The effect of carbon dioxide on the absorption activities of roots of wheat, maize and rice plants was studied. The following results were obtained: (1) Bubbling carbon dioxide through the solutions reduced significantly the absorption of water, and of all nutrient elements tested, by all three experimental plants. (2) Bubbling air through the solution, in general, increased the absorption of water and nutrient elements over the control plants, but the increment was much less prominent and consistent than the reduction moused by the carbon dioxide. (3) In a single experiment bubbling commercial nitrogen gas through the solutions did not affect the absorption of either water or nutrient elements by wheat plants. (4) Covering the solution with a thin layer of paraffin oil caused no great reduction in the absorption of water, Ca, Mg and P when compared to the control plants, but the depressing effect on K and possibly on N absorption was significant. (5) The reduction in water absorption caused by carbon dioxide was consistent and striking for all three plants, averaging 32 percent. (6) Among the cations, K absorption was most affected by carbon dioxide, loss of K to the solution being obtained in many jars. The absorption of Ca and Mg was reduced by 47 and 36 percent respectively. (7) The absorption of the anions NO3 and PO 4 also was sharply reduced by carbon dioxide, average absorption being, 27 and 44 percent of the control. (8) Adjusting the acidity of solutions to pH 4.0 failed to produce a reduction in water absorption by rice plants. The absorption of total salt, including NO3, was reduced significantly by lowering the pH, but the absorption was still significantly higher than in the plants treated with carbon dioxide. (9) The results of these experiments support the conclusion that carbon dioxide has a specific effect upon root protoplasm which decreases its permeability to water and minerals. Reductions in the absorption of water, calcium and magnesium can be explained by such decreased permeability alone. The greater decreases in nitrate and phosphate absorption and negative absorption of potassium suggest changes in the metabolism and possibly in the structure of the protoplast. (Abstract shortened by UMI.)