Pteridium aquilinum (PROSEA)
Pteridium aquilinum (L.) Kuhn
- Protologue: Cryptogamae vasculares. In: C.C. von der Decken, Reisen in Ost-Afrika 3(3): 11 (1879).
- Family: Dennstaedtiaceae
- Chromosome number: 2n= 52 (diploid), 104 (tetraploid, most common), 208 (octoploid)
Pteris aquilina L. (1753), Pteris esculenta Forst. (1786), Pteridium esculentum (Forst.) Nakai (1825).
- Bracken (fern), pasture brake, eagle fern (En). Fougère aigle (Fr)
- Indonesia: andam dangdeur, paku geulis (Sundanese), pakis gemblung (Javanese)
- Malaysia: pakis gila
- Philippines: anamam (Tukukan, Bontoc), sigpang (southern)
- Thailand: kut kia, kut kin (northern), chon yai (peninsular)
- Vietnam: quyết.
Origin and geographic distribution
The origin of P. aquilinum is thought to be in the tropics. Now it is found worldwide in all temperate and tropical regions and is one of the most widely distributed vascular plants. It is absent in hot or cold desert-like regions, but on farmed land in temperate regions (e.g. United Kingdom) it has become a troublesome weed.
P. aquilinum is the fern with the most diverse uses, although its positive economic importance lies one or two centuries back (at least in Europe), now being (again in Europe, particularly in United Kingdom) an economic nuisance as a persistent weed in pastures. In South-East Asia it can be a weed in tea and other plantation crops. Due to this weed aspect, bracken is also one of the best investigated ferns of the world. Nevertheless, its various uses are worth mentioning, because in most parts of the world (including South-East Asia) it is not often a common troublesome weed. The young tender leaves and rhizomes are used as food and feed. The rhizomes were favoured by the New Zealand Maoris, who soaked and pounded them to obtain a starchy material, resembling that of arrowroot. In other countries, the raw or dried and ground rhizomes are eaten after roasting or baked into a poor-quality bread in times of food scarcity. The young, still curled leaves ("croziers" or "fiddleheads") are eaten as a vegetable in many countries. They are regarded as a delicacy in Japan where canned croziers are sold ("warabi" or "zenmai"). Leaves and rhizomes mixed with grass are eaten by livestock. Silage is also possible. However, cases of bracken fern poisoning of livestock and human stomach cancer make the food value of rhizomes and leaves questionable.
The ash, having a high potash content, was formerly used extensively in Europe for glass and soap production. Bracken ash was particularly useful for making clear colourless glass. For soap, the potash is combined with oils and fats. Wettened bracken ash was sold in balls to wash clothes and to bleach linen.
Leaves are used as fuel, for thatching, as litter and as compost. The thatch was praised because it is naturally dry and not apt to ferment like straw, whereas the use as litter for livestock has perhaps been the most important application of bracken. The litter provides a warm bedding with high absorbency and decomposes more slowly than straw and may be used as compost. In Java, leaves are placed over nurseries of Cinchona and other plants for protection against the sun. This shelter decays slowly with the increasing growth of Cinchona and gradually disappears.
Bracken has also numerous medicinal uses. In Europe the dried and powdered rhizome or a decoction of the fresh rhizome is taken as an anthelmintic, has diuretic and sedative activity and activates blood circulation. A decoction of the leaves is taken against whooping cough, tuberculosis, bronchitis and costal pain. Leaf juice is given to women in labour to promote uterus contractions and the entire or powdered leaf is used as a dressing to cure wounds. Decoctions of rhizome and leaves are also used against chronic disorders arising from obstruction of the intestines and against melancholy. A bed of green leaves was believed to be a sovereign cure for rickets in children. In China, a tincture made from a decoction of the rhizome and petiole basein wine is drunk against rheumatism. In Indo-China and Thailand, roasted and powdered rhizomes mixed with sesame oil or Selaginella , are used to treat snake bite. In Papua New Guinea, petiole juice is applied against toothache and mouth infections.
In Europe, P. aquilinum rhizomes have been used as a substitute for hops in beer brewing (one third rhizome, two thirds malt) and in leather tanning. Leaves were used to protect plants against frost and to pack and store fruits. Petioles have been applied as fibre to make cord and paper. In Kalimantan (Indonesia) fiddle-strings have been made from the petioles.
