Chinese brake
The quick-growing fern Pteris vittata originates from Asia but is widely naturalised throughout the tropics and subtropics. In its native habitat it favours open sites on limestone, but in urban environments it grows well on walls and in concrete. Pteris vittata is adapted to a variety of soils, whilst its ability to accumulate arsenic has meant it has attracted the attention of people interested in using plants to extract heavy metals from soils; phytoremediation or phytoextraction.
Ferns are an ancient plant group that produces spores, rather than seeds. Many stages of the fern reproductive cycle are hidden. The most visible stage of a fern's lifecycle produces spores and is called a sporophyte (literally 'spore plant'). Fern spores are released from sporangia which are clustered into sori. Sori are usually found on the underside of fronds, the shape and arrangement of these sori varies hugely, and beautifully, among fern species. In Pteris vittata sori are arranged in unbroken, copper-coloured ridges along both margins of the frond. Up close the sporangia resemble minute pearls clustered amongst threads of yarn.
When the dust-like spores of Pteris vittata are released from the sporophyte and germinate they grow into minute, fragile, green, membranous structures called prothalli. The prothallus which produces the gametes is called the gametophyte (literally 'gamete plant'). The gametophyte usually grows tight against the surface of the soil, producing female (archegonia) and male (antheridia) sex organs on its lower surface. If environmental conditions are appropriate, male sperm will move from antheridia to archegonia, fertilising the egg cell. The embryo grows into a new sporophyte. The lifecycle is complete, with an alteration between two morphologically distinctive stages. This is one example of the alternation of generations; a lifecycle common to all groups of plants.
Complex biological interactions, involving associations of Pteris vittata with microorganisms, and phosphate and arsenate transport within the fern, mean it is able to concentrate arsenic in its fronds. Given the ease with which Pteris vittata grows across a wide-range of anthropogenic habitats, and its natural ability to accumulate arsenic, it has been intensively investigated as an 'environmental cleaner'. Trials on contaminated agricultural and industrial soils, and on waste resulting from mining activity, are being used to determine whether Pteris vittata is efficient enough to have significant commercial applications. Furthermore, Pteris foliage can be harvested several times a year, without affecting either the fern's health or its capacity to accumulate arsenic.
Further reading
Lombi E et al. 2002. Arsenic distribution and speciation in the fronds of the hyperaccumulator Pteris vittata. New Phytologist 156: 195-203.
Wang J et al. 2002. Mechanisms of arsenic hyperaccumulation in Pteris vittata. uptake kinetics, interactions with phosphate, and arsenic speciation. Plant Physiology 130: 1552-1561.
Zhu L-J et al 2014. Characterization of arsenic-resistant endophytic bacteria from hyperaccumulators Pteris vittata and Pteris multifida. Chemosphere 113: 9-16.
Kate Pritchard