PLANTS IN THE SAME HABITAT 183 



woodlands is largely mesophytic in character, though some 

 xerophytic plants are to be found in the drier, upland woods, 

 and some hydrophytic in the swampy or boggy portions of low- 

 land woods. Moors, on the other hand, have a xerophytic type 

 of vegetation, and rivers, ponds, lakes, marshes, a hydrophytic. 



In studying Plant Associations it is usual to try and determine 

 the dominant species. In woods, as we have seen, the chief tree 

 is noted ; then the sub-dominant species, and lastly the herbaceous 

 undergrowth. This method may be applied to any natural area, 

 as a wood, a hedge, a pond, a moor, etc. The dominant species 

 gives the name to the Plant Associations. Thus the plants found 

 growing together with the Heather form a Heather association ; 

 those with the Cotton-Grass, a Cotton-Grass association, and so on. 

 Sometimes it is difficult to decide on a dominant species, for two 

 plants may be competing with each other and be almost equal 

 as regards the number of the individuals. In some commons 

 it would be difficult to say whether the Heather or the Gorse was 

 the dominant species ; the association would then be called a 

 Heather-Gorse. In woods it is often impossible to fix on one 

 particular tree as dominant above all others ; such woods are 

 named after the two trees which are most conspicuous, as Oak- 

 Birch, or Oak-Hazel woods. The sub-dominant species are those 

 which are most abundant next to the dominant. In a wood it is 

 often some shrub, such as the Hazel or the Blackberry, or Rosebriar ; 

 these would all be grouped together as sub-dominant. In a Cotton- 

 Grass moor, the Heather might be the sub-dominant species, 

 together with certain Sedges and Rushes. The lesser herbs, which 

 in a wood often comprise a large number, form the herbaceous 

 undergrowth. 



During the last ten or twelve years attempts have been made 

 to map the vegetation of various districts. The object of these 

 vegetation maps is to indicate what plants grow together under 

 more or less uniform conditions of soil and climate ; what plants 

 stand in any relationship to each other, and above all, how the 

 present vegetation of any given area has arisen. This grouping 

 together of plants in associations, according to the nature of their 

 habitat, received a strong impetus from the work of Professor 

 Warming, the Director of the Botanical Gardens at Copenhagen, 



