248 DISTRIBUTION OF PLANT COMMUNITIES 



Most of the grassland occurs where the rainfall is somewhere 

 between 25 and 35 inches with nearly three-fourths of it falling in the 

 warmer half of the year. However, grasslands occur under a very 

 wide range of conditions. Some are found where the rainfall is only 

 10 inches or less and some where no more than 10 per cent of the rain 

 falls in the summer season. The range of temperature is also very 

 great in different parts of the grassland formation. In the north it 

 is subject to winter temperatures far below zero while in the south 

 there are places in which the freezing point is seldom if ever reached. 

 The essential requirement seems to be merely that there shall be 

 sufficiently frequent rains during the growing season to enable 

 grasses to bloom and produce seeds. 



The tall grass prairie formerly formed a belt along the western 

 edge of the deciduous forest formation and occurred as islands within 

 the forest. It also occurs in sandy areas farther west, especially in 

 the sand-hill area of Nebraska. These sand-hill areas are the best 

 places to see tall grass prairie at the present time since elsewhere it 

 has practically all been destroyed in order to utilize the land for 

 agricultural purposes. The most important dominants of the tall- 

 grass prairie are the beard grasses (Aridropogon furcatus and A. 

 scoparius) and Indian grass (Sorghastrum nutans). These are tall, 

 sod-forming grasses that grow 5 to 8 feet tall. Associated with them 

 are many other grasses and many herbs which form seasonal societies 

 at blooming time. 



The true prairie, like the tall-grass prairie, has been largely 

 destroyed by cultivation. Originally it is believed to have formed a 

 quite distinct belt between the tall-grass prairie on the east and the 

 mixed prairie or short-grass plains on the west. The true prairie 

 is composed largely of grasses that are intermediate in height. The 

 most characteristic dominants are species of Stipa and Sporobolus 

 which are bunch grasses but some of the sod-forming genera are also 

 represented. Since the change of climate from east to west is 

 gradual the lines of separation between the true prairie and the tall- 

 grass prairie on the one hand and between true prairie and mixed 

 prairie on the other, as might be expected, are not lines that can })e 

 drawn accurately on a map but are broad belts or ecotones where the 

 species of neighboring types of grassland are mixed. The true 

 prairie probably contains a larger number of aspect societies, com- 

 posed of beautiful flowering herbs, than any other type of grassland. 

 The sequence of spring, summer, and fall societies produces an ever 



