Ferns are the most advanced extant non-seed plant group. In variety and breadth of distribution they are very successful. Primitive ferns, Lycophyta, and Sphenophyta were conspicuous members of the flora of the Carboniferous world. Many were tree-like. There are about 200 extant genera of ferns and about 12,000 species. Some of these ferns appear very ancient while many are of more recent origin. It may be that the more advanced leaf structure found in this group has been significant in their success.

Like other primitive vascular plants, extant ferns have the same simple life cycle with an independent sporophyte and independent gametophyte generation. They differ from the other primitive vascular plant groups in other characteristics.

(1) Features of extant ferns relating to success on land:

Derived features

• 1. Large leaves with numerous veins.

  Ancestral features

• 1. Water needed for fertilization.
• 2. Two independent generations.
• 3. Gametophyte restricted to wet places.
• 4. Slow growth of gametophyte.

(2) Fern Gametophytes

Examine a fern gametophyte (Rust, fig. 33-34; V & C fig. 6.51) using a dissecting microscope, then mount it in water and examine it using higher power. Compare with a prepared slide. Note its size and shape. How many cells thick does the gametophyte thallus appear to be? Locate the archegonia and antheridia (see figures). How do their structures and locations compare to those of the bryophytes? Does there appear to be significant evolutionary differences between primitive vascular plants and bryophytes in terms of their reproductive structures and way of exchanging gametes? Can you tell how the sperm find the archegonia and their eggs? Where will the embryonic sporophyte develop?

(3) Fern Sporophytes

Examine material showing young sporophytes growing out of gametophytes. Does the initial growth of a sporophyte in ferns differ from that found in mosses? In what ways is it similar? Does the young sporophyte appear to be dependent upon the gametophyte for its initial growth?

An array of fern sporophytes is available for your examination. These demonstrate the rhizome, or underground stem, which sometime forces its way out of greenhouse pots. The rhizome produces young leaves at its tip. These leaves develop as a tightly coiled mass and "unroll" as they mature. This fiddlehead (Rust, fig. 30b; V & C fig. 6.47) shows circinate vernation, which is the unrolling developmental process. Examine the backs of mature leaves (Rust, fig. 31a).

In most ferns sporangia are organized into clusters called sori, (singular = sorus) (Rust, fig. 31 b-d, 32 a-b; V & C fig. 6.44-6.45, 6.48-6.49). Remove a few sporangia and examine them with a microscope. Within the sporangium, a number of cells (spore mother cells) will undergo meiosis and the resulting cells will develop into spores. These spores will have a hard protective outer coat and will be able to withstand drought conditions. How does the sporophyte compare with that of the bryophyte? In what ways does the sporophyte differ with regard to the production of sporangia? Note the presence of an annulus on the sori. How does it function?