136 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
movements of desert plants. From his analysis it is seen that within 
the area selected for special study there have been collected 449 species, 
belonging to 269 genera, included in 68 families of plants, and it is safe 
to assume that this list, as regards the flowering plants, approaches very 
closely to completeness. 
Of the 449 species no less than 264 are limited in their distribution to 
the southwestern United States and Mexico, in other words, at least 
59 per cent of our flora as it exists to-day is to be thought of as belong- 
' ing distinctively to the desert region of southwestern North America. 
It is of much interest to note, as pointed out by Professor Thornber, 
that of the species constituting the remaining 41 per cent a large pro- 
portion occur on the flood-plain, which may be considered the least 
distinctively desert part of the area studied. 
The classification of genera on the basis of their geographical distri- 
bution indicates that the flora of the Laboratory domain and the adjacent 
valley is composed of the following elements: 
(1) Genera belonging exclusively to the southwestern United States and 
Mexico, or occurring very exceptionally beyond those limits. Such are 
Fouquerta, Harpagonella, Keberlinia, Olneya, Riddellia, Simmondsia, and 
others, 39 in all. 
(2) North American genera, comparatively few in number, whose 
‘‘centers of dispersal,’ judged by criteria given by Adams (1902, p. 122) 
are in the northwestern and western United States. Eviogonum is a 
good example of these. 
(3) Genera well represented in high northern latitudes of both hemi- 
spheres, but occurring southward along the Alleghenies and the moun- 
tains of western North America. These are represented by Anemone, 
Delphintum, and others. They are far more numerous on the Santa 
Catalina and other neighboring mountains, where they constitute a large 
and characteristic element of the flora. 
(4) Tropical or subtropical American genera, including those that 
occur chiefly in the tropics, from Mexico southward, but exceptionally 
or not at all northward of Arizona. Examples of these are Condalia, 
Janusia, Martynia, Pectis, and others. 
(5) Genera ranging from the southwestern United States to extratrop- 
ical South America, e. g., Amsinckia, Argemone, Bowlesia, Cryptanthe, 
Encehia, Krameria, Larrea, and Trixis. ‘These constitute a characteristic 
element of the desert floras of Chile and Argentina. 
(6) Genera of both eastern and western hemispheres, many being 
widely distributed in both, such as Abutzlon, Acacia, Atriplex, Clematis, 
Ephedra, Hibiscus, Lycium, Sambucus. 
The natural interpretation of these data harmonizes with the well- 
known views of Gray, Engler (1882), and others. We may assume with 
regard to the geographical origin and movements of the plants here 
