yal 
humus was added to the sand, increasing both the stability and water- 
holding capacity of the sandy soils. This made conditions favorable 
for the growth of plants characteristic of the prairie. Short bluestem 
first became abundant and crowded out most of the pioneer plants 
(Plates LXXII, LXXIV). With further increase of humus, stability, and 
water content of the sandy soils tall bluestem came in and crowded out 
the short bluestem (Plate LXXV). This succession of grasses, leading 
ultimately to tall bluestem on the sand deposits, was not completed on 
all the sand areas of the state when the first settlers arrived. All grada- 
tions from the pioneer to the climax could be found. 
Starting with the pioneer soil-conditions left by the melting glaciers, 
the accumulation of organic matter from vegetation and the work of 
erosion have been gradually changing the topography, water content, 
and composition of the soils of the state. The change in water content 
of the soil was the most potent factor underlying the changes in prairie 
vegetation discussed above. With a decrease in water content of the 
areas covered by sloughs and an increase in the water content of clay 
slopes and sand accompanying the increase in humus and stability of 
the soil, more and more of the prairie area became covered by tall blue- 
stem. This grass was the most abundant and prominent prairie plant 
on all the well-drained and sufficiently moist prairie areas of the state. 
In all the successions of prairie plants accompanying the filling and 
draining of sloughs, the wearing down of clay slopes, and the stabiliza- 
tion of sandy soils, the ultimate vegetation of the prairie is dominated 
by tall bluestem. This means that tall bluestem is the climax grass of 
the prairies of Illinois. It will remain the dominant prairie plant under 
natural conditions unless the water content of the soil is again seriously 
changed. Where the turf is broken by eroding heads of ravines it 
may be succeeded by a forest. A glance at Dr. Brendel’s map (page 526) 
shows that much of the forest area of the state has developed in this 
way and is generally distributed along all the creeks and rivers. 
None of the pictures accompanying this report give an adequate 
conception of the former bluestem prairies. This grass forms a close 
sod and under good growing condition may reach the height of 10-12 
feet. The early settlers say that they were unable to locate cattle on the 
prairie except by climbing some elevation and watching for the waving 
of the tall grass as the cattle walked through it. Its growth was suf- 
ficiently dense to crowd out most of the other prairie plants, among the 
most prominent of which were plants of the mint, bean, and sunflower 
families. On eroding clay slopes and along forest borders the yellow, 
blue, and white flowers of these coarse herbs became conspicuous in late 
summer and autumn. A glance at Plate L may give some impression of 
the rank growth of the prairie grasses. 
The humus accumulated on our prairie soils does not supply food 
to our crop plants. Like all green plants they make their food chiefly 
out of water and carbon dioxide. In the manufacture of carbohydrates 
