The Vegetation of North America 327 



the native moves on and clears another area. The sequel of 

 migratory agriculture is the tropical jungle, with its dense, 

 tangled, and almost impenetrable masses of vegetation. 



In Dominica, Trinidad, Venezuela, and other northern states 

 of South America, are remnants of the original broadleafed ever- 

 green tropical forest. These forests are noted for their great 

 variety of tree species and their freedom from dense undergrowth. 



The prairie formation. Extending from North Dakota to 

 Texas and eastward to Indiana is a roughly triangular region 

 in which vast areas of level and rolling uplands formerly were 

 covered with tall grasses from 3 to 10 feet in height, while decidu- 

 ous forests dominated the river valleys. These are the true 

 prairies. Toward the western margin the prairies were well 

 drained, or even overdrained, but to the eastward they were 

 interspersed with sloughs and temporary ponds which were also 

 dominated by grasses. 



During the summer the prairies were studded with the brightly 

 colored flowers of scattered perennial herbs. In the fall the 

 prairies were a vast sea of highly inflammable grasses, and often 

 they were swept by fires that destroyed everything in their path. 

 In winter they were bleak and exposed to the full sweep of the 

 wind and drifting snow. 



The dominance of the prairie grasses over this great area and 

 the absence of forests was made possible by the climate. The 

 most important climatic factors influencing plant growth are 

 rainfall, temperature, humidity of the air, and wind velocity. 

 The first — rainfall — represents the source of the water supply 

 in the soil. The other three factors determine the rate of evapo- 

 ration from a water surface. In the prairie region the rainfall is 

 less than the amount of evaporation ; it is about six tenths as 

 great on the western side and eight tenths on the eastern border. 

 The prairie region is characterized by high summer temperatures 

 and summer droughts. Another characteristic feature of the 

 prairie climate is the uneven distribution of the rainfall during 



