- 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.
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.
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
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.
2 November 2014