28 



outer end to at the inner end, which is .2 atmo- 

 sphere per 100 centimeters, or .002 atm. per cm. The 

 total pressure is assumed constant at one atmosphere. 

 Substituting these numerical Values in the above for- 

 mula we have : 



V = .197 x .007854 x 3600 x .002 

 or V s .01114 cubic centimeter per hour. Under the as- 

 sumed conditions, therefore, a little over one one- 

 hundredth of a cubic centimeter of oxygen would dif- 

 fuse through a root-passage one millimeter in diameter 

 and one meter long. It is obvious that this will not 

 provide any large oxygen supply for the respiration of 

 tissues lying beyond the one-meter point. 



This numerical value must not be considered 

 as having any quantitative precision or as being 

 more than an indication of the general order of the 

 quantity of oxygen which would diffuse. The necessary 

 simplifying assumptions have simplified all reality 

 out of the case. However, the value given is probably 

 above rather than below the truth. The assumed diameter 

 of one millimeter is unusually large for actual root- 

 passages. It will be noted that the rate of diffusion, 

 other things being equal, is proportional to the area 



1. The composition of the atmosphere is 

 assumed to be 20 percent oxygen and 80 percent nitro- 

 gen, by volume. Seepage , below. 



