Abstract: Quantitative estimations of downward oxygen transport from aerial to subterranean parts in intact seedlings were carried out in the present investigation with the respiratory hydrometer specially designed by us for this purpose. The chief object of the investigation is rice, a crop which is notable for its marshy habitat and whose submerged roots are in particular demand of such transport. Some other common plants (wheat, pea, water cress, etc.), either of marshy or of mesophytic habitat, have also been included in the investigation for comparison. Although rice has long been known for its capability of downward oxygen transport, as has often been revealed by various qualitative demonstrations and indirect estimations; yet, data of direct quantitative measurement of the actual amount transported, so far as we are aware, have been very scanty. The few attempts of bringing about such quantitative measurement in an intact plant are made by enclosing its shoot and root in two adjoining compartments respectively, and gas analysis is made on samples taken from each compartment at intervals. The procedure is so elaborate and tedious that estimations on a large scale could not be readily carried out and the results have often been rendered unreliable by mishandling of the plant and air leakage between the compartments. Proposals to the path and mechanism of downward oxygen transport in higher plants have largely been based upon such scanty quantitative approximations and various qualitative observations, and the conclusions derived therefrom are contraversial and far from being convincing. The presentation in this communication of a simple yet accurate experimental method for the quantitative determination of this kind might be opportune and appropriate. The basic principle of the respiratory hydrometer employed in this investigation has been given previously (Lou et al., 1963). Seedlings raised in water culture are inserted into the vessel of the hydrometer (Fig. 1) with its aerial part in the air space above and roots in the water passage below. As the diffusion rate of oxygen in water is about 1/300,000 that in air, the submerged roots of an intact rice seedling practically have their immediate oxygen supply cut off and have to rely upon the oxygen transported from above. Downward oxygen transport in intact seedlings can be easily estimated through the following procedures and the results thus obtained are summarized below: 1. The difference between two consecutive determinations of the oxygen absorbed by the aerial parts of intact seedlings made before and after their roots are severed gives the amount of oxygen transported downwards to roots. For the marshy plant (rice, water cress), it is about 50% (range: 30%–70%) of the total amount absorbed; whereas for ordinary land plants raised in water culture (wheat, pea), it is 20%–30% of the total. 2. The above results are in good agreement with those obtained by determining the respiratory quotients of intact seedlings first in air (e.g.R.Q. ≌ 1 in case of rice seedling) and then with their roots submerged in water (R.Q. ≌ 0.5). The difference between the two consecutive determinations again gives the fraction of oxygen transported downwards. 3. Either by varying the oxygen supply to the aerial part (from 1/4 to twice the oxygen content in air) or by increasing the oxygen consumption of the root through temperature increase or DNP stimulation, the oxygen concentration gradient along the vertical axis of the plant can be steepened or lessened at will. When such experiment is carried out in rice seedlings, the amount of oxygen transported downwards increases with the gradient.