Vegetation of Northwest Mexico 
[9 
Shreve. In addition to the plant investigators listed above, 
there have been several specialists who have visited north¬ 
western Mexico while obtaining specimens and data. Among 
the best known of these are Trelease for yuccas, agaves, and 
oaks; Hitchcock for grasses; Britton for cacti; Shaw for 
pines; Safford for plants of economic significance; and Stahl 
for cacti and agaves. 
Vegetational Studies 
The vegetation of northwestern Mexico has been but im¬ 
perfectly studied and described, despite the extensive botan¬ 
ical investigations in the region. The wealth of floristic 
data has not been synthesized and organized in terms of 
associations and formations except in a very general man¬ 
ner. Only a few investigators have attempted to describe 
the entire vegetational landscape, including climatic and 
edaphic factors involved. These studies have been of small 
and scattered areas within the region. 
In working up this study, use has been made of 
published floras, botanical monographs, reports of military, 
naval, and boundary commissions, travelogues, consular re¬ 
ports, descriptions of landscapes incorporated in geologic 
reconnaissances, and varied scientific reports which include 
data on vegetation, e.g., reports of archaeologic, zoologic, 
and ethnologic surveys. The following paragraphs are 
devoted to a consideration of the principal sources utilized. 
The vegetation of northwestern Mexico has been de¬ 
scribed in eighteen pages by John Harshberger (Phy to geo¬ 
graphic Survey of North America , 633-648, 656-658, New 
York, 1911, which appeared in Die Vegetation der Erde 
series edited by Engler and Drude). A map of North 
America, on the scale of 1:40,000,000, shows northwestern 
Mexico divided into five vegetational areas: Chihuahuan 
Desert, Sonoran Desert, Western Sierra Madre, Rocky 
Mountain, and Jaliscan. Limits of pine, mangrove, and 
yucca are indicated. In the symposium Naturalist's Guide 
to the Americas (edited by V. E. Shelford, Baltimore, 1926), 
