334 Part III. Chapter 4. 
Arctic and Arctic-Alpine Flora. An analysis of the floral elements of the 
north-south arctic and arctic-alpine flora distributed along the elevated conti- 
nental axis from Alaska to Cape Horn and broken only from the southern 
downfall of the Guatemalan highland ı5° N. to the Colombian Andes 5°N. 
shows the following interesting phenomena. ı. The flora of the higher Rocky 
and Sierra Nevada Mountains, the Mexican Cordilleras (8,000 to 12,000 feet) 
of the Guatemalan tierra fria and the tropic Andes (above 12,500 feet) is 
one of northern extraction, abounding in genera associated with the colder 
zones of North America, Europe and Asia. Such for example, are: Ranun- 
culus, Anemone, Berberis, Geranium, Spiraea, Geum, Rubus, Ribes, Saxifraga, 
Hydrocotyle, Gaultheria, Vaccinium, Veronica, Eritrichium, Gentiana, Pole- 
monium, Hieracium etc. 
2. While possessing many genera in common, by far the greater per cent 
of species in the Mexican cordilleras are endemic, as are those of the alpine 
Andes. This points to a long and effective isolation of the Mexican and 
South American Andes from each other and from the Rocky Mountains. 3. The 
arctic-alpine genera are the most common which belong to the element common 
to the Himalayan and east Asiatic regions and the Rocky Mountains from 
Alaska to Colorado. Such genera occur sparingly in the Mexican and tropic 
Andes, and then with endemic species. There is an increase of this element 
in the extra-tropic Andes toward the Straits of Magellan. Here should be 
noted the fact that certain species of the Rocky Mountain arctic-alpine region 
reappear in the extra-tropic Andes toward the southern extremity of South 
America, being so far as known absent from the Mexican and tropic Andes. 
Among these are: Gentiana prostrata, Trisetum subspicatum, Primula farinosa, 
and var. magellanica, Draba incana (D. magellanica ), Aopecurus alpınus 
(A. antarcticus), Saxifraga caespitosa (S. cordill;rarum ), Polemonium micran- 
thum (P. antarcticum), Collomia gracilis g" 
Warm Temperate and Subtropic Xerophilous Elements. The warm temperate 
and sub-tropic xerophilous elements common to North and South America 
deserve special emphasis. They embrace for the most part, the flora of 
the arid regions of the western and south-western states and north Mexico. 
This flora occupies the mountain slopes of the transition climatic zone and 
plateaus of the upper Sonoran and the hot deserts of the lower Sonoran 
climatic zones. This area has been the field of development of many groups 
peculiarly American. It is the region of xerophytic composites, Nyctaginaceae, 
Polygonaceae-Eriogoneae, Onagraceae, Amaranth Gomp! ‚Malvaceae, 
Boraginaceae-Eritricheae, Gilias, the Yucca and the Agave kinships (Fig. 19, 
20) and the Cactaceae. 
When this peculiar flora was in the vigor of its development and occupa- 
tion of new territory, the climatic conditions seem to have exerted a pressure 
1) See ante page 302, and compare the views of Gapow: Altitude and Distribution of Plants 
in Southern Mexico. Journal Linnaean Society. Botany. XXXVIII: 429—440. 
