51 



The Opuntia which is sensitive to aeration, grows only 

 on the well-drained slopes and hill sides. The Prosopis, 

 which is less sensitive, prefers the stream banks and 

 valley-flats where water is more plentiful and aera- 

 tion less good. 



The detailed application of these concepts to 

 problems of plant distribution is impossible in the 

 present ignorance of the responses of most species to 

 aeration differences. The methods of the present in- 

 vestigation, or some equivalent methods, must be applied 

 in detail to a wider range of species just as actual 

 tests of temperature and moisture responses have been 

 applied already. However, the present data justify 

 the conclusion that these aeration responses cannot be 

 neglected and suggest that they may furnish the key 

 to some of the facts of plant distribution which are 

 anomalous on the present theoretical basis. 



