The ferns constitute an ancient division of vascular plants, some of them dating back 360 million years ago to the Carboniferous Period. There are an estimated 12,000 fern species, approximately 75 of which are members of the Polypodium genus. In addition to acting as common ornamental plants, humans utilize many of the Polypodium ferns for medicinal purposes. For instance, the common polypody, P. vulgare, which is found on shady rocks and in mountainous areas of the United States, may help eliminate parasites, act as a purgative, and aid in treatments for pulmonary and hepatic diseases. The rhizome of the plant is considered to be the pharmaceutically active part and has a long history of use.