70 THE STORY OF PLANT LIFE 



formation which we are studying, not the species, 

 though each species forms a unit of the whole 

 formation and has to be considered analytically, just 

 as the formation must be studied synthetically also, 

 as a unit on the broader basis, and in its relation to 

 other plant communities. 



Plant formations are the largest units the plant 

 ecologist has to deal with. They are determined by 

 the main factors such as soil, water, altitude. Climate 

 in a small degree^ influences plant formations, but as 

 a rule plant formations do not extend over wide 

 enough areas for the influence of climate to be 

 reflected in their component elements. 



It is the larger zones of vegetation that are espe- 

 cially affected by climate. Humidity,! however, in an 

 insular area, such as the British Isles, does influence 

 the types of plant formation and their distribution. 



The British Isles being much more humid along 

 the west coasts than in the Midlands or on the east 

 coasts, are characterised on the west by a greater 

 extent of bog and moorland types of formation which 

 are less dependent on the soil than others, being 

 regulated by the distribution of peat and a heavy 

 rainfall. It is in such western and northern areas 

 that Cotton-grass, Whortleberry, and other types of 

 wet moorland plants are developed. Dry, sandy 

 heaths are conversely more widely distributed over 



* So far as the British Isles is concerned. But even here geo- 

 graphical position plays a not unimportant part. 



t Regulated by the course of the Gulf -stream, and the distribution 

 of the harder Palaeozoic rocks. 



