Submersed aquatic plants in streams, lakes, ponds, bays, and estuaries do more than take up nutrients and provide habitat for fish and other organisms, they produce oxygen during photosynthesis. Here we see tapegrass (Vallisneria) in bright sunlight releasing a visible string of oxygen bubbles, an emission known as “pearling”. British chemist, theologian, and philosopher Joseph Priestly (1733-1804), who spent his final decade residing along the Susquehanna in Northumberland, Pennsylvania, isolated oxygen during experiments in 1774 by exposing mercuric oxide to direct sunlight. During the following year, Priestly published his findings in “An Account of Further Discoveries in Air”, describing what he called “dephlogisticated air”, the gas later named oxygen. To observe and record the effects of pure oxygen in the absence of atmospheric air, Priestly first tested it on a mouse, then breathed it himself.
