Overview: When flowering plants evolved, they displaced most of the world's more primitive spore producing plants. The most varied and successful of the survivors are the ferns. The fossil record shows that some seed producing ferns arose but all became extinct for reasons unknown. In tropical areas, many ferns may grow into trees 40 feet or more in height.
A closer look: Ferns are members of an archaic plant group that dominated the landscape about 360 millions year ago during the Carboniferous period. Ferns are one of the three vascular plant groups---the other two being lycophytes, and horsetails---that lack seeds and flowers but rather reproduce by spores. Strangely enough, in the three groups of seedless vascular plants, the sperm is flagellated and swims to fertilize the egg. Ferns are the most evoluntionarily successful seedless vascular plant group in terms of abundance---more than 10,000 species worldwide.
Ferns are predominately found in shady moist areas but some species are adapted to dry rock crevices when sheltered from the full sun. They can be aquatic, terrestrial herbs, large trees, and ephiphytes (growing on trees.) Aquatic species include free floating plants such as mosquito fern or rooted species whose leaves float at the surface of the water like clover fern. Tree species of ferns still living can acquire a height of 40 feet, although the fossil record shows that some now extinct tree species grew even taller.