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in roots sinoe the root-passages form a closed system 

 open only at one end. 



At first sight it might seem that the meta- 

 bolic removal of oxygen from the atmosphere of the root- 

 passage might create an inward draft of air, provided, 

 of course, that all the carbon dioxide produced were ex- 

 creted outward into the soil and did not have to diffuse 

 backward through the passage. This would be true if 

 the gas supplied at the outer end of the passage were 

 pure oxygen. But it is not. It is air, and the removal 

 of the oxygen leaves approximately 80 percent of nitro- 

 gen. Very soon, therefore, the atmosphere within the 

 root passage becomes nitrogen plus what oxygen can be 

 supplied by diffusion. 



The actual cases of oxygen diffusion into root 

 passages are too complex for quantitative treatment but 

 it is interesting to examine a simple case which is 

 somewhat analogous. We imagine a single air passage 

 one millimeter in diameter and one meter long. We assume 

 that this is supplying oxygen to lower-placed root tis- 

 sue only; that is, no oxygen is absorbed or otherwise 

 removed during passage through this one meter length. 

 We assume that the inner end of this passage communicates 

 with a section of the root-spaces from which all the oxy- 

 gen which arrives is immediately removed and that the 

 outer end communicates with air. Temperature is assumed 



constant. 



