Fern (Ptisana purpurascens)

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Ptisana purpurascens mature plant
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Fern fact file

Fern description

KingdomPlantae
PhylumPolypodiophyta
ClassFilicopsida
OrderMarattiales
FamilyMarattiaceae
GenusPtisana (1)

A large fern, Ptisana purpurascens has broad, sturdy fronds that can reach up to one metre in length (2). The dark green fronds are divided into as many as three branches, each with six pairs of leaflets, known as ‘pinnules’ (2). Ferns produce spores and these develop within structures known as ‘sori’, which line the edges of the pinnules (2).

Synonyms
Marattia purpurascens.
Size
Frond length: up to 1 m (2)
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Fern biology

Ferns are ‘primitive’ plants that spread by releasing spores rather than by producing flowers and fruits. The distinctive frond stage of the fern lifecycle is asexual; spores are released from the fronds, which then germinate into minuscule heart-shaped structures known as prothalli’. It is here that the sexual stage of the lifecycle occurs; male and female organs on the prothallus produce sperm and eggs respectively. If the female eggs are fertilised successfully, a new fern plant will begin to grow and the cycle starts again (4).

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Fern range

Ptisana purpurascens is endemic to Ascension Island in the South Atlantic, where it is now restricted to a single population on Green Mountain (3).

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Fern habitat

Ptisana purpurascens is found growing in open grassland on the exposed side of Green Mountain and amongst bamboo thickets near to the summit (3).

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Fern status

Ptisana purpurascens is classified as Critically Endangered (CR) on the IUCN Red List (1).

IUCN Red List species status – Critically Endangered

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Fern threats

The main threats to this rare fern are habitat loss due to the planting of evergreen trees and degradation due to invasive plants (1). Ptisana purpurascens is also under threat from competition with introduced species and grazing by sheep, although this is not currently a common occurrence (3).

Its highly restricted distribution makes Ptisana purpurascens particularly vulnerable to any catastrophic events. For example, a land slide could do serious damage to its main population in a single narrow ravine (1).

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Fern conservation

Ptisana purpurascens is the largest of Ascension Islands’ native plants and its population is being monitored by Ascension Conservation (3). The entire population of this species in found within Green Mountain National Park, which may offer it some protection (1).

Ptisana purpurascens is currently being grown in local nurseries and is also held at the Royal Botanic Gardens, Kew, in the United Kingdom. Small numbers of cultivated plants have been planted out, and it is hoped that this programme will be expanded in the future (1).

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Authentication

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:
arkive@wildscreen.org.uk

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Glossary

Endemic
A species or taxonomic group that is only found in one particular country or geographic area.
Evergreen
A plant which retains leaves all year round. This is in contrast to deciduous plants, which completely lose their leaves for part of the year.
Prothallus
A small, gamete-producing structure that germinates from certain spores.
Spores
Microscopic particles involved in both dispersal and reproduction. They comprise a single or group of unspecialised cells and do not contain an embryo, as do seeds.
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References

  1. IUCN Red List (August, 2012)
    http://www.iucnredlist.org/
  2. Ashmole, P. and Ashmole, M. (2000) St. Helena and Ascension Island: a Natural History. Anthony Nelson, England.
  3. Gray, A. (2003) Red List Assessment Form. Ascension Conservation.
  4. Australian National Herbarium (September, 2003)
    http://www.anbg.gov.au/
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Image credit

Ptisana purpurascens mature plant  
Ptisana purpurascens mature plant

© Alasdair Cameron

Alasdair Cameron
http://www.themushypea.blogspot.com/

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