49 



ECOLOGICAL CONCLUSIONS. 



The ecological significance of the results here 

 reported lies in the wide differences found to exist 

 between different species in the oxygen requirement of 

 their roots. Coleus and heliotrope are injured by a 

 very slight deficiency of oxygen. Oleander is injured 

 after a time but is much less sensitive. Willow and eucalyi 



tus can live with very little soil oxygen, possible with 

 none. A similarly wide difference exists in the behavior 

 of the roots of Coleus and willow toward carbon dioxide. 

 The experiments of Cannon already described show simi- 

 lar differences between Opuntia versicolor and Prosopis 

 velutina . the Opuntia being more sensitive to low oxy- 

 gen and high carbon dioxide than is the Prosopis. This 

 conclusion of the sensitiveness of Opuntia is confirmed 

 by the single test of this species which is included 

 in the present experiments. 



The ecological bearing of these facts is ob- 

 vious and has been discussed already by Cannon and Free. 1 

 If differences as wide as this exist in the response of 

 different species to the composition of the soil air 

 it is manifest that soil aeration may be a much more im- 

 portant ecological factor than is generally assumed, 



1. Science 45: 178-180 (1917). 



