Polypodiophyta (pŏlˌēpōˌdēŏfˈətə) [key], division of the plant kingdom consisting of the plants commonly called ferns. The ferns are vascular plants with stems, roots, and leaves. The small and inconspicuous gametophyte and the large spore-producing fern plant are quite independent of each other. The sporophyte plant, which is the plant form popularly recognized as a fern, may have an erect stem of more than 50 ft (16 m) in height, or a prostrate stem lying in or on the ground. The anatomy of fern stems, especially in the arrangement of the vascular bundles, differs greatly from group to group and is used as a means of interpreting the evolutionary relationships of the various groups. Typically, the leaf, or frond, is large and much divided, although many ferns have simple leaves, i.e., leaves with the blade undivided. Fern leaves generally unroll as they develop from a coiled early bud stage called the fiddlehead or crozier. Sporangia, the spore-producing structures, are generally found on the back of the leaf, but occasionally occur on special structures, which are probably evolutionarily modified leaves. In the great majority of ferns, the spore cases, or sporangia, are produced in groups, with each group called a sorus. These sori can often be seen on the back of the leaves. The sporangia in the sorus are usually protected in some manner, sometimes by an umbrellalike structure, the indusium, and sometimes by the inrolling of the leaf edge. The sporangium consists of a jacket of thin cells, partly surrounded at one side by a row of very thick-walled cells, the annulus. When the spores are mature, a springlike mechanism in the annulus serves to tear open the sporangium and eject the spores. Germinating spores produce a green, thin, sometimes heart-shaped, gametophyte, or prothallus, on the lower side of which are produced the sex organs, the sperm-producing antheridia and the egg-producing archegonia. The gametophytes are thin and delicate and thrive only in moist places; the lower side usually has a film of water, which facilitates the swimming of the motile, flagellated sperm from an antheridium to the neck of an archegonium and to the egg within it. The resulting diploid zygote develops slowly into a mature sporophyte with stem, root, and leaves. Ferns were abundant in Carboniferous times, and many are known only from fossil records. Of the 10,000 known species of living ferns, almost all belong to the order Filicales (true ferns). The grape ferns and the adder's-tongue ferns, of the order Ophioglossales, are very few in number. The genus Marsilea, of the order Marsileales, and the genera Salvinia and Azolla, both of the order Salviniales, have complex life histories. Marsilea grows in wet places, and Salvinia and Azolla float on the surfaces of ponds and lakes.