Ferns

  • Over 10,000 species
  • Most are found in the tropics where tree ferns — with their above-ground stems — may grow as high as 40 feet.
  • In temperate regions, the stems of ferns — called rhizomes — grow underground.
  • The leaves — called fronds — grow up from the rhizome each spring.

Alternation of Generations

The Sporophyte Generation

The plant we recognize as a fern is the diploid sporophyte generation.

(Link to a general discussion of the alternation
of generations in all plants.
)

Sori form on the fronds. Each contains many sporangia mounted on stalks.

Within each sporangium, the spore mother cells undergo meiosis producing four haploid spores each.

When the humidity drops,

  • The thin-walled lip cells of each sporangium separate.
  • The annulus slowly straightens out.
  • Then the annulus snaps forward expelling the spores.


The photo shows the sori on the underside of the leaflets of Polystichum acrostichoides, the Christmas fern.

The Gametophyte Generation

If a spore is blown to a suitable moist location,

  • It germinates into a filament of cells.
  • This grows into a prothallus with
    • rhizoids, which absorb water and minerals from the soil;
    • archegonia, which produce a single egg (by mitosis) or
    • antheridia, which form swimming sperm (again, by mitosis) or
    • both.

Fertilization

If moisture is plentiful, the sperm swim to archegonia — usually on another prothallus because the two kinds of sex organs generally do not mature at the same time on a single prothallus.

Another method for promoting cross-fertilization: The first spores to germinate develop into prothallia with archegonia. These prothallia secrete a gibberellin into their surroundings. This is absorbed by younger prothallia and causes them to produce antheridia exclusively.

Fertilization restores the diploid number and begins a new sporophyte generation.

The embryo sporophyte develops a foot that penetrates the tissue of the prothallus and enables the sporophyte to secure nourishment until it becomes self-sufficient.

Although it is tiny, the haploid fern prothallus is a fully-independent, autotrophic plant.

Welcome&Next Search

2 November 2014