CHAPTER XV. 

 PLANTS OF DIFFERENT REGIONS. 



211. Original Distribution of Plants. — Plants occur in 

 some form, either on the land or in the water, from the 

 equator to the poles. Where they first originated on the 

 earth, or whether independently in more than one locality, 

 is not known. It is known, however, that when they had 

 advanced sufficiently in the course of their evolution to 

 form tissues which could be preserved as fossils, they were 

 fairly uniformly distributed throughout all latitudes, the 

 earth down to Middle Tertiary time having a temperate cli- 

 mate, even within the arctic circle. After that time, snow 

 and ice accumulated in and beyond the arctic circle, and 

 extended far southward in the form of moving glaciers. 

 Land plants had then to move southward by means of 

 their seeds and spores, or become exterminated. When 

 the ice again retreated toward the poles, only those exiled 

 plants which were able to adapt themselves to extremes of 

 temperatures moved northward. We accordingly find that 

 the floras of the tropical, temperate, and frigid zones are 

 dominated by different kinds of plants. 



212. Factors governing Floras. — Within each zone there 

 are factors, other than temperature, which sort the vege- 

 tation into groups of various characters. These factors 

 are water supply, relative humidity of the atmosphere, 

 winds, and nature of the soil. 



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