Flora of Northern Mexico. 295 
Great Basin flora and is perhaps older to the region in point of time than 
the flora of the Colorado Desert, for from a previous statement, it is not pre- 
sumptious to suppose that the Mohave desert supplied to the drying Great 
Basin many of its most typic plants. 
Flora of Northern Mexico. The flora of northern Mexico is characterized 
by the absence of thirty nine natural orders, which are represented in central 
and southern Mexico, the absent orders being essentially tropic or consisting 
of hygrophilous plants. Three orders, the Frankeniaceae, Elatinaceae and San- 
talaceae, represented by one species each, are not known to occur in the 
central and northern positions of the country. The Cruciferae, Polygalaceae, 
Malvaceae, Malpighiaceae, Leguminosae, Crassulaceae, Onagraceae, Cactaceae, 
Compositae, Asclepiadaceae, Polemoniaceae, Hydrophyllaceae, Boraginaceae, 
Convolvulaceae, Solanaceae, Scrophulariaceae, Labiatae, Nyctaginaceae, Ama- 
rantaceae, Chenopodiaceae, Polygonaceae, Euphorbiaceae, Cupuliferae, Coniferae, 
Amaryllidaceae, Gramineae, and Filices are the orders predominant in the 
number of species. Such genera as Beschorneria, Fouguiera and Nolina have 
their center in north Mexico and probably originated there. The number of 
Mexican species extending northward west of the Mississippi is almost double 
that extending northward east of that river. The extensions into eastern 
North America are largely south Mexican plants and not north Mexican. Such 
genera as Magnolia, Asimina, Tilia, Robinia, Ligwidambar, Tlex, Diospyros, 
Bumelia, Ulmus, Morus, Ostrya, Carpinus and Carya (Hicoria) are absent 
from the Pacific forests, but all these genera are represented in Mexico, yet, 
so far as we know, only four out of the fourteen in north Mexico. Sometimes 
the species of eastern North America and in the mountains of south Mexico, 
are identic Ziguidambar styraciflua, Ostrya virginiana, and Carpinus ameri- 
cana. In the belt of oaks in south Mexico according to ENGLER are found 
Ulmus, Alnus, Clethra, Cornus, Vıburnum, Deutzia, Triumfetta, Rubus, Vitis, 
genera which occur in the Atlantic states of the American Union, and this 
relationship between two widely separated floras extends to many genera of 
herbaceous plants: 7halictrum, Ranunculus, Anoda, Hypericum, Desmodium, 
hexia, Cuphea, Lobelia, Salvia, etc. Many of the species inhabiting pine- 
woods belong to genera which are widely prevalent in the extra tropic portion 
Of the northern hemisphere: Veratrum, Salix, Trifolium, Arenarta, Polygala, 
Helianthemum, Castilleia, Gerardia, Chelone etc. 
here is, therefore, ample evidence that north Mexico is the center of a 
Special xerophilous flora which, there are good grounds for assuming originated 
in this area, though this flora by migration now has considerable northward 
and southward extensions. A considerable part of the flora of California, still 
more of Nevada, Utah and western Texas and, yet more, that of Arizona and 
er Mexico may be regarded as a northward extension of the flora of the 
Mexican plateau En 
en 
!) Hemstey, W. Bortig: Botany in Biologia Centrali-Americana, Vol. IV, 1888: 306-315. 