As a dye bracken has been variously used: an olive-green dye was made from leaf tops mordanted with alum (double sulphate of aluminium and potassium); wool is coloured light green in Ireland by cooking young bracken tops for 20 minutes and after adding the wool, simmering for 40 minutes; in France, silk was dyed grey-green using bracken croziers; in the United States weavers and Indians obtained a brown or black dye with the rhizome and a yellow-green or lime-green dye with the croziers.
Hairs on the rhizome and crozier have been used for stuffing pillows, and a decoction of the leaves was an effective insecticide in roses. In China, water in which bracken has been soaked is applied as a vegetable pesticide. In the Philippines, leaves are used as body covering by women working in pond fields during rain; leaves are also used by women as a carrying-pad on the head. Smoke of burning bracken is an effective insect repellent.
Bracken has many superstitious uses as well. In the Philippines bracken leaves are worn in the hair or attached to a load believing they protect against lightning. In Europe, the bracken spores were believed to enable people to become invisible and burning of bracken was believed to attract rain.
Production and international trade
Statistics are scarce on production and trade of P. aquilinum . In 1970 over 300 t bracken croziers were consumed in Tokyo alone and in the whole of Japan annual consumption may have reached several thousand t. For other uses the importance of bracken is mainly local and often only historical. At present all attention and research in the United Kingdom is focused on the eradication and control of bracken.
Starch from the rhizomes of P. aquilinum for consumption requires careful preparation and adequate washing to remove its bitterness. The nutritional value of rhizomes is poor since they contain much tannin, mucilage and indigestible fibre. Dry rhizomes contain about 40-70% soluble carbohydrates and 2-20% albuminoids, but amounts vary strongly with the season, being highest in the (winter) rest stage. The curled young leaves are said to have a peppery taste and to be slimy and stringy, only the hairs are sometimes problematic. In Japan it is a popular food but it is suspected that it contributes to a high incidence of stomach cancer. Particularly older leaves, reaching the spore stage, are thought to be poisonous and are also the cause of much poisoning of livestock. In general, all parts of bracken (including the spores) may contain a wide range of secondary plant substances as well as flavonoids which are important for the production of tannins. The secondary substances include sesquiterpenoids (ptaquiloside and related substances, in general called pterosins, having insecticidal and carcinogenic activity), ecdysones (terpenoid compound, identical to the moulting hormone of insects), cyanogenic glycosides (same function as alkaloids), tannins (broad spectrum antibiotic by inhibiting a large number of enzymes) and phenolic acids (acting as fungicide, insecticide and playing a role in protection against other plants by allelopathy). All these constituents may have some activity as a feeding deterrent, affecting predators such as man, animal and other plants.Shikimic acid has also been found in all parts of bracken, especially in the rhizome. It has carcinogenic activity and may explain the occurrence of a fatal haemorrhagic syndrome in cattle and stomach cancer in man after eating bracken foliage. In cattle also thiaminase I, present in bracken, is responsible for neurotic syndromes because it destroys thiamine. Flavonol glycosides isolated from P. aquilinum include: rhamnetin 3-O-β-laminaribioside, isorhamnetin 3-O-β-laminaribioside, quercetin 3-O-fructoside, quercetin 3-O-β-laminaribioside and isoquercitrin, kaempferol 3-O-β-(6-caffeoylglucoside), kaempferol 7-O-rhamnoside-4-O-glucoside and the acylated flavonol glycoside kaempferol 3-O-(5-feruloylapioside). The occurrence and concentrations of the various chemical constituents vary with the subspecies and varieties. Air-dried bracken contains about 5.9% ash, of which 43% K2O, 5% Na2O, 6% SiO2and 14% CaO.
