792 TEXTBOOK OF BOTANY 



cause of the mountains, and the vegetation varies from deciduous dry 

 forest to tree savannas, scrub, and desert. 



The products of the tropical forest region inchide bananas, cotton, 

 sugar cane, cacao, coffee, and many kinds of fruits. The banana planta- 

 tions are at lower elevations, the coffee at elevations of two or three 

 thousand feet. 



One must not forget that on the mountains of tropical regions there is 

 a temperate belt and even cold temperate summits where frosts occur 

 regularly, and, on a few of the highest peaks, permanent snow. Unlike 

 the continental temperate regions the climate is more uniform, the day- 

 light period constant; the most variable factor is the length of the rainy 

 and dry seasons. 



The wettest forests in the rainy tropics are those on mountain slopes 

 in the cloud belt. The trees are usually of low stature, and variously 

 gnarled and twisted. They are shrouded with epiphytic orchids, bro- 

 mehas, filmy ferns, mosses, lichens, fungi, and algae which accumulate 

 and continuously destrov the plants beneath. Constant moisture, mod- 

 erate light, abundant mineral salts in the plant debris, and nearly con- 

 stant temperatures foiTn an environment so nearly "ideal" that hundreds 

 of plant species compose an ever-changing living mass of vegetation. 



Two centuries of destruction. Except along the sea coasts, most of the 

 United States was a wilderness less than two hundred years ago, and 



Fig. 421. Effects of sheet erosion in a forest in Oklahoma started by overgrazing 

 of the grasslands higher up the slope by sheep. Photo from U. S. Soil Conserva- 

 tion Service. 



