Photosynthesis replenishes the earth’s atmosphere with oxygen. The two variables that affect the rate of photosynthesis are (1) the production of oxygen (O2) and, (2) consumption of carbon dioxide (CO2). The buoyancy of the leaf is an indirect measurement of the net rate of photosynthesis in the leaf tissue. The more oxygen the leaf contains, the more buoyant or the more it floats in the water.