INTRODUCTION. 3 
to place side by side the plants of Tumamoc Hill and those of Mount 
Lemmon, a few miles away, to realize how essentially different in their 
fundamental characteristics the two floras are. 
In dealing with the former, it is found that with it are closely asso- 
ciated, though in more or less distinct contrast, the plants of the neigh- 
boring valleys, and it has seemed necessary, therefore, to include with 
the Laboratory domain the natural setting of Tumamoc Hill, that is, the 
ground immediately adjacent, extending from the bed of the Santa Cruz 
River on the east to the boundary of the Laboratory reservation on the 
west, and from the flood-plain of the river on the north to the same topo- 
graphic horizon on the south, in this way delimiting the area studied by 
natural rather than artificial boundaries. Thus bounded, this area includes 
approximately 4 square miles and exhibits all of the characteristic plant 
associations within the immediate neighborhood of the Desert Laboratory. 
As already indicated, attention will be given in this paper chiefly to 
problems of distribution as related to causes now in operation. The 
physical factors which modify and more or less control the associations 
of plants are under investigation by different members of the staff of the 
Desert Laboratory, but for the most part can not as yet be formulated 
quantitatively. At this stage of progress it is a distinct advantage to 
be able to make use of the more exact knowledge in certain departments 
which, by the courtesy of my colleagues and other specialists, is given in 
the accompanying contributions. The section on the origin of desert 
floras has been written by Dr. D. T. MacDougal; that on geological his- 
tory by Prof. C. F. Tolman; and that on soils by Dr. B. E. Livingston. 
The list of species, and their arrangement in vegetation groups, has been 
_ prepared by Prof. J. J. Thornber, who has otherwise rendered valuable 
aid. Dr. W. A. Cannon has generously placed at my disposal, in advance 
of publication elsewhere, his observations on the root topography of 
certain species. Prof. Bruce Fink has contributed an account of the 
lichens; and the maps of local distribution, with accompanying notes, 
have been prepared by Mr. J. C. Blumer. 
Through this cooperation it is fair to say that there is perhaps no simi- 
lar area that has been more thoroughly studied. Yet it must be frankly 
admitted that, at the present time, interpretation of the data thus far 
gained is only possible in part. It would be nothing but the baldest 
pretense, for example, to tabulate observations of light intensity or tem- 
perature, even though, as with the latter, they have been laboriously 
continued through many months, and to attempt to correlate these accu- 
rately with observed facts of distribution; nevertheless, it is precisely 
by the multiplication of such data that we hope for the ultimate attain- 
ment of results that may be expressed with mathematical exactness. 
The justification of publishing so far in advance of the attainment of this 
goal lies in part in the necessity of preserving vanishing data and in part 
