculiari- 

 ties. The eastern sky is frequently seen lit up on summer evenings by flashes 

 of lightning, and by day a bank of cloud may often be observed hanging low 

 along the eastern horizon while all is dry and arid at the observer's station. Or, 

 looking to the westward, heavy showers may be seen to gather upon the moun- 

 tains, only to thrust a few spent cloud-columns over the parched regions below. 



The prime cause of the changes in the vegetation above pointed out is there- 

 fore of course not far to seek. The scanty rain-fall would seem alone sufficient to 

 account for it. After passing the western boundary of Kansas, the annual precip- 

 itation is but sixteen, and falls as one proceeds farther westward to twelve inches. 

 The same is true throughout western Nebraska and Dakota and eastern Montana. 

 At Denver it is 14.09; at Cheyenne, 13.77 ; at Fort I.yon, Col., 12.56; at Fort 

 Stevenson, Dak., 11.84, ^.nd at Fort Rice, Dak., 11.39 inches. 



Any country whose annual rain-fall is less than ten inches is a true desert, 

 and we see how closely this great region approaches this condition at certain 

 points. It is only rarely, and this either in natural depressions or in the vicinity 

 of rivers, that green patches of buffalo grass, Bontelona oligostachya or Buchloe 

 dactyloides, may be seen, possessing anything that can be called turf, and the face 

 of the country has, to a large extent, the general aspect of a desert. 



But if this deficient rain-fall accounts for the absence of grass and the other 

 herbage characteristic of the East, is it sufficient also to explain the absence of 

 trees? And if so, why is the country not timbered out to near the looth 

 meridian ? 



The first of these questions should, I think, be answered affirmatively. As 

 to the second, it has already been answered at the beginning of this paper. It is, 

 I believe, indeed true that there are no trees on the plains because they will not 

 grow there, and it may be this opinion, (which /^ somewhat prevalent,) that Prof. 

 Meehan has confounded with a similar explanation of the more eastern prairies ; 

 for at the beginning of his paper he remarks that "The most prevalent belief had 

 been that trees would not grow on these prairies," a statement which is very 

 wide of the truth for the prairies proper. 



This view may be substantiated by a few facts : 



I. It is not possible to account for the absence of trees on the plains on the 

 theory of fires. Fires do indeed occur but not frequently or extensively. The 

 vegetation is scarcely dense enough to support them. The testimony is conflict- 



