94 



may differ widely in their response to oxygen defi- 

 ciency in the soil or to the presence of excess of 

 carbon dioxide. Prom this fact Cannon makes certain 

 ecological deductions to which it will be necessary 

 to refer below. Another significant result in Gannon's 

 wori is the fact that root growth of Prosopis stops in 

 pure carbon dioxide and is slowed in the mixture with 

 10 percent of air although a specific effect of car- 

 bon dioxide is improbable since growth continued nor- 

 mally in mixtures containing 75 percent of this gas. 

 Apparently the stoppage of growth in the higher concen- 

 trations of carbon dioxide must be ascribed to defi- 

 ciency of oxygen rather than to excess of carbon 

 dioxide and, except for the experiments of the present 

 investigation this is the first piece of real evidence 

 that plant roots are injured by anaerobic conditions. 

 It appears that Opuntia is injured specifically by ex- 

 cess of carbon dioxide since growth stopped in a mix- 

 ture of 5U percent carbon dioxide and 50 percent oxygen. 

 Probably Opuntia roots are as sensitive to deficiency 

 of oxygen as are those of Prosopis hut this cannot be 

 determined from the data since the specific effect of the 

 carbon dioxide intervenes. 



The experiments which will be described in 

 the following chapters have been noted in several ad- 



