Photosynthesis within a corn crop canopy was measured (wright and Lemon 1966) on a clear day, August 1, 1961 at various heights in a densely planted four-hectare (10 acre) field at Ithaca, New York. The crop was 220 cm in) and had not reached the tasseling or reproductive stage. At noon when light penetration into the crop was greatest, the lowest level of CO2 was 260 ppm measured at 130 cm (51 in) above the soil. CO2 levels in this zone were restored between 0500 and0600 hours (Table 9-3). Lemon and Wright stressed that wind to cause Co, movement is essential; that the percentage Co2 fixed in photosynthesis closely paralleled wind intensity and that in this study the maximum hourly change of CO2 was less than 10 ppm under conditions moderate winds. The low rate of CO2 replenishment was attributed to the iact that such a tall crop resisted air movement within the canopy. Between 53 and of CO2 fixation, depending on the time of day, was accounted for in the 175- to 900-cm (69-79 in) zone probably because this represented the zone of greatest light interception and photosynthetic surface, although at noon, more CO2 was fixed in the 20- to 175-cm in) penetrated the canopy. Highest CO2 concentrations were found at the 20-cm (8 in) height but it is uncertain if this was due to low photosynthetic usage, effective transfer of CO2 into this zone by air currents, or release of CO2 from the soil Much speculation remains on the significance of CO2 release from the soil for photosynthesis.