537 
water content of this soil at a depth of 7.5 cm. was below the wilting 
coefficient during most of the period from the first of July to the last of 
September, and at a depth of 25 cm. the water content was but a little 
above the wilting coefficient for the same period. Plants growing 
throughout the season, therefore, must be able to maintain a suitable 
water balance through the two oppo- 
Poa pratensis site extremes of soil-water content. 
Since the roots of many prairie plants 
grow deeper than 25 cm., we need 
more data on the wilting coefficient in 
relation to depth of root systems of 
Poa compressa the dominant plants in the different 
associations. 
More than 4,000 acres of this prai- 
rie are included in this survey. Fig- 
Agrostis alba ure 3, shows the trend of associations 
from a small pond near Chicago 
Lawn. The prominence of the cul- 
tivated grasses, Poa and Agrostis, in 
; this locality is very probably the re- 
Glyceria nervata sult of the suburban habit of “staking 
cows out to pasture”, but, as will be 
shown later, other factors may also 
be important here. 
In Figure 4 an attempt is made 
to summarize the relations of the prin- 
cipal associations on the Chicago prai- 
rie including those of cultivated 
grasses. No attempt is made to in- 
clude all the associations in the deeper 
swamps. These have been described 
in detail by Sherff (23) and Gates 
(6). 
The data show that there is a far 
greater complexity of associations on 
this prairie than on those discussed 
above. In the first place the appear- 
ance of associations dominated by 
Agrostis alba and by the blue grasses, 
Scirpus fluviatilis Poa pratensis and Poa compressa, 
Fic. 3. Diagram showing the succes- represents a condition in this prairie 
eed anes Gee pean eta not characteristic of the natural prai- 
on the old lake bed of Lake Chi- ries of the state. There is no evidence 
were Se ee ee in the older descriptions of the prai- 
here, of associations dominatedby rie.that either of these species was 
Glyceria and cultivated grasses. . eels oe 
See Plate LIV. present in the original prairies of the 
Calamagrostis canadensis 
Carex stricta 
Spartina Michauxiana 
