2. Limited in southward distribution by mean temperature 
for hottest part of year. 
Note condition on Pacific coast. 
a. High sum total effective heat. 
b. Low mean temperature in summer. 
Limiting factor for east and west distribution in U. S. 
I. Mountain ranges. 
2. Low precipitation in interior. 
3. Methods and Causes of Plant Migration. 
Advantages of plant migration. 
Increases factor of safety for existence. 
1. Larger number of individuals. 
2. Safety of some assured in case of disaster in certain 
regions. 
Structural characters favoring plant migration. 
Bes Seeds. 
a. Buoyancy. 
b. Grappling structures. 
c. Food for animals. 
d. Floating seeds. 
Ze nuit. 
a. As food for animals. 
b. Exploding fruits. 
aa bimbles weeds. 
4. Floating of broken branches. 
5. Prostrate or creeping plants. 
6. Underground creeping stems or roots. 
Causes of plant migration,—2 factors. 
ia, Biotic factors. 
a. Influence of animals. 
b. Initiated by plants 
Ist, Fertility of species—brings pressure. 
2nd, Centrifugal habit of propagation. 
3rd, Factor of adaptation, or acclimatiza- 
tion. 
2. Physcial and climatic factors. 
a. «Wind, “water, etc. 
b. Tensions in fruits. 
c. Climatic pressures, as in glacial epoch. 
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