y3 



Using this method experiments have been made 

 with roots of Opuntia versicolor (cactus) and of 

 Prosopis elutina (mesquite) in air, in carbon dioxide 

 and in various mixtures of carbon dioxide with air and 

 with oxygen. It was found that the roots of Opuntia 

 are much more sensitive to carbon dioxide than are 

 those of Prosopis* A mixture of 50 percent carbon 

 dioxide with 50 percent oxygen stopped the growth of 

 Opuntia roots while those of Prosopis were unaffected 

 in growth rate by concentrations of carbon dioxide as 

 high as 75 percent. An atmosphere of yO percent carbon 

 dioxide and 10 percent air (containing, therefore, only 

 2 percent oxygen) slowed the growth rate of Prosopis 

 roots but did not stop it entirely. However, both 

 Prosopis and Opuntia ceased to grow in pure carbon 

 dioxide. Aeration by drawing air through the tube in- 

 creased the growth rate of Opuntia but had little ef- 

 fect on that of Prosopis. Collateral experiments by 

 other methods have shown that the development of the 

 root system of Opuntia is favored by good soil aeration 

 while Prosopis is relatively little effected. The roots 

 of the oc%till% ( ffouguieria sp lendens ) are also stimu- 

 lated by good aeration* 



The most significant thing in these experiments 

 is the shajjp difference discovered between the responses 

 of upuntia and Prosopis. This strongly reinforces the 

 inference from other literature that different plants 



