328 General Botany 



the growing season. During one season the heavy rains occur 

 in the spring, during others in midsummer or autumn. This 

 leads to annual droughts either preceding or following the rains. 



The soils are for the most part clays and sandy loams upon 

 which there has accumulated since glacial times, through the 

 comparatively slow decay of the prairie vegetation, several inches 

 to several feet of black humus. But the nature of the soil was of 

 less importance in the maintenance of the prairie than the cli- 

 matic factors which controlled the moisture content of the soil. 

 The prairies that were low and poorly drained were unfavorable 

 to the growth of trees because of too much water in the spring 

 and early summer. The more western prairies were subjected 

 to too intense droughts in summer to favor the growth of trees. 

 In the years that have passed since the prairies were first settled, 

 thousands of miles of tile drains and ditches have drained the 

 ponds and sloughs, and the ground-water table is today several 

 feet lower than it was originally. This has made possible the 

 growth of trees in the wet prairies, where formerly they were 

 absent. The absence of fires is also favorable to the extension 

 of the forests. 



Although there were several species of grasses common on the 

 prairies, by far the most important is the '' big bluestem." This 

 grass formed an almost pure growth over the large areas, and in 

 late summer was so tall and dense that cattle were lost to sight 

 in it and their position could be told only by the swaying of the 

 grass tops as they moved about. 



On the sandy and more exposed dry prairies " bunch grass," 

 or " little bluestem," 2 to 3 feet high, was most abundant.-^ As 

 humus accumulated and the soil moisture was increased, these 

 areas were invaded and often occupied by the " big bluestem." 



In the wet areas of the prairie the '' slough grass," 6 to 10 feet 

 in height, was dominant. This grass was used frequently by the 

 pioneers to thatch the roofs of their smaller farm buildings. 



^Big bluestem is Andropogon furcatus ; little bluestem is Andropogon scoparius. 



