Any of numerous seedless vascular plants belonging to the phylum Pterophyta that reproduce by means of spores and usually have feathery fronds divided into many leaflets. Most species of ferns are homosporous (producing only one kind of spore). The haploid spore grows into a small, usually flat gametophyte known as a prothallus , which is undifferentiated into roots, stems, and leaves. The green prothallus anchors itself with hairlike extensions known as rhizoids and bears both archegonia (organs producing female gametes) and antheridia (organs producing male gametes). The male gametes require the presence of water to swim to the female gametes and fertilize the eggs. Normally only one embryo is produced, and it then grows out of the gametophyte plant as a diploid sporophyte plant that has roots, stems, and leaves and conducts photosynthesis, while the smaller gametophyte withers away. The leaves of these sporophytes eventually produce sporangia (in some species occurring in clusters known as sori ). Under dry conditions, the sori burst releasing hundreds of thousands or millions of spores. Ferns were abundant in the Carboniferous period and exist today in about 11,000 species, about three-quarters of which live in tropical climates.