148 C. E. Moss, W. M . Rankin and A. G. Tansley. 
natural one to adopt, but a wood is a complex plant-association, and 
the ground-vegetation is subject to other influences than those 
which determine the dominant tree. Thus one may get exactly the 
same ground-flora in a wood of Quercus sessiliflora as in a wood of 
Q. Robur, because the surface soil is of the same type, though the 
underlying rock determines the occurrence of a different dominant. 
In view of the prevailing uncertainty and confusion in the 
nomenclature of units of vegetation, we have avoided so far as 
possible the use of terms signifying units of a definite rank. The 
term association is the only term of this kind used. By an association 
we mean a community of plants occupying a definite habitat, with 
constant dominants and definite floristic composition. 1 
In the scheme on p. 149 an attempt has been made to exhibit 
the relations of the dominant species of tree as well as those of the 
conditions determining the general character of the associations. 
The Alder-Willow series stands at the base, connected with 
both the damp lowland types, the Oak (Q.r.) and the Ash-oak (Fr. 
Q.r.). A vertical line divides the Beech and Ash series on the left 
from the Oak and Birch series on the right. The lowland types 
occur at the bottom, and the upland types at the top of the diagram. 
Various lines lead up to different examples of birch associations, 
the birch being the tree which is dominant at the highest altitudes. 
One line within the Oak and Birch series represents a series of 
damp woods, the moist condition being generally determined by 
soil in the lowland types, largely by climate in those of the uplands. 
To the right are the associations of acid, peaty soils, to the 
left those characterised by mild humus. The lines represent 
affinity, not necessarily genetic derivation, except where they have 
arrowheads. 
The largest capitals are employed for the names of the great 
primary series; the next size of capitals is used for groups of 
associations and for the varied oak-birch-heath association. Medium 
capitals are used for the great basal associations. Small capitals 
for the less important associations, and italic capitals for the semi¬ 
artificial types of woodland. 
1 For a full discussion of the concepts and nomenclature of vege¬ 
tation units see Moss, “ The Fundamental Units of Vege¬ 
tation,” New. Phyt., Vol. IX., p. 18, 1910, 
