■ 65 

 Importance of water from its varlabilitv and nnoertalnty. One 



of great ca^^ses for difference in water is difference in soil. 



Another, evaporation ; others, openness of soil and slope. Soil 



water is an edaXphic factor. 



2. Light . 



Sunlight "both edaphic and climatic. Zones of the earth divided 



iDecaiise of light being distributed differently. In a forest difference 



comes from the influence of light as an edaphic factor. Succession 



of forest trees ; first aspens, and white birches ; then piiies, 

 then oaks and lastly beeches. Such succession found in Michigan 



caused very largely by difference' in light. First are light-loving 



trees and they grade down to those which can grow in shad^?. Other 



influences of light. Suggestions in water plants, as different 



colored Algae. 



3. Heat — most important climatic factor from north to south. Of 

 comparatively little importance as an edapliic factor. Difference in 

 temperature of soils. (A) Slope. (B) Amount of water. Do not 

 look for early spring flowers in a swamp. It is too cold and wet. 



4. Air. One of the most important factors, but not ecologically 

 because of its evenness. Movements often radically change the flora 

 of the region. Importance of air in the water. In stagnant water 

 plants have been unable to get proper gaseous food through the ordinary 

 Qjiannels, hence they ''seem to adopt extraordinary methods like the 

 Utricularia. 



