The results with increased CO2 concentrations in enclosed environments suggest that normal CO2 levels in the air may limit photosynthesis and yield is logical to conclude that higher CO2 levels under field conditions would lead to higher yields. Under field conditions, however, the direct addition of CO2 seems impractical because of the volume of air involved, and winds and air turbulence limit control, although they serve to replenish depleted supplies of CO2. Tisdale and Nelson (1966) nevertheless suggested that it may not be unreasonable to suppose that atmospheric enrichment of CO2 for certain field crops in certain locations may someday be practiced. Whether feasible or not, CO2 is a key element in photosynthesis, and if c producers understand its importance, they may be able to effect changes and practices