The Soil as Direct Source of Carbon Dioxide for Ordinary Plants

About the middle of the nineteenth century, when the photosynthetie process in chlorophyll-bearing cells was becoming appreciated, the question naturally arose as to whether the CO2 necessary for that process in ordinary plants might be in part derived from the soil as well as from the free air. It was soon shown that the free air is generally the main source of the CO2 of photosynthesis in green plants with aerial exposure, and that conclusion has remained well supported up to the present time ; there is now no reason to question its correctness. But some experiments were reported which indicated that a portion of the CO2 decomposed in green leaves had been derived not from the free air about those organs but from other parts of the plant body and even from the soil. Many recent writers appear to have ignored the results of those experiments, as well as the basic physical principles involved, and it is probably safe to suppose that the free air is now generally regarded as the only source of CO2 for ordinary plants. That only a relatively small portion of the CO2 actually absorbed can be supposed to enter the plant through the roots ought not to lead to the complete neglect of root absorption of CO2, especially when it is recalled that the soil solution has generally a considerable concentration of carbonate ion, that large amounts of water are absorbed through root systems, and that almost all of the absorbed water is regularly given off in transpiration through green tissues. We submit a brief summary of the status of this fundamental question, together with some discussion of the logical possibilities and an account of a few experiments. To conserve space we shall refer to the carbonate ions (HCO^ COf ) in aqueous solution as dissolved CO2. Since gaseous CO2 cannot penetrate to active chloroplasts (which are surrounded by and thoroughly impregnated with water), it is safe to suppose that all CO2 decomposed in green cells must be in watery solution before its reduction may occur. It is not necessary for our present purpose to attempt to distinguish between the hypothetical carbonic acid (H2CO3), the carbonates, and the carbonate ions, nor is it necessary to dwell on the consideration that a very small portion of