48 
C. E. Moss. 
Conclusion. 
I should have liked, had space permitted, to have also traced the 
development of the concepts of those minor units of vegetation 
which are subdivisions of associations. These are important from 
two points of view. First, they are the outstanding vegetation 
units to the student of vegetation who is investigating the detailed 
structure of a very limited area. That such intensive studies may 
yield results of great value is seen from a perusal of such works 
as those of Woodhead (1906) and Brockmann-Jerosch (1907), and 
from unpublished results by several British ecologists. It is 
becoming usual in this country to speak of the subdivisions of the 
association as plant societies (cf. Clements, 1905: 296). Graebner 
(1895, 1901) and some other continental ecologists have termed them 
“ facies ” ; but the term “ facies,” like the term “ society,” has been 
used in several other senses. In fact, so many terms have been 
used by ecologists and plant geographers with so many different 
significations that it would appear to be impossible to find any term 
to which the above objection does not apply. The intensive study 
of plant societies permits of a rigorous and quantitative deter¬ 
mination of habitat factors, as has been done by Hedgcock (1902), 
Livingston (1906), Yapp (1909), and others, and in unpublished 
work by Professor Oliver, Mr. Crump, and Mr. Adamson. 
Woodhead’s useful generalisation with regard to “ competitive ” and 
“complementary” communities (1906: 396, 7, etc.) is also an out¬ 
come of this kind of work. Such work also lends itself to the 
investigation of the physiological responses of plants to definite 
habitat factors. The latter work has scarcely been begun by 
ecologists; but it is certain that a rich reward of results awaits 
investigators who undertake such research. 
Secondly, the investigation of plant societies is necessary to the 
study of the formation; for several writers have displayed an 
unfortunate facility in mistaking the one vegetation unit for the 
other. Pound and Clements, Drude, Clements, and others have 
protested against the tendency of certain writers to elevate to the 
rank of a formation any phase of a succession or any slightly 
different patch of vegetation. This tendency has to be guarded 
against in all intensive studies of vegetation; and the quasi 
definiteness of the local patches of vegetation in a restricted area 
can only be corrected by comparative observations in other districts. 
The study of vegetation needs workers from all the departments 
