CHAPTER VI. 
REVIEW AND DISCUSSION. 
In conducting the investigations which are discussed in the present 
paper the work was carried out as follows: first, a survey of the Labora- 
tory domain was made, in order to recognize by actual observation its 
natural associations of plants, and to gather up, in the most general 
way, through such observation, whatever evidence could be obtained 
regarding the factors by which these associations have been determined; 
second, a more thorough study of certain constituent species of the asso- 
ciations was undertaken, in which detailed mapping, records of local 
distribution, and investigation of the relation of certain physical factors 
to choice of habitat on the Laboratory domain were supplemented by 
comparative observations at various points in New Mexico, Arizona, and 
southern California; finally, the aid of specialists was called in for the 
purpose of gaining more exact knowledge of the soils, of geological history, 
and of the plants themselves, in their biological and genetic relations, so 
far as these might be expected to throw light on the problem in hand. 
Less reference, possibly, than might have been expected has been made 
thus far in regard to the general subject of plant migration. This, 
however, has not been neglected; and the more important results, so far as 
they pertain to the immediate subject, have occasionally been cited. 
Further reference to these, suggested in part by the analysis of the flora, 
will be made in the following pages. 
We are nowin a position tocall attention to facts that may be regarded 
as satisfactorily established, and to point out their more obvious relations 
to distributional problems. 
THE PLANT ASSOCIATIONS. 
It has been seen that the plant associations and habitats of Tumamoc 
Hill and the adjacent valley fall naturally into four well-defined groups, 
namely, those of the river, of the flood-plain, of the slopes, and of the hill. 
Associations of the rvver.—These are two in number: first, the aquatic 
plants of the river itself and the irrigating ditches fed from it, and second, 
the species of the river-banks. These latter include not only willows 
and cottonwoods, but arrow-weed and some other species characteristic 
of the arid Southwest. The small number of species belonging to both 
of these associations, in comparison with those of corresponding habitats 
in regions of greater rainfall, is significant. It is apparently due to two 
causes; first, the greater difficulty which seedlings encounter in starting 
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