The most common ferns are tropical, living in wet, shady spots, but the wide range of fern species reaches into just about every habitat. There are aquatic ferns, and there are ferns that grow on the bark of trees. There are ferns that grow in the wettest rainforests, and ferns that cling to rocks in harsh climates. Even with all their variation, ferns remain unlike most other plants, and a primary aspect of their uniqueness is their leaves.
Fern leaves are usually called fronds, and although some ferns have fronds that consist of a single blade, the fronds are usually divided into smaller leaflets called pinnae. Sometimes those pinnae are divided themselves into even smaller pinnae, and in some ferns, the frond is divided into still smaller leaflets. The fronds of giant tree ferns can be several meters in length, while the fronds of a mosquito fern are only a few millimetres long.
"Circinate vernation" is the term used to describe the way that new fern fronds emerge, and although it sounds complicated, it simply means to turn green in a circular manner. As the new frond emerges, its tip is curled in a tight spiral to protect the tender, youngest part of the new growth. The coiled end of the frond is called a crosier, because of its resemblance to a shepherd's crook, or a fiddlehead, in reference to the spiral scrollwork at the head of a violin.
Ferns don't produce flowers to reproduce the way most plants do, but rather reproduce via spores, single cells that are able to grow into a new plant if they land somewhere moist. Spores are produced in tiny sacs called sporangia, which are grouped together, usually on the underside of the fronds, in clusters called sori. The ferns expel these spores so they can find a place to grow. A small fern frond can produce 750,000 spores, and a tree fern frond can shed 750 million spores.
Fern fronds are extremely diverse. Of the nearly 12,000 species of fern, most can be differentiated by the unique form of their leaves. They range greatly in size, from massive tree ferns to species whose leaves are only one cell thick, and sometimes they have a function beyond simple photosynthesis. Some ferns produce specialised fronds that carry the spores rather than having sori on all their leaves; these spore-bearing fronds are as close as ferns get to having flowers.