Fossil plants that somewhat resemble whisk ferns have been found in Silurian geological formations (Table 21.1). These formations are estimated to be as much as 400 million years old. One group of these fossil plants, of which Cooksonia and Rhynia are examples (see Fig. 21.22), had naked stems
and terminal sporangia. Cooksonia is the oldest plant known to have had xylem. A second group of fossils, represented by Zosterophyllum, had somewhat rounded sporangia produced along the upper parts of naked stems. Zosterophyllum and its relatives first appeared during the Devonian period (Table 21.1). They are thought to be ancestral to the club mosses, discussed in the next section, "Phylum Lycophyta—The Ground Pines, Spike Mosses, and Quillworts."
Whisk ferns are of little economic importance. Their spores have a slightly oily feel and were once used by Hawaiian men to reduce loincloth irritations of the skin. Hawaiians also made a laxative liquid by boiling whisk ferns in water.
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