these plants, carbonic acid gas is broken up, the carbon becomes fixed in the form of carbohydrates, and oxygen is evolved which rises as a stream of bubbles from the plant. The rate of evolution of oxygen thus measures the rate of assimilation.
Numerous difficulties were encountered in making this method practical; they have been completely removed 
Fig. 103. The Plant-Vessel and the Bubbler.
S, stop-cock; B, Bubbler; O, mercury valve.by the Automatic Recorder. A piece of water plant, e.g., Hydrilla verticillata, is placed in a bottle completely filled with tank-water containing sufficient CO2 in solution, the open end of which is closed by a
special Bubbler apparatus, for measuring the oxygen evolved. The Bubbler consists of a U-tube, the further end of which is closed by a drop of mercury acting as a valve. The oxygen evolved by the plant, entering the U-tube, produces an increasing pressure, which eventually lifts the mercury valve and allows the escape of a bubble of the gas. The valve then immediately closes until it is lifted once more for escape of another equal volume of gas (fig. 103). The movement of the mercury completes an electric circuit, which either rings
