1906] SHANTZ—VEGETATION OF THE MESA 199 
the Muhlenbergia gracillima consocies almost entirely replacing the 
earlier stages of the succession. A very little Boebera papposa and 
Schedonnardus paniculatus remained, while about an equal amount 
of Bouteloua oligostachya was invading and will after a number 
of years replace the Muhlenbergia. 
As long as the ants are alive, they remove all vegetation for some 
distance around their hills. In low places Cleome serrulata may 
form a semicircular zone on the lower side of this denunded area. 
Helianthus annuus, H. petiolaris, Stipa Vaseyi, and many other 
species may also be found in this situation. The most common plant 
to develop in this area is Munroa squarrosa, which often forms a 
perfect zone. 
Erosion. 
The dry soil is easily washed away by heavy rains. This forms 
loose soil at the base of the hills and also leaves broken places from 
which the soil is removed. On the hillsides there are often produced 
natural terraces, each of which ends in a broken edge. During the 
rains these terraces are cut back and new soil is exposed. These 
places may be occupied by Boebera papposa, Salsola Tragus, or other 
annuals, but generally Stipa Vaseyi is the invading species. Here 
it serves to bind the soil and prepare the way for Muhlenbergia 
gracillima and Bouteloua oligostachya. 
In the low draws there is generally a hollow washed out below 
the terrace, and as a result of the falling in of the soil when dry there 
is found both loose and undisturbed soil. These places are marked 
by a growth of Sti pa Vaseyi, with Boebera papposa, Solanum ros- 
tratum, and often Salvia lanceolata, Helianthus annuus, Verbena 
bracteosa, Salsola Tragus, Xanthium commune, and Leptilon cana- 
dense. Places such as this are very much like the ordinary draw 
where more or less of the soil which was washed down from the hills 
is deposited. Conditions here are also almost the same as in the 
alluvial fans which are formed at the bases of all the ravines, whether 
they be small or large. Here new soil is deposited during every heavy 
rain, and as a result the slow growing grasses such as Bouteloua 
oligostachya and Muhlenbergia gracillima cannot thrive. Stipa 
Vaseyi is the most successful plant of such habitats. It marks the 
