790 TEXTBOOK OF BOTANY 



rhizomes are difficult to suppress. Consequently the area is soon aban- 

 doned and the native moves to a new area. This kind of "migratory 

 agriculture" has been going on for several thousand years, and few are 

 the areas in which secondary forests have actually reached a completely 

 stable condition. One has only to travel through our southeastern states 

 to see comparable effects of migratory agriculture in this country. 



Fig. 419. Tropical jungle on a mountain slope in western Cuba. The palm-like 

 trees are cycads (Micwcijcas) . Photo by O. W. Caldwell. 



Tropical muddy coasts are characterized by mangrove swamps. These 

 small trees have extensive root systems which interfere with wave action 

 and increase the deposition of sand and mud by the shore currents. 

 Certain species also have embryos that genninate and grow more than 

 a foot in length before they drop from the tree into the mud below. 

 Mangrove swamps consequently cause the extension of land seaward. It 

 is said that this process has added 2500 square miles to the west side 

 of the Florida peninsula. Succession of plant associations occurs; and the 

 mangroves may be followed by grasslands as in Florida, or by scrub or 

 rain forest as along the Central American coasts. 



The western slopes of Central America have decreased rainfall be- 