Terrestrial fern, with up to 2.5 m tall, finely divided leaves. Rhizome long, creeping deep in the soil, repeatedly branched, covered with fine, pale brown hairs, vascular system of two concentric, complex dissected solenosteles. Leaves alternate, appearing on short rhizome branches, never very close together; petiole thick, up to more than 1 m long, pale, in cross-section showing a horse-shoe pattern of vascular bundles; lamina large, in outline ovate-triangular, up to 2 m × 1 m, 2-4-pinnate and pinnatifid; all axes grooved on upper surface and often hairy; basal pinnae usually subopposite, the larger with a dark callose spot on the rachis near the base which in developing leaves serves as a nectary; basal pair of pinnae up to 70 cm × 40 cm, upper pinnae and pinnules gradually reduced and confluent; ultimate divisions pinnately compound or lobed, often with a long, entire, apical portion; segments oblong, obtuse, adnate, often with winged expansions at base, often interspersed with smaller and short lobes; leaf tissue firm, hairy to glabrescent; veins close, free, forked, raised beneath, hairy; margin entire, always revolute. Sori submarginal, linear, mostly continuous on marginal vein connecting the lateral vein ends; sporangia borne between the outer indusium consisting of the reflexed segment margin and the thin inner indusium attached just below the receptacles. Spores trilete, tetrahedral-globose, 23-35 mm in diameter, irregularly granulate.
Growth and development
Spores of P. aquilinum germinate in almost any light intensity, nearly always in newly exposed open virgin habitats such as fire damaged sites or after forest logging, but never in closed vegetation including its own canopy. The role of spores seems to be invasion rather than maintaining an existing population since maintenance is by rhizome growth. Once prothalli are established young sporophytes may appear within 2 months and growth is rapid. Within one year a clump of 0.5 m in diameter can be formed with numerous leaves and extended rhizomes. The development of a leaf is characteristic of bracken: the pinnae unroll completely one by one, starting at the base of the lamina, and in this way the apex of leaves grows for a considerable period of time and a developing lamina may show segments treacherously resembling those of other fern species. At the base of the lower pinnae nectaries are present and exude droplets of petiole sap, attracting ants; they are only active in leaves that are developing and their function is unknown. P. aquilinum shows a gradual decrease in fertility with an increasing degree of shade, although vegetative growth is not visibly hampered by heavy shade. Other factors that affect spore formation are not well known but might include ecological stress, age and maturity of a clone. When fertile, however, spore production can be enormous, up to 1 g spores per leaf (about 300 million spores). When mature, spores are mechanically ejected 1-2 cm in the air during dry weather and further carried and dispersed by wind currents and often deposited with the first rain thereafter. P. aquilinum is known as a colonizer, e.g. of areas of cooled lava after volcano eruptions such as Krakatau in Indonesia. Here 11 fern species, including P. aquilinum , had become firmly established within 3 years after the eruption, later largely displaced by other plants which caused too much shade and competition for P. aquilinum .
Other botanical information
A cross-section of the rhizome of P. aquilinum shows, with some imagination, 2 spread eagles, hence "aquilina", the Latin name for eagle, given by Linnaeus to this fern. Because of its wide distribution, the history of P. aquilinum is long and shows great variability, resulting in numerous names and subdivisions. Only Tryon made a worldwide taxonomic revision and survey of Pteridium , whose opinion will be followed here. Tryon accepts only one species, P. aquilinum , which he subdivides into 2 subspecies:
- subsp. aquilinum . Ultimate segments adnate or equally decurrent and surcurrent, without a farinaceous appearance below, pubescence of the lower surface absent or woolly, mainly in the northern (temperate) hemisphere and in Africa.
- subsp. caudatum (L.) Bonap. Ultimate segments mostly decurrent, underside with farinaceous appearance with straight, appressed or arachnoid pubescence, mainly in the southern hemisphere, but not in Africa.
In subsp. aquilinum 8 varieties and 4 in subsp. caudatum have been distinguished, mainly based on characters of the rhizome hairs, hairiness of the rachis and leaf underside, hair density, angle of pinnules to their mid-vein, dissection of the leaf lamina, shape of the ultimate segments, their division and mode of attachment, the relative sizes of the indusia and the sequence of the unrolling of the leaf lamina. In South-East Asia only 2 varieties occur:
- subsp. aquilinum : var. wightianum (Ag.) Tryon (synonyms: Pteris revoluta Blume, Pteris recurvata Wall. ex Ag. var. wightiana Ag., Pteridium revolutum (Blume) Nakai). Pinnules sessile, segments contiguous, terminal lobes small or indistinct, rachis hairy in the groove above; distributed from the Himalayas throughout South-East Asia to Taiwan, in South-East Asia mainly at higher altitudes.
