Divisions Horsetails and Ferns

There are about 10 000 fern species in the world; most are perennial herbs with a rhizome, and tree ferns grow only in the tropics and subtropics. Their long, feather-like cut leaves are rolled up at the tip when young and unroll as they grow. They differ from horsetails in having large leaves and no spore-bearing cone. Spores ripen in sporangia inside brown bumps (sori) under or at the edge of the leaf. A spore that lands on damp soil grows into a green, heart-shaped gametophyte about 1 cm across bearing antheridia and archegonia; fertilization takes place in rain water, and the zygote grows into the new fern, the sporophyte. The maidenhair fern is a perennial rhizome herb with leaves 10–40 cm long, 2–3 times feather-divided, with sori under the leaf; it grows mostly in damp, shady places. The water fern (Salvinia) is an annual rootless plant: each joint has 3 leaves, two floating on the surface and the third thread-like and submerged, bearing the sori.