At this point in time, we have measured the atmospheric CO2 concentrations in our six experimental tanks a total of three times a week - Monday, Wednesday, and Friday - and we have computed the average carbon dioxide concentration of the air in each tank for the first week of the experiment.  Over this period, the only change we have noted in the Pothos plants is the initiation of root growth in Tank 6 (see Figure 1); and by placing a ruler against the front glass of the aquarium, we have measured the lengths of each of these new primary roots (see close-up of plant root in Figure 2).

From these two sets of data - mean biospheric airspace CO2 concentration and mean primary root length per plant - we have produced the graph of Figure 3.  This graph shows that root growth began first in the tank with the highest CO2 concentration; but it is possible that this earlier initiation of growth may be nothing more than an expression of natural plant variability that is unrelated to the air's CO2 content.

Another thing to note at this stage of the study is that the two tanks with completely sealed tops (Tanks 1 and 2) still have airspace CO2 concentrations that are higher than that of the outside atmosphere.  As time progresses, however, the Pothos leaves in these tanks should draw more CO2 out of their isolated airspaces, causing their CO2 concentrations to drop to lower levels.

Note also that the CO2 concentrations of Tanks 3 and 4 are almost identical.  To remedy this situation, we have reduced the sizes of the holes in the polyethylene top of Tank 3 by covering portions of them with clear tape.  By thus further restricting the exchange of air between this tank and the higher-CO2-content air of the room in which it resides, the plants in Tank 3 should be able to draw its airspace CO2 concentration down below that of Tank 4.