Ferns grow in damp, dark places. They are known for their uniquely shaped leaves, called fronds. In the mid to late Paleozoic, they covered forests and swaps. The leaves usually have delicately cut toothed or lobed edges. Most ferns grow from a creeping underground stem called a rootstock. Ferns, which grow from spores, come out in early spring. When they first appear, their leaves are tightly curled. Slowly, they uncurl, taking on a variety of shapes as they develop. About 10,000 species of ferns live around the world.