2 Raymond John Pool 
the author measured very carefully the physical factors which 
were operative in the production of the structures which she 
recorded. 
However, most of the above studies have been more general 
than the one here reported. The authors have attempted to cover 
a greater mass of vegetation rather than to confine themselves to 
a single unit or a subdivision of vegetation. As indicated in the 
title, the study given here was made in a definite area or unit of 
vegetation—the Artemisia formation. This study does not in- 
clude all of the species of the formation, but twenty-eight of the 
prominent ones. 
The original thought was to make a careful study of the leaf 
histology of some of the more typical plants of this formation in 
relation to the physical conditions and to determine to what ex- 
tent the leaf structure of the plants of the formation coincided 
with the present conception of xerophytic anatomy. An attempt 
was also made to discover the range of variation in physical fac- 
tors, especially of water-content, of the formation and the bear- 
ing of this variation upon structural phenomena. 
Since the term formation has often been used loosely without 
any definite or commonly accepted application, it is well that, 
before going further, I should define the term as I understand it 
and as it has been used in this paper. The confusion caused by 
the various conceptions or usages of such terms as formation, 
association, society, etc., is well known to every ecologist, and 
hence it is unnecessary to enter here into a detailed historical 
account of the various controversies over these matters. A plant 
formation as here regarded is the unit of vegetation. It does not 
take a well-trained botanist to recognize that certain areas of 
vegetation are marked out by certain conditions of the habitat. 
Any one can tell that certain areas are very clearly delimited 
by great differences in physical conditions. In other words, it is 
a commonplace observation that the distribution of plants is con- 
trolled by some more or less well-defined character of the sub- 
stratum, and no one has the slightest difficulty in pointing out a 
swamp, a wet meadow, an open prairie, a coniferous forest, or 
a lichen covered cliff. The term plant formation is applied to 
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