Growing cells of the hypocotyl of Helianthus seedlings were used, employing conditions identical with those of previous experiments cited above. The uppermost thirty-five millimeters of hypocotyl were cut in seven segments of uniform length (5 mm.). The region in which the cells were embryonic was not more than one millimeter long. At a greater distance than two millimeters there were no embryonic cells but all were in the stage of elongation. In the seventh or basal zone all cells were mature. The respiration of each of the whole five-millimeter sections was determined. In a series of ten supplemental experiments, the respiration of the uppermost two-millimeter section was measured in order to determine possible differences in the respiratory rate of embryonic and elongating cells. In some series of experiments the sections were taken from the same hypocotyl. In such cases the hypocotyl was either left on the rootstock or thirty-five millimeters of it were cut away at once and left lying in a moist chamber. When it was left on the rootstock the zones were cut away successively beginning at the tip. Otherwise the zones were cut away successively beginning below. The respiration of each zone was determined in direct succession. In other series of experiments only one section was taken from any one hypocotyl. By this combination of procedures an effort was made to compensate for possible errors that might result from the injury done to the hypocotyl by sectioning. A modification of the differential micro-respirometer previously described was used (8). The modification consisted in sealing a graduated capillary to the "experimental chamber." The pressure was kept constant by raising a mercury column into the graduated capillary which, by special arrangement, could be levelled with ease. The respirometer has these distinct advantages: it occupies little space; its temperature does not readily change in the constant temperature water bath ; it has few separable parts ; it is relatively easy to manipulate; and it is highly sensitive. Not more than two minutes elapsed from excision and first reading to verify equilibrium. A period of fifteen minutes was allowed for the establishment of equilibrium, after which records were made.