- subsp. caudatum : var. yarrabense Domin (synonyms: Pteris aquilina L. var. esculenta (Forster) Bedd., Pteridium esculentum (Forster) Nakai). Pinnules shortly stalked, segments rather widely spaced, terminal lobes distinct, rachis subglabrous; distributed from the Himalayas throughout South-East Asia to Australia, in South-East Asia mainly in the lowland.
Many authors do not agree with the one species concept and distinguish several species. For example in Australia 2 species ( P. esculentum found all over, P. revolutum in northern Australia only) and an intermediate hybrid (var. yarrabense from above) are distinguished.
In Europe, P. aquilinum has been a camp-follower of man and often it is an indicator of old forest. Deforestation promoted bracken in competition with other communities which have replaced forest and share similar habitats with bracken, notably grassland and moorland. Bracken is generally inhibited by bog, marsh, clay or waterlogged soils, probably due to the lack of oxygen. Leaves and superficial rhizome buds do not withstand frost, so in many regions the duration of the growing season may be determined by early and late frosts. Bracken is characteristically found on acid, nutrient-deficient, deep soils and is usually absent on nutrient-rich and calcareous soils. In areas where bracken dominates and the gain of litter continues to exceed the loss, bracken will ultimately degenerate and create possibilities for other species to take over. In woodland an equilibrium is often maintained between bracken growth and the environment. In South-East Asia, P. aquilinum is never present in rain forest, but is common particularly as var. wightianum , usually at higher altitudes (var. yarrabense occurs from sea-level up to 2500 m altitude), in clearings, open areas and forest edges. It needs a deep soil, and this is not provided where the soil has been cultivated and abandoned. In hilly country in Borneo, var . wightianum survives the periodic burning of secondary growth and there it can grow abundantly; in the Philippines it grows between 800-2000 m in open locations; in Java between 1800-2800 m on exposed mountain flanks in open Casuarina junghuhniana Miq. (synonym C. montana Jungh. ex Miq.) forest; in New Guinea it occurs on Mount Wilhelm at 2600-3400 m. The 2 varieties sometimes grow side by side in Thailand, where it is common in pine forest but rather rare on limestone soils. In Peninsular Malaysia var. yarrabense is restricted to the lowlands.
Propagation and planting
Propagation of P. aquilinum is mainly by the creeping, underground rhizome, but in favourable localities (not too dry, not too dark), propagation by spores and subsequently sexual reproduction can be common and effective. Spores are easily transported by wind over long distances to new areas which explains its worldwide distribution.
P. aquilinum is not cultivated commercially. If planted as a garden plant, it should preferably be on poor, barren, sandy soil, half in the shade. In Scotland, spraying of 4.4 kg/ha asulam (methyl (4-aminobenzenesulphonyl) carbamate) on the foliage is an effective means of controlling bracken.
Diseases and pests
Corticium anceps, Fusarium spp. and Septoria aquilina are parasitic fungi found on P. aquilinum . On the Canary Islands Orobanche species grow parasitically on P. aquilinum rhizomes. Bracken has almost no serious insect enemies because it contains ecdysone which interferes with the growth processes (causing moulting) of insects. In Vermont (United States) the Japanese beetle Popillia japonica attacks bracken.
For any use, P. aquilinum is harvested when needed and when possible. In temperate climates, leaves can only be harvested in spring and summer.
Only a few statistical data are available on the yield of P. aquilinum . There is one estimate that annual rhizome production for animal feed may amount to 17 t/ha. For fertilizer use of the ash, it has been calculated that 50 t of dried bracken are required to produce one t of potash.
Genetic resources and breeding
P. aquilinum is so widespread that it does not seem to be in danger of genetic erosion. Germplasm collections and breeding programmes are not known to exist.
P. aquilinum has the most varied uses of all the ferns. Because of its carcinogenic activity, human and livestock consumption of rhizomes and leaves should be discouraged. The medicinal and pesticidal qualities need further investigation. Its uses as a dye, fertilizer, thatch and litter most probably will remain of local importance only.
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Titien Ngatinem Praptosuwiryo & P.C.M. Jansen