smooth scouring rush, Equisetum laevigatum, California, White Mountains, Cottonwood Basin, Poison Creek, Fishlake Valley drainage, elevation 3016 m (9895 ft).

The 15 or so living species of Equisetum, the scouring rushes and horsetails, are the sole remaining members of a whole class of plants (Equisetopsida) that first appears in the fossil record during the late Paleozoic Era more than 300 million years ago, and came to dominate forest understories during the age of the dinosaurs. In contrast to the great age of its lineage, Equistetum laevigatum is thought to be one of the most recently diverged species in the family, having originated sometime during the last 2.5 million years of the Quaternary period.

As is true of many spore-dispersed plants, smooth scouring rush is a widespread species, covering the entire western and north-central United States and adjacent southern Canada and northern Mexico, but apparently absent from New England, the Atlantic seaboard, and the southeastern U.S. It reaches as high as 3500 meters (11500 feet) elevation, and appears to share common ancestors with certain central and south American species.