Vegetation of Northwest Mexico 
[ 15 
Flora 
The flora of northwestern Mexico is quite diversified, 
as would naturally be expected in a region situated on both 
sides of a tropic (22°N.-38°N.), and extending from a 
coast line nearly a thousand miles long to the crests of a 
mountain range which average over 8,000 feet in elevation. 
High continental-interior and coastal deserts merge into 
grassy steppes and shrub savannahs, which are succeeded 
by upland coniferous and lowland tropical forests. With 
great extremes and ranges of temperature and precipitation 
are associated a variety of soils which differ markedly in 
organic content, salinity, and other factors conditioning the 
physiologic processes and morphology of the native flora. 
The Compositae outnumber every other order in gen¬ 
era and species; grow in almost every variety of situation; 
are often very numerous individually; and especially char¬ 
acterize the dry districts. The Leguminosae, Gramineae, 
Cactaceae, Euphorbiaceae, Felices, Labiatae, Amaryllida- 
ceae, and LUiaceae are most numerous in species and most 
prominent in the landscape, with the exception of the Sierra 
Madre area where the Coniferae are the dominant group in 
the landscape. Most of the arid climate genera and families 
(Yucceae, Cactaceae, Agave, Fouquiera, Nolina Dasyliron, 
etc.) of North America presumably originated in the Chi¬ 
huahua Desert area, and advanced northward with the with¬ 
drawal of the northern glaciation. The species of these 
genera and families should be considered the native vege¬ 
tation of northern Mexico. Rosaceae, Fagaceae, Coniferae, 
etc. of northern forms have invaded the upland portions 
of the region and maintain themselves to as low as 3,000 feet 
elevation. This spread of boreal plant forms took place from 
the southern Rocky Mountain area as the California flora 
extends into Mexico only in Lower California. Although the 
prairie vegetation of the Great Plains has entered northern 
Mexico to some extent, the present direction of movement 
seems to be of Mexican species north-ward. Certain Yucca 
