Acrostichum
The common names of Acrostichum L. are the leather ferns or leather swamp ferns (“Acrostichum: Leather ferns,” n.d.). The genus Acrostichum has a number of species; Acrostichum aureum (Golden Mangrove Fern), Acrostichum chrysophyllum, Acrostichum conforme (Deer’s-tongue), Acrostichum grande, Acrostichum repandum, Acrostichum speciosum (Mangrove Fern) and Acrostichum velleum (“Acrostichum L.,” n.d.). This genus is native to Western Australia (“Acrostichum aureum L.,” n.d.). This genus gave rise to the word acrostichoid, due to the tightly packed sporangia on the abaxial side of the lamina (Tindale, 2009). The presence of sporangia and the absence of sori differentiate Acrostichum from other ferns (Hill, 2006).
The International Union for Conservation of Nature (IUCN) has declared Acrostichum as a genus of least concern (LC) (Ellison et al., 2010). In Western Australia, Acrostichum is Priority One (P1). This indicates that it is a poorly-known genus (“Acrostichum aureum L.,” n.d.).
A threat to Acrostichum is that it cannot tolerate prolonged immersion in water (e.g. flooding) (Tan, 2001). While there are no major threats to Acrostichum, it is instead considered a threat to other mangroves, as it can become a major problem in mangrove systems that have been disturbed. Acrostichum can be a threat to other mangrove areas because once it engulfs an area it is difficult to eliminate (“Acrostichum speciosum,” n.d.).
Acrostichum is a fern that grows on wet ground, usually on mud-covered or salty surfaces. The roots are strong and can run along the ground or be vertical, with broad scales and thick, strong prop roots. The fronds can be many metres high with the stem being up to one-third the length of the frond, which is covered in scales at the base. The lamina is ridged, leathery and hairless, and has three or more leaflets positioned along the stalk. The stalk has many slender oblong to oval-shaped pinnae (divisions of the frond) that rise clearly off the stalk. The upper pinnae are small and fertile, whereas the lower pinnae are infertile. The length of the lowest pinna is occasionally smaller than that of the upper pinnae. All of the boundaries on the pinnae are complete. There are no secondary veins and the tertiary veins form a tight network of sloping, elongated hexagonal networks (apart from the veins on the edge) without veinlets. Sporangia cover the lower surface of the fertile pinnae or are located at the tip. The sporangia have many sterile, multicellular filaments that are threadlike, with darkened cells at the end. The spores are comprised of four or more individual reproductive cells (Kramer & McCarthy, 1998).
Plants in the Acrostichum genus grow in terrestrial and marine habitats where the soil is nearly water logged. The plants can grow in full sun or partial shade and require large amounts of water especially in sunnier conditions (VanZile, 2014). Although the plants thrive in rich, organic soil, they also have a high saltwater tolerance (Tan, 2001). These plants can be cultivated and do not require large amounts of fertiliser (VanZile, 2014).
The evolution of ferns and fern species involves natural hybridisation (Zhang et al., 2013). The current view that horsetails and ferns are an intermediate evolutionary step between bryophytes and seed plants has been refuted by the study by Pryer et al (2001). Pryer et al (2001) has stated that horsetails and ferns (as a monophyletic group), are the closest living relatives to seed plants. This study highlights the controversy around the identification of class and phylogeny of the leptosporangiate ferns (including Acrostichum). Ferns have a long history back to the Devonian period (Pryer et al., 2001). Since that time, fern evolution has either been convergent or parallel and multiple extinctions of species has increased the difficulty in finding the relationships between groups (Hasebe et al., 1994).
Kingdom: Plantae
- This kingdom is made up of photosynthetic organisms that have adapted to terrestrial life. A common characteristic of this kingdom is the existence of an embryo during the spore-producing phase of the life cycle (Raven, Evert, & Eichhorn, 2013).
Phylum: Tracheophyta
- Tracheopytes are vascular plants with roots, stems and leaves. Tracheophytes diverged from bryophytes with the development of vascular tissue (xylem and phloem) (“Tracheophytes,” n.d.).
Class: Polypodiopsida
- Polpodiopsida are also known as the leptosporangiate ferns due to the sporangia forming from a single cell (“Class polypodiopsida [inactive],” n.d.).
Order: Polypodiales
- This order contains most modern ferns (or true ferns) and are different to other orders of ferns due to their small sporangia having a defined number of spores (Vestal, n.d.).
Family: Pteridaceae
- This large, ecologically diverse family is identified by the location of sporangia along the veins or at the tips of the vein (Yatskievych, 2013).
Genus: Acrostichum
- This genus is characterised by clusters of spore-producing receptacles (sporangia) located on the underside of the blade(Editors of Encylopaedia Britannica, n.d.).
(Hogan, n.d.)
In Western Australia, Acrostichum has been recorded in the Kimberleys region, concentrated in coastal areas (Figure 1) (Department of Fisheries, 2012). In other parts of Australia, Acrostichum can be found in the coastal areas of the Northern Territory, Queensland and New South Wales (Figure 2) (“Acrostichum L.,” n.d.). The species of this genus are widespread and can be found in many tropical regions of the world such as the Caribbean, southern Asia and East and West Africa (Ellison et al., 2010).
Figure 1: This image shows the distribution of Acrostichum in Western Australia. This image was retrieved from https://florabase.dpaw.wa.gov.au/browse/map/20889
Figure 2: This image shows the distribution of Acrostichum in Australia. This image was retrieved from http://bie.ala.org.au/species/Acrostichum
Acrostichum grows in tropical climates with heavy rain (Hill, 2006). It thrives in coastal conditions but will not grow in arid coastlines (Hill, 2006). Acrostichum grows in semi-aquatic areas such as fresh or saltwater swamps, salt marshes and brackish water (Tan, 2001). Acrostichum is halophytic (tolerates salty water), however the salinity in the soil cannot be more than 50ppt (parts per thousand) otherwise it will not grow (Hill, 2006). It may tolerate salty water, but it requires fresh water to establish itself in the environment (Hill, 2006). Acrostichum is shade tolerant and grows in almost all light conditions that range from full sun to dense shade (Hill, 2006). To grow, this genus requires wet roots (Smithsonian Tropical Research Institute, n.d.) with protection from frequent tidal influxes and prefers elevated positions (Hill, 2006).
Acrostichum has been used in human medicine and cuisine for many years. For example, in Malaysia, Acrostichum aureum is used to heal wounds, boils and ulcers by grounding up the rhizomes and applying to the affected area. Acrostichum fronds are used in India to treat victims of venomous snakebites. Acrostichum aureum is used in Fiji to remedy sore throats, elephantiasis, constipation and chest pains (“Acrostichum aureum,” 2011).
In India and Sri Lanka, the young shoots of Acrostichum are edible when cooked or raw. In Vietnam and in the Pacific, the older leaves are dried and can be used as thatching on roofs. The leaves can also be used to stem the flow of blood from an open wound (Tan, 2001).
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