176 



THE AMERICAN NATURALIST [Vol. XL VI 



real tropical flora. Along the coast there are extensive Man- 

 grove swamps and the strand flora includes many West Indian 

 types. Among the strand plants is the ubiquitous Ipomcea pes- 

 caprce found on every tropic beach throughout the world. An- 

 other very widespread tropical plant occurring in the swamps 

 along the shore is the stately fern AcrosticJ/um aureum. This 

 is also a common and conspicuous plant in the Nipa swamps of 

 the East Indies. 



A feature of southern Florida is the presence in the grassy 

 lands of the everglades and in the dry sandy pine forests, of 

 areas covered with hardwood trees. These wooded areas are 

 known as "hammocks," and the vegetation, both the trees and 

 lower growing plants, are mainly tropical types, for the most 

 part West Indian species. There are many epiphytic Orchids 

 and Bromeliads as well as tropical species of ferns, Palms, Figs, 

 Mahogany and other West Indian types. In this region is found 

 the only Cycad (Zamia floridana) occurring in the United 

 States. 



Proceeding westward, the rich forest vegetation of the Atlan- 

 tic Gulf states and the eastern Mississippi Valley begins to thin 

 out with the falling off in the rainfall toward the interior. In 

 western Michigan, Indiana and Illinois there are already en- 

 countered open prairies, and the forest is much less luxuriant. 

 Open groves of Oaks, Hickories and AValnuts, the so-called "oak 

 openings," are characteristic formations of this region. Fur- 

 ther west these disappear, and the whole country forms a con- 

 tinuous open prairie. How far the formation of the prairie is 

 due to climatic causes, i. e., insufficient rain and cutting winds, 

 and how much is due to compact soil (loess) which often under- 

 lies them, and which seems unfavorable for tree growth, is not 

 quite clear; but throughout the prairie region trees are practi- 

 cally confined to the beds of the streams and the sheltered gullies 

 between the hills. 



Still further westward the prairie insensibly merges into the 

 semi-arid plains, which end abruptly at the foot of the great bar- 

 rier of the Rocky Mountains. 



Compared with the eastern third of the continent the flora of 

 the great central plains is scanty. Grasses predominate, but 

 there are a good many showy flowers which adorn the prairie in 

 the spring and summer. Conspicuous among these are numer- 

 ous Composite, Rosin Weed, Rudbeckia, Sunflowers, Asters, 

 Golden-rods, and many others. 



