Igo BOTANICAL GAZETTE [SEPTEMBER 
- These quadrats are all on steep slopes where the soil is more or 
less broken. The first quadrat is in pure disintegrated limestone; 
some gravel has been washed into the second quadrat; while the 
third is a mixture of gravel, clay, and lime. The difference in soil 
is of no importance in this connection; since in other places the same 
succession occurs on the pure disintegrated limestone. In the first 
quadrat the water content varies from 19 to 6%; in quadrat 3 it 
varies from 19 to 2%; while in the second quadrat the per cent. of 
water is intermediate. As one passes from the first quadrat to the 
second and then to the third, the facies of the Bouteloua formation are 
found making their appearance; in fact, the flora, aside from a few 
ruderal species and Lesquerella alpina, is decidedly of the Bouteloua 
formation. 
The native species, which are referred to as ruderal, show the great- 
est ability to occupy new ground and they are the most important 1m 
secondary succession. It seems reasonable to suppose - that they 
were also very active in invading the newly formed alluvium, and 
that, if any of the existing species have taken part in the primary 
succession, these plants are to be sought among the native ruderals. 
This can be more clearly understood after a consideration of sec 
ondary succession. 
SECONDARY SUCCESSION. 
The repeated changes which have taken place in the formation 
of the great plains have manifestly been accompanied by changes 
in vegetation. What these changes have been can only be inferred 
from the changes which are now taking place wherever, in the pre? 
of erosion, there is a cutting away or deposition of material, These 
successions in a certain sense are primary, but will be discussed 
under secondary successions. 
Biotic agencies. 
There are so many chances for observing secondary successions 
that the experimental denuded quadrat was not deemed necessat» 
although several of these are now under observation. There 47 
many trails which lead through the Bouteloua formation where the 
ground has been but slightly disturbed (fig. 12). The travel has 
simply worn off and killed the original vegetation. After having 
