An apparatus is described which permits continuous monitoring of the concentration of both the total and radioactive carbon dioxide around an experimental plant. The apparatus consists of a closed circuit of tubing into which are introduced a plant chamber, an infrared CO2 analyzer, a Geiger tube, and a pump. Continuous records are obtained rom which are calculated the amounts of total carbon dioxide and C14 absorbed or evolved by a plant. With an apparatus 2.03 liters in volume, evolution or absorption of amounts of CO2 as small as 0.01 mg were measured. The time necessary to observe a change in the concentration of C14 in the chamber (i.e. the lag time) is less than 2 seconds. The corresponding value for the total carbon dioxide is 7 seconds.Experiments on the respiration and photosynthesis of detached shoots of Pinus resinosa in an atmosphere containing C14O2 are described as an example of the use of the apparatus. The external C14O2 was continuously diluted with C12O2 in both light and dark. However, evolution of carbon dioxide in the light (light respiration) was only about 20% of that in the dark (dark respiration).

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