1906] SHANTZ—VEGETATION OF THE MESA 205, 
the trees, Populus angustifolia and P. deltoides. This condition has 
obtained for only about three years, and the effect thus far has been 
destructive. The bank species, with the exception of those that 
are able to exist in more xerophytic habitat, have died, and aside 
from the entrance of a very few ruderals there has been no marked 
succession as yet. 
Due to irrigation. 
The changes produced by irrigation are complete and varied. 
Where a ditch passes through the Bouteloua formation in places 
where there is seepage, the first stages of the succession which lead to 
the irrigated meadow formation arefound. The effect here is usually 
the better development of A gropyron occidentale, and at the same 
time the disappearance of Muhlenbergia gracillima. Bouteloua 
oligostachya also grows more rank than where not irrigated. This 
Succession is exceedingly variable, probably because of the variable 
water supply, and the stage of the succession is an index to the amount 
of water added. In some places the meadow flora is reduced largely 
to rushes and other hydrophytic plants, while in other places Boute- 
lowa oligostachya, Erigeron flagellaris, Vicia americana, and Astraga- 
lus hypoglottis are most important. 
n the bank formation the conditions are varied. Between 
the water and the xerophytic formation through which the stream 
or ditch runs, there are all gradations from hydrophytic to xerophytic. 
But to divide this formation in regard to water content of the soil 
Would not be practicable. Another condition makes the formation 
more complex, In the ditch formation, there is, besides the invasion 
of the mesophytic and hydrophytic plants, the entrance of the ruderal. 
When the ditch is first made the ruderal formation makes its appear- 
ance. At almost the same time thé mesophytes and hydrophytes 
hag The ruderal formation always tends towards self destruc- 
Bee Here the result is that the outer portion of the ditch is ulti- 
te 4 
on th Occupied by the characteristic Bouteloua formation, while 
I : inner the more mesophytic bank species become established. 
ra os ultimate bank formation the line of separation from the grass 
‘mation is very distinct. 
" Many of the irrigation ditches a condition prevails which 
