IFZ THE AMERICAN NATURALIST. [VOL. XXXII. 
For fifty kilometers the vegetation is of this monotonous char- 
acter, the monotony intensified by the straw color now assumed 
by the dry vegetation. Only when we cross the broad valley 
of Bluewater Creek do we find a marked departure from the 
Agropyron-Stipa-Bouteloua-Lepidium formation. The increased 
moisture has enabled other grasses to push their way in, es- 
pecially Sporobolus aroides, and this with Agropyron, which 
here is taller and quite green, give a refreshing color to the 
stretch of level land on each side of the creek. We cross a 
stretch of rounded sand hills over which the vegetation is still 
more sparse, but yet it is only a modified form of the Agropyron- 
Stipa-Bouteloua-Lepidium formation. As we enter and as we 
pass out of these dry hills, we cross a belt, or zone, of Arte- 
mista filifolia, which begins and ends with marked abruptness. 
The valley of the North Platte, thirty kilometers from the 
western boundary of the state, is from six to sixteen kilometers 
in breadth, and here, on account of the general introduction of 
cultivated plants under irrigation, the botanist finds little of 
the original vegetation. The river banks and the sandy islands 
scattered here and there in the rapid current are fringed with 
young willows (probably Sa/zx nigra) and buffalo berry, but 
there is no heavy body of woodland, as we should find under 
similar conditions in the eastern part of the state. Doubtless 
the greater elevation (1100 meters) above sea level has much 
to do with this absence of trees along the banks of the great 
river, which is fully a kilometer in breadth. 
Upon crossing the river we soon begin the ascent of Chey- 
enne Ridge. In its narrow cafions, which open to the north, 
we find cottonwood (Populus deltoides), Rydberg’s cottonwood 
(Populus acuminata), almond willow (Salix amygdalotdes), box- 
elder (Acer negundo), plum (Prunus americana), hackberry (Celtis 
occidentalis), and red cedar (Juniperus virginiana), while on the 
sides of the bluffs were scattered specimens of pine (Pinus pon- 
derosa scopulorum). Here grow side by side the western shrubs, 
Rhus trilobata, Prunus demissa, Rosa fendleri, Ribes aureum, 
Lepargyrea argentea, and the eastern Symphoricarpos occiden- 
talis, Parthenocissus quinquefolia, and Vitis vulpina. Passing 
still further up the side of the ridge, we find the smaller moun- 
