Ecological factors 5 



Edapliic factors, on the other hand, depend upon the 

 features of the soil in which the plants grow, such as water- 

 content, food-content, aeration, the presence and amount 

 of humus and of certain minerals, acidity, and so on'. 



Not all the features of a habitat can be reckoned as 

 eifective ecological factors. Thus two soils, alike in many 

 respects, may differ in others, such as the presence or 

 amount of certain salts. But unless the differences have 

 an actual effect on the vegetation they cannot be con- 

 sidered as ecological factors. Similarly one locality may 

 have twice the rainfall of another, and yet the same type 

 of soil may bear the same plant-formation in both. 



The effective ecological factors which separate one 

 formation from another we shall call the determining, 

 differentiating or master-factors. 



In the British Islands the most striking effect of climate 

 is seen on the higher mountains. The various climatic 

 factors involved in differences of altitude determine for 

 instance a limit above which woodland does not extend; 

 and the rock-vegetation near mountain summits is another 

 effect, though partly an indirect effect, of increased 

 altitude (see Chapter XIV). There is also a considerable 

 difference in the climate of the eastern and western 

 portions of Grreat Britain, and this is probably related to 

 the prevalence of moors in the west as opposed to heaths 

 in the east. 



Characteristic but subordinate features of other plant- 

 formations also depend on climate, for instance the abun- 

 dance of ferns in the woods of the west and north as 

 contrasted with their paucity in the east and south-east. 

 Certain species of plants are confined to parts of the 

 country characterised by a definite cUmate, and their 

 absence elsewhere may often be due to climatic factors, 



1 It must be understood that nothing approaching a detailed treat- 

 ment of ecological factors, and of their relations to one another, is 

 possible in the present work. 



