Final Results
Final Results: Part 2
Before closing the book on this experiment, there is one other aspect of the data that should be considered. Specifically, how do the numbers and lengths of the primary, secondary and tertiary roots vary as a function of biospheric airspace CO2 concentration? 
In broaching this question, we utilize the data obtained from the bulk-gravel plants that did not suffer root restrictions of the type experienced by the plants that were sandwiched between the black felt and aquarium glass, as the latter situation (although useful for illustrative purposes) is too unnatural a condition for exploring the behavior of roots that normally grow in a three-dimensional matrix.
Figure 15 depicts the mean number of primary roots per plant as a function of mean biospheric airspace CO2 concentration; while Figure 16 similarly depicts the mean lengths of these roots. 
Multiplying the corresponding ordinate (y-axis) numbers of the two graphs and plotting the results in like fashion produces Figure 17, which shows how the mean total primary root length per plant increases as a function of atmospheric CO2 concentration. Likewise, Figures 18, 19 and 20 are the analogous results we obtained for secondary roots; and Figures 21, 22 and 23 are the similar results for tertiary roots.
In viewing these three three-figure sets of graphs, several observations can be made. First of all, except for the two lowest CO2 concentrations (which produced only primary roots), there were far more secondary roots than either primary or tertiary roots; and the numbers of secondary roots, as well as their total lengths, increased with atmospheric CO2 concentration in a manner closely akin to the total plant biomass results of Figure 14. 
In the case of the primary roots, however, there was no increase in their numbers above 513 ppm CO2, and no increase in total length at the highest CO2 concentration, presumably because the real "worker roots" at this stage of the plants' development were the secondary roots, and the more-CO2-enriched plants were pouring more of their photosynthetically-produced resources into this category of root, which at this stage was absorbing the bulk of the water and nutrients the plants needed to sustain their greater CO2-induced growth potential. 
In the case of the tertiary roots, on the other hand, the greatest increases in all root properties occurred at the highest CO2 concentration, presumably because tertiary roots are ultimately destined to join the ranks of, or become more like, secondary worker roots as time progresses, and the more-CO2-enriched plants were much further along in their development in this regard. 
Consequently, it can be seen that not only does atmospheric CO2 enrichment stimulate plants to produce more biomass, it stimulates them to allocate that biomass to the various plant organs in such a way and at such a rate as to further enhance their growth and development.
Last of all, we have added the ordinate values of Figures 17, 20 and 23 to obtain the mean total lengths of roots of all types (primary, secondary and tertiary) for our several biospheric airspace CO2 concentrations and plotted them in Figure 24, the resulting relationship of which is again very similar in shape to the total plant biomass results of Figure 14. 
One further note of interest to be gleaned from this graph is the fact that each of the bulk gravel plants in the highest CO2 concentration produced over 9.3 meters of roots. Hence, if the roots of the five plants in the bulk gravel behind the black felt of Tank 6 were placed end-to-end, they would stretch over 46.5 meters long, which is just over half the length of an American football field!