The acronym for scanning electron microscope. The SEM uses electrons to image specimens. It provides images of the surface of the silica cell wall, or frustule, while the light microscope (LM) provides images of the transparent features of the diatom.

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Image Credit: S. Spaulding
The specimen chamber of a scanning electron microscope (SEM). Here, the Quanta FE450 at the U.S. Geological Survey in Denver, CO.