Class:
Filicopsida
Order:
Filicales
These are the true ferns. They are also known as the "Leptosporangiate ferns", since all members of this class produce the delicate leptosporangia. The number of genera and species of leptosporangiate ferns is far greater than the number of eusporangiate ferns. Some 300 genera and 9000 species have been described. These are most abundant and diverse in the tropics, but they are also well represented in the temperate zone. The Order Filicales contains numerous genera of homosporous genera. In this family the sporophyte axis is usually long and rhizomatous, the fronds are very diverse from simple to pinnately compound. The sporangia are small, thin walled and scattered or grouped into "sori" which are marginal or abaxial on the fronds. The vernation is circinate.
The gametophytes (often called
prothalli), are usually found on moist soil or in rock crevices in the
vicinity of the parent fern colonies. The
prothallus usually has a flattened foliar appearance and is often
heart-shaped. It is only one cell
thick at its margins and only several cells thick near its central region.
The underside is covered with numerous rhizoids.
The prothalli are usually monoecious, but usually somewhat "protandrous",
(i.e. male gametes are produced earlier than those of the females in
monoecious species). The
antheridia are protuberant and the necks of the archegonia project from the
thallus surface. The sperm cells
are multiflagellate. It has been
demonstrated that the movement of the sperms towards the archegonia is not
fortuitous, but the result of a chemotactic response.
The moist habitat of the gametophytes, along with the ventral position
of the sex organs, enhance the opportunities for fertilization.
Further growth of the gametophyte ceases after its archegonia have been
fertilized.
Genus: Polypodium
We
will use the genus Polypodium as an
example of a "typical" filicalian fern.
From the following slides construct the life history of a typical fern. Compare the slides to the live and preserved material.
Gametophyte:
Fern prothallus, whole plants showing antheridia and archegonia
Fern prothallus (slide #57) x.s. (antheridia)
Fern prothallus (slide #58) x.s. (archegonia)
Fern prothallus w.m. (fertilization)
Fern prothallus w.m. (sporophyte)
Sporophyte:
Rhizome x.s. (slide #63)
Rachis x.s. (slide #64)
Leaflet x.s. with sporangia (slide #62)
Sporangia (whole mount)
Examine the prepared slides of sporangia of Adiantum, Asplenium, Cyrtomium and Osmundia. Compare the
structure with those of Polypodium.
Asplenium sp.True Indusium
Cyrtomium sp. True Indusium
Osmundia sp.
There is a good collection of Filicales in the
herbarium. Examine these
specimens and note, the structure of the fronds and rhizomes, position of sori,
and presence or absence of indusia. (PLEASE
HANDLE HERBARIUM SPECIMENS CAREFULLY).
Make slides (wet mounts) of the sori from the
live specimens provided.
Examine the sporangia, noting the stalks, annulus, lip cells, thickness
of cell walls, presence or absence of indusia and the spore shape.