PLANT HABITATS AND COMMUNITIES 317 



also to a rarefied atmosphere. These changes of altitude, 

 slope, climate, aspect, soils, and water-supply sort out, 

 as it were, groups of plants, each with its own character- 

 istics, and so completely is this done that it imprints 

 itself on the landscape. 



The diagram, Fig. 208, is a transect extending from the 

 summit of a hill to the valley below, and shows the striking 

 changes in the types of vegetation. The high ground is 

 treeless. To the west is a wet, peat-covered Cotton-grass 

 moor (A). Rising above is a rocky summit with stony 

 slopes, dominated by Bilberry (Vaccinium Myrtilkis) (B), 

 and along with it are Ling (Calluna vulgaris) and rolled- 

 leaved Grasses. These moorland species extend down the 

 slope, but here the most conspicuous species is the Bracken 

 (C). The zone of Oat cultivation (D) now begins, but the 

 area is largely given up to pasture. In the higher part of 

 this zone the Oak reaches its upward limit. At the lower 

 level is the zone of Wheat cultivation (E), but, as in the 

 Oat zone, pasture-land predominates. 



Factors affecting the distribution of vegetation. In 

 Britain the factor which has the greatest influence 

 on plants is the soil, and some of its constituents are 

 much more effective in determining the distribution of 

 plants than others, e. g. water-content, humus, acidity, 

 and lime, which are therefore called determining factors. 

 A given habitat will, with the varying seasons, be subject 

 to a more or less definite cycle of climatic conditions, e. g. 

 temperature, rainfall, atmospheric humidity, and wind. 



The conditions affecting plant growth due to topography, 

 e. g. large mountain masses, altitude, exposure, and slope, 

 are known as topographic or physiographic factors ; those 

 due to soil-conditions are edaphic factors ; temperature, 

 precipitation (i. e. rainfall and humidity), and winds are 

 climatic factors. 



The vegetation of a given area will thus be under the 



