Pallavicinia lyellii (Hook.) Carruth.

Liverworts, hornworts and mosses form a group of simple plants called bryophytes (1). Bryophytes lack many of the more complex structures of the higher plants, such as a vascular system, and flowers. They do not have roots, instead they have structures called 'rhizoids' which absorb water and anchor the plant to the substrate. In liverworts these rhizoids each consist of a single elongated cell (2). Bryophytes have an interesting life cycle consisting of two main parts, called the gametophyte and sporophyte generations, the gametophyte stage is dominant (3). Plants that are in the gametophyte stage can reproduce sexually. Male organs (antheridia) produce antherozoids which move to the female organs (archegonia). Fertilisation occurs and a 'sporophyte' develops, this structure remains attached to the plant. The sporophyte releases spores which disperse and develop into a new plant (3). This species tends to grow in summer, and sporophytes are only rarely produced (1). Some UK populations appear to consist of just one sex, so spores cannot be produced; this may cause problems with dispersal and colonisation (1). All liverworts are so named because the lobed types were thought to resemble the liver. During Medieval times, common belief held that the appearance of a plant indicated which part of the human body it could cure; liverworts were thought to cure liver ailments (4).

Five remaining populations occur within Sites of Special Scientific Interest (SSSIs), one of which is a National Nature Reserve (NNR) (3). Veilwort is a UK Biodiversity Action Plan (UK BAP) priority species, and a Species Action Plan has been produced (3).

Veilwort is a thalloid liverwort, meaning that these liverworts do not have stems and leaves, but consist of a 'thallus', a flattened 'body' just a few cells thick (2). Veilwort is pale green in colour (1) and easily recognised by its obvious raised midrib (2), which has a strand of elongated conducting cells passing along the centre (1).

Tends to inhabit the edges of raised bogs or cut-over areas, growing amongst clumps of purple moor grass or rushes (1). It has also been recorded from the bases of trees in carrs, on wet sandstone, on the banks of sandy or peaty ditches and streams, on rotting wood, and even on rotting leather, including an old boot on Wimbledon Common (1).

This species appears to have undergone a significant decline, particularly in the north of England. It has become extinct in North Yorkshire, South Lancashire, West Sussex and Westmoreland (1). The number of sites in England and Wales that support the species has been reduced from 26 around 200 years ago, to 17 in 1950 and just 10 in 1970 (1). Veilwort has a wide distribution globally, being particularly widespread in tropical areas, but it is vulnerable in continental Europe (1).

Classified as Vulnerable in Great Britain (1).

The threats to the species are not understood (1), but are thought to include drainage, over-shading caused by scrub encroachment, and climbing activities where the species occurs on sandstone (3).

Pallavicinia lyellii, the ribbonwort or veilwort, is a dioicous bryophyte plant in the liverwort family Pallaviciniaceae. Often seen in moist situations on rocks and soil, with a worldwide distribution.

Pallavicinia lyellii là một loài rêu trong họ Pallaviciniaceae. Loài này được (Hook.) Gray mô tả khoa học đầu tiên năm 1821.