Final Results
The accompanying figure depicts the final results of the experiment. As in Experiment #3, the first data point was derived by assigning a dry weight of zero to the airspace CO2 concentration at which death occurred. The other data points were obtained by direct measurement of the weights of dry matter produced by the plants in the three experimental units over the course of the study.
As can be determined from the relationship defined by the data of the figure, a doubling of the CO2 concentration characteristic of the outdoor urban environment (365 to 400 ppm) produces about a doubling of the dry weight of the radish plants. This is approximately the same type of response we obtained for pea plants in Experiment #3. As we indicated in discussing those results, however, it is likely that both radishes and peas growing under more normal outdoor conditions would probably not be quite as responsive to atmospheric CO2 enrichment as is indicated by our two experiments; for plants under stress or lacking certain resources (such as normal high light intensity) tend to respond more to elevated CO2 levels than do plants growing under ideal conditions.
Were we to repeat this experiment, we thus would probably not fill the 3-liter bottles with as much sand as we did here, so as to be able to confine the plants to a single-aquarium experimental unit. Then we could locate the two lights above the plants much closer to them. We would also probably direct additional lights towards the plants from the front and back sides of the top portions of the single-aquarium units.