CHAPTER XIV 
PLANT GEOGRAPHY 
The physical characteristics of the vegetation of a given region 
are largely due to environmental conditions, while the system- 
atic relationships depend to a great extent on the past or pres- 
ent geological connections or barriers. When two regions have 
been separated for a considerable length of time by barriers 
such as high mountain chains or wide seas, which it is impossi- 
ble for most plants to cross, the systematic composition of the 
vegetation in the two regions will be very different. If the 
environmental conditions are similar, however, the vegetation 
of two areas, whether separated or not, is likely to have the 
same general appearance because in most cases similar external 
conditions produce associations of plants whose fundamental 
physical characteristics are much alike. In the present chapter 
the vegetation of the world will be considered from the stand- 
point of the physical types of vegetation found in various en- 
vironments. The most favorable environmental conditions for 
plant growth are found in those lowland regions of the tropics 
where moisture is abundant and where there is no pronounced 
dry season. If from moist tropical lowlands we proceed either 
to colder latitudes, to higher altitudes, or to drier regions, the 
environmental conditions become less favorable and the vegeta- 
tion is less luxuriant. Forests of moist tropical lowlands there- 
fore afford a convenient starting point for a discussion of the 
vegetation of the world. 
Tropical rain forests. Tropical lowlands, where soil condi- 
tions are favorable and where there is no distinct dry season, 
or where the dry season is not long and severe, produce very 
luxuriant forests (Figs. 6, 488). Typically, the forest canopy 
is composed of three stories characterized by different types of 
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