Southern Extension of North American Flora. 335 
to the southward which geologic conditions favored, with the consequence of 
carrying a great richness of forms into the South American region. There 
has also apparently, been an encroachment of elements developed in South 
America northward, as shown in the Loasaceae [Mentzelia) and species of 
Prosopis, whose great development occurs in the 
Chilean and Argentine regions respectively. 
Greater details of distribution may be discussed 
as follows: (1) the mountain forms; (2) forms of the 
arid basins and valleys of the lower Sonoran 
climatic zone; (3) sub-tropic xerophilous forms of 
Gulf distribution. According to BRAY, the mountain 
Fig. 20. Beschorneria yuecoides Hook., 
a Mexican xerophyte frequently eultivated. 
‚ Stiff spiny-margined leaves and flower-spike After Die Natürlichen Pflanzenfamilien 
?—3 m long. After Die Natürlichen Pflanzenfamilien II 5, p. 116. 
5, P- 72. 
