539 
be considered representative of original conditions. The data in Figures 
2 and 5 show that grazing results in the coming in of cultivated grasses. 
A knowledge of the amount of grazing this prairie suffered during the 
early development of Chicago is wanting. During the last forty years 
the grass has been cut annually in July. This is rather more detrimental 
to the late fall grasses than to the earlier grasses in that it destroys 
their vegetative growth shortly before time for fruiting and usually pre- 
vents them from seeding. In addition, fires started by trains contribute 
to the disturbance of the composition of associations which obtained un- 
der natural conditions of competition. Artificial drainage has lowered 
the water-table and extended the seasonal period during which the water 
content of the soil is below the wilting coefficient. During the slow read- 
justment of the associations of native species to the disturbing condi- 
tions other species have a chance to obtain a temporary foothold. These 
cultivated grasses are not able to invade the undisturbed native associa- 
tions, and the period of their dominance in disturbed areas is dependent 
upon the continuation of the disturbances. 
In the second place a number of coarse herbs become more con- 
spicuous, notably Silphiwm laciniatum, Cacalia tuberosa, and Liatris 
spicata, which usually follow Spartina and Calamagrostis and may be 
found in pure or mixed stands of various combinations now occupying 
the greater part of local areas. Apocynum cannabimum var. hyperici- 
folium is also occasionally abundant at this stage. On the slightly drier 
prairies Silphium terebinthinaceum and Eryngium yuccifolium are some- 
times abundant. 
Finally, the occurrence of a mixed association dominated largely 
by Andropogon scoparius and Sporobolus heterolepis following Agrostis 
alba and Panicum virgatum on the low prairies seems out of harmony 
with the xerophytic nature of these grasses. The same species also 
dominate a portion of the Calumet beach near Chicago Ridge where 
it has been deforested. In this latter situation these species are in a 
- xerophytic habitat characteristic of them, but on the low prairie they 
appear in the hydrarch succession which is not characteristic of them. 
Isolated clumps and small patches of Andropogon furcatus occur with 
them. In my opinion the above factors explain the occurrence of culti- 
vated grasses in these prairies and also account for the Andropogon- 
Sporobolus association in the hydrarch succession and for the notable 
abundance of coarse herbs. During the period of readjustment follow- 
ing disturbance, the -speed of migration becomes an important factor 
which in turn depends upon the proximity of plants producing an abun- 
dance of readily dispersed seeds. Andropogon furcatus is not nearly as 
abundant on the hills and sand dunes near Chicago as are the other grasses, 
and the latter, therefore, have a chance to obtain the first foothold on 
drained and denuded areas. It appears that the composition of the 
associations on the Chicago prairie is scarcely more representative of the 
composition of the earlier natural associations than are those found on 
railway rights-of-way. 
