1898 | FLORA OF LOWER SONORAN AND ARID ZONES 143 
objects (¢. g., feet of birds) and thus secure transportation. So 
far as I was able to examine, all the South American species of 
Polemoniacee possess these mucilaginous seeds. Giha minima 
and dwarf forms of G. involucrata (Chili) grow in little dense 
mats in arid spots. On the addition of moisture the seeds are 
gradually pushed up from the dense enclosure of bracts until 
they stand exposed and ready to adhere to any disturbing 
object. In the section Navarretia the bract development itself 
would be sufficient to bring about extended distribution by cling- 
ing to the hair of mammals. The absence of these genera from 
both north and south Mexico is noteworthy. 
SUMMARY. 
1. Most of the genera just considered are of pronounced 
xerophytic or halophytic character. 
2. Characteristic American groups, such as Zygophyllacee, 
Guajacinee, Borrag.-Eritrichiee, Amarant.-Gomphrenee, and 
Loasacez, tend to a development in both extra-tropical 
xerophytic zones, often with the same, and generally with 
nearly related species in both zones. 
3. For some genera each zone has its characteristic group 
of endemic species, indicating an independence from the other 
reaching into prehistoric time, é. g., Malvastrum, Chorizanthe, 
Larrea, etc. 
4. The halophytic genera in particular indicate that in some 
cases no distribution has occurred from one region to the other 
under present geological conditions, ¢. &:, Frankenia Palmeri, 
F. Jamesii, F. triandra, Niederleinia juniperoides, Spirostachys, 
Larrea, etc. 
It is evident from a study of the plants concerned that dis- 
tribution by natural methods has occurred and is occurring under 
present physical conditions. It is further evident that distribu- 
tion has been greatly facilitated by what may be called, in con- 
tradistinction, artificial means, namely, as a result of commerce. 
Again, one must suppose the present conditions, or others as 
favorable, to have endured far back into the history of the pres- 
