Ferns and their allies are in a major division of the Plant Kingdom called Pteridophyta, and they have been around for millions of years. At one time they were a dominant part of the earth's vegetation. Ferns tend to be distributed in wetter parts of the world but it is not uncommon to find ferns in very dry or severely cold conditions. The majority inhabit wet, tropical forests and can be found from sea level to mountains over 15,000 feet in altitude. There are over 250 different genera of ferns and about 12,000 species. Ferns differ from other plants in that they do not produce flowers or seed, instead they reproduce from spores. Spores are one-celled microscopic structures that cannot be seen with the naked eye. These spores are found most often on the underside or edges of the frond, and in some cases are borne on separate stalks.