CHAPTER IV. 
VEGETATION GROUPS OF THE DESERT LABORATORY DOMAIN. ' 
The following list includes as nearly-as possible the plants growing on 
Tumamoc Hill, the fenced area of the mesa-like mountain-slopes lying 
to the west, and the Santa Cruz flood-plain between Tumamoc Hill and 
the Santa Cruz River on the east, together with the hydrophytic species 
growing in the Santa Cruz River and adjacent irrigation ditches. For 
convenience the areas noted above have been designated as follows: 
(I) Tumamoc Hill; (11) Mesa-like mountain-slopes; (III) Santa Cruz 
flood-plain; (IV) Santa Cruz River and irrigation ditches. 
The species of each of the above areas have been arranged alphabeti- 
cally under the following vegetation forms: (1) trees; (2) shrubs; (3) 
woody climbers; (4) dwarf shrubs; (5) half-shrubs; (6) perennial herbs; 
(7) biennial herbs; (8) annual herbs including (a) long-lived annuals; 
(b) winter annuals; (c) summer annuals. ‘The occurrence of a species in 
any quantity in an area other than the one of which it is characteristic is 
shown by a Roman numeral indicating the area over which its secondary 
distribution obtains. Besides the four groups of plants corresponding to 
the four areas above noted it is thought desirable to include a fifth com- 
posed of miscellaneous introduced species which have become established 
here by virtue of certain inherent qualities or characters. ‘These exotic 
species are limited almost wholly to area III, though a fewoccur in I and II 
In questions of nomenclature the recommendations of the Vienna 
Congress of 1905 have been followed as closely as possible. For con- 
venience synonyms are given as they stand in the second edition of Heller’s 
Catalogue of North American Plants. 
The writer gratefully acknowledges assistance from numerous sources 
in the preparation of this list of plants. Most of the Graminee have 
been passed upon at one time or another during the last few years by 
Professors Scribner, Hitchcock, and others of the Bureau of Plant Indus- 
try, U. S. Department of Agriculture. Dr. J. N. Rose, of the Smithsonian 
Institution, who during his trip through the Southwest last spring spent 
some time in studying the Cactacez in the field, has determined several 
species which had hitherto remained unnamed. Prof. P. Beveredge 
Kennedy, of the University of Nevada, has kindly verified the species 
of Atriplex; while Mr. Paul C. Standley, of the New Mexico College of 
1Prepared by request and Contributed by J. J. Thornber, A. M., Professor of Botany 
in the Arizona Experiment Station. 

103 
