Ferns

A fern plant generally consists of one or more fronds attached to a rhizome. A frond is simply the leaf of the fern. A rhizome is a specialized, root-like stem. In most temperate-zone species of ferns, the rhizome is subterranean and has true roots attached to it. Fronds are generally connected to the rhizome by a stalk, known technically as the stipe. The structures of the frond, rhizome, and stipe are important characteristics for species identification.

The sizes of ferns and their fronds vary considerably among the different species. Tree ferns of the Cyatheaceae family are the largest ferns. They are tropical plants which can grow 60 ft (18 m) or more in height and have fronds 15 ft (5 m) or more in length. In contrast, species in the genus Azolla, a group of free-floating aquatic ferns, have very simple fronds which are less than 0.2 in (0.5 cm) in diameter.

The fern frond develops from a leaf bud referred to as a crozier. The crozier is coiled up in most species, with the frond apex at the middle of the coil. This pattern of coiled leaf arrangement in a bud is called circinate vernation. Circinate vernation is found in a few other seed plants, but not in any other free-sporing plants. During growth of a bud with circinate vernation, the cells on one side of the coil grow more rapidly than those on the other, so the frond slowly uncoils as it develops into a full-grown leaf.

The horsetails (phylum Sphenophyta) and club mosses (phylum Lycodophyta) are known colloquially as fern allies. The fern allies also reproduce sexually by making spores and have stems with vascular systems. However, there are two principal differences between ferns and fern allies. First, unlike the ferns, the leaves of fern allies, known technically as microphylls, are small, scale-like structures with a single mid-vein. Second, fern allies make their spores at the bases of their leaves or on specialized branches. There are about 1,500 species of fern allies in the world.

The reproductive cells of ferns are microscopic spores which are often clustered together in the brown spots visible on the fronds' undersides. Since fern spores are microscopic, fern reproduction was not well understood until the mid-1800s. This led some people to attribute mystical powers to the ferns. According to folklore, ferns made invisible seeds and a person who held these would also become invisible. Even Shakespeare drew upon this folklore and wrote in Henry IV; "we have the receipt of fern seed; we walk invisible." Nowadays, anyone with a simple microscope can tease apart the brown spots on the underside of a fern frond and see the tiny spores.