BOTANICAL GEGGRAPHY 333 
Besides the comparatively well-known and readily seen 
larger algz 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 algze known as diatoms (Fig. 176). 
402. Botanical Geography of the United States. — All of 
the continuous territory of the United States? 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. The 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. 
