1898 ] FLORA OF LOWER SONORAN AND ARID ZONES 127 
Sec. IV. DENDROMENTZELIA, one species, a large shrub or tree, Mexico. 
BICUSPIDARIA, four Californian species, extending to Arizona and 
north Mexico. 
s 
SEC. 
Sec. VI. BARTONIA, six species. 
Two species in western North America. 
Ba. One species in western North America. 
Bb. Three species; 7. adbescens in Argentine, Texas, and Mexico ; 
two species in central and gulf states. 
It appears from this synopsis, that although the development 
of the Loasace has proceeded in a manner to result in endemic 
genera in both the Sonoran zone and Chili-Argentine, still for 
Mentzelia, in particular, there is rather an intimate relation in 
distribution between the two regions. 
The mechanical devices for distribution are such as to secure 
ready transportation by mammals or birds. The stems are 
exceedingly brittle, and all the younger parts are thoroughly 
beset with stout barbed pubescence, so that a very slight dis- 
turbance suffices to fasten the branch or capsule to the disturb- 
ing object. 
LEGUMINOS&. 
The Leguminose and also the Composite deserve special 
study in their distribution and relationships in the arid regions 
of North and South America. It suffices here simply to bring 
forward an illustration, and the genus selected is Prosopis, 
because in the remoteness of regions occupied and as a zonal 
genus it agrees with certain Zygophyllacex, ¢. §., Larrea, 
Porlieria, and Bulnesia. Leaving out of account the two Asiatic 
and the two African species, there remain more than twenty 
New World species whose geographical center is in Argentine. 
These fall into two sections: 
Avcarosia. Nineteen species, mostly in Argentine, but 
including the noteworthy P. judiflora, the mesquite, which in its 
distribution has come to occupy all of the subtropical and warm 
temperate, more or less arid districts of the western hemisphere. 
Apparently the Lower Sonoran zone would be very accurately 
