XII, LAWS AND LIMITS OF PLANT MIGRATION 
Brief discussion of natural laws of movement of plants over 
albu 
I. “Distribution” generally used to mean area already 
occupied by different species. Sometimes also used 
to mean migration within own limits, or to dis- 
tant regions. 
2. Migration (also two-fold significance) in general refers 
to,— 
1. Movement plants to new territory. 
2. Movement plants back and forth in own terri- 
tory. 3 
I. Relation Plants to Earth’s Surface as a Whole. 
Northern hemisphere. 
Southern hemisphere. 
Land hemisphere (or continental). 
Water hemisphere (or oceanic). 
Opportunity for migration between continents. 
Discontinuity of land sets barrier. 
2. Life Regions, Zones and Areas. 
Lines of plant migration. 
I. Aiong lines of least resistance. 
% 
2. “A complex problem. 
Lines of least resistance. 
a. Belts of like temperature. 
b. Belts of like moisture content. 
c. Belts vary for different plants. 
4. Life zones. Belts favorable for growth and reproduc- 
tion of plants and animals. 
a. In general, transcontinental, not coincide with 
lines of latitude. 
b. Compare tropics. 
Life regions. 
I. Physiological constant of a species. 
2. More accurate than isothermal. 
Isothermal lines. 
Regions first established in Northern Hemisphere by Alex. 
von Humboldt. 
1. Boreal, or Northern. 
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