Light micrograph of a section through the stem of the primitive pteridophyte Psilotum sp., showing an open stoma (the gap in the epidermis at top), with its paired guard cells. The plant is one of the last surviving representatives of an ancient lineage, the Psilopsida. Many fossil psilopsids are found in the rocks of the Devonian period (395-345 million years ago), but Psilotum is one of only two genera which have survived to the present day. The plant produces aerial shoots, bearing occasional scale-like leaves, from a rootless rhizome. Magnification: x400 at 35mm size.