BOTANICAL GEOGKAPHY 333 



Besides the comparatively well-known and readily seen 

 larger algse there is a great amount of vegetation floating 

 in what is known as the plankton. This is a mass of 

 microscopic animals and plants, found floating scum-like 

 or submerged in fresh and in salt water and often accu- 

 mulated in great quantities near shores, to which it is 

 swept by the action of the wind and waves and currents. 

 Much of the plant life of the plankton, both of fresh and 

 of salt water, often consists of the flinty-shelled one-celled 

 microscopic algae known as diatoms (Fig. 176). 



402. Botanical Geography of the United States. All of 

 the continuous territory of the United States l lies in the 

 north temperate zone. There is material for a large vol- 

 ume in the discussion of the distribution of plants over 

 our territory in this continent alone, but it is possible to 

 sum up a mere outline of the matter in a very few words. 

 Excluding the floras of many single mountains and moun- 

 tain ranges, the land surface of the country may for botan- 

 ical purposes be divided into four great areas, as follows : 



1. TJie Forest Region. This occupies the eastern and 

 central portion of the United States. It is bounded on 

 the west by an irregular line, most of which lies to the 

 eastward of the hundredth meridian. In some places this 

 forest boundary extends eastward across the Mississippi 

 River, while in others it recedes from the river five 

 hundred miles or more to the westward. 



2. The Great Plains Region. This extends westward 

 from the region above named to the Rocky Mountain 

 Plateau. 



1 That is, not counting in Alaska, our West Indian possessions, the Sand- 

 wich Islands, or the Philippines. 



