MIGRATION AND DISTRIBUTION OF PLANTS 435 



of fertile seeds with the best means for seed distribution not only 

 gain distribution through the seed, but by the crowding of cer- 

 tain areas bring about pressures. Second. The centrifugal habit 

 of self-propagation by runners, by layerings, or by the propaga- 

 tion of stems from separate roots. Third. The factor of adapta- 

 tion to environment, or acclimatization. Other causes are found 

 in physical and climatic factors. Some of these have already 

 been mentioned under the head of structural characters favoring 

 plant migration, ist : There are certain physical factors, as 

 wind and water, which float seeds of various plants to great dis- 

 tances. Then the increase of depth of water or the lowering 

 of depth of bodies of water forces to a limited extent migiation 

 of plants along other shores. 2nd: Tensions in fruits; for exam- 

 ple, exploding fruits. 



618. Changes in climate cause migrations of plants. 

 Movements of plants are caused by changes in the climate which 

 extend over long periods of time. The most noted of these 

 influences upon plant distribution occurred in what is known 

 as glacial times. During this epoch of the earth's history a great 

 ice-sheet formed in Canada and British America, flowed down 

 across the border and over a great portion of the northern United 

 States. This great change in the climate, the intense cold for so 

 many ages gradually extending southward, forced the plants of 

 northern North America southward. Those which were not able 

 to migrate in advance of the glacier perished. As the ice-sheet 

 reached into the temperate regions it forced in advance of it the 

 species from the temperate regions southward. Then as the 

 glacier retreated northward the plants which were able to survive 

 by southward migration again migrated northward. Geological 

 evidence goes to show that there were a number of movements back 

 and forth of the ice-sheet. This great climatic pressure, therefore, 

 fluctuated for long periods, forcing the plants southward, then 

 again yielding and allowing the plants to take up their former 

 positions, when again they would be forced southward, and so on. 



619. Evidences of plant migration in glacial times. In 

 studies of the distribution of the plants of North America, Europe 



