Liverwort

• 1. Introduction

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  Liverworts are among the most ancient and overlooked plants on Earth. They blanket rocks, creep across tree bark, carpet the floors of tropical forests, and colonize the margins of Arctic ice. Yet most people walk past them without a second glance, because the typical liverwort is no wider than 2 to 20 millimetres and no longer than 10 centimetres. What exactly are these small, persistent plants, and why do scientists regard them as a key to understanding how life first moved onto land? Those are the questions this documentary sets out to answer.

  Liverworts belong to the division Marchantiophyta, a name drawn from the genus Marchantia, which was itself named by the French botanist Jean Marchant after his father. Scientists estimate about 9,000 species exist, distributed across almost every terrestrial habitat on the planet, from shaded greenhouses where certain species are considered weeds, to desert soils and polar regions. The answers to what makes them tick run all the way down to a single set of genetic information inside each cell.

• 2. Two Kinds of Body

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  Marchantia polymorpha, the liverwort most likely to turn up uninvited in a gardener's pot, illustrates one of the two main body plans. It grows as a flat, ribbon-like or branching sheet called a thallus. Plants built this way are called thallose liverworts. Most liverwort species, however, take a different approach, producing flattened stems lined with overlapping scales or leaves arranged in two or more ranks. These are leafy liverworts, and at first glance they look strikingly like small mosses.

  The single most reliable way to tell a liverwort from a moss is to look at its rhizoids, the hair-like anchoring structures that grip the substrate. In liverworts these are single-celled; in mosses they are multicellular. Beyond that, leafy liverwort leaves are often deeply lobed or segmented, and they tend to be arranged in three ranks rather than spiralling uniformly around the stem. Frequent dichotomous branching, where the stem forks into two equal parts, is another pointer. In some cases, the similarity between mosses and leafy liverworts is close enough that an experienced bryologist, the specialist term for a student of these plants, or a microscope is required to settle the identification.

  One feature of liverworts has no parallel anywhere else in the plant kingdom. Most liverwort cells contain unique membrane-bound oil bodies filled with isoprenoid compounds. In every other plant on Earth, lipid droplets in the cytoplasm are unenclosed. The function of these oil bodies is still a subject of study, but their presence alone marks liverworts as genuinely distinct from all other living plants.

• 3. A Life Cycle Turned Upside Down

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  Spores scattered from a ruptured capsule are where the liverwort life cycle begins. Each haploid spore germinates into a protonema, either a tangle of thread-like filaments or a flattened thallus, and the protonema is only a passing stage. From it grows the mature gametophore, the structure that will produce the plant's sex organs.

  The male organs, called antheridia, manufacture sperm cells. Clusters of antheridia are wrapped in a protective layer of cells called the perigonium. The female organs, called archegonia, each contain a single egg cell at the base of a slender hollow neck. Fertilisation depends on water: liverwort sperm are biflagellate, meaning they carry two whip-like tails and can swim, but only if at least a thin film of moisture is present.

  Researchers have recorded liverworts firing sperm-containing water up to 15 centimetres through the air, a mechanism that allows fertilisation of female plants growing more than a metre away. Rain splashing into the shallow cups that hold gemma discs in Marchantia polymorpha can scatter those asexual reproductive bodies up to 120 centimetres from the parent plant.

  Once fertilisation succeeds, the resulting diploid sporophyte develops three distinct regions: a foot that anchors it and draws nutrients from the parent plant, a spherical or ellipsoidal capsule where spores are produced, and a seta, the stalk connecting the two. The seta elongates, rupturing the archegonium and pushing the capsule out into open air. Inside the capsule, elater cells act as spring-like mechanisms that force the capsule wall open when it bursts, scattering spores into the surrounding environment.

  This arrangement stands in stark contrast to the life cycle familiar from trees and flowering plants. In seed plants, the haploid phase is represented only by pollen and the ovule, while the conspicuous plant body is diploid. In liverworts, the situation is reversed: cells carry only one set of genetic information for the majority of the plant's life, and the diploid sporophyte is short-lived, withering away shortly after releasing its spores. Mosses keep their sporophytes for longer, and hornworts disperse spores over an extended period, making the liverwort version the most fleeting of the three.

• 4. Deep Roots, Deeper Fossils

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  Among the earliest fossils that researchers tentatively assign to the liverworts are compression fossils of Pallaviciniites, found in the Upper Devonian of New York State. These fossils bear a resemblance to modern species in the Metzgeriales. A second Devonian fossil, Protosalvinia, also looks liverwort-like, but its relationship to other plant groups remains uncertain.

  In 2007, bryologists announced what were then the oldest fossils assignable to the liverworts: Metzgeriothallus sharonae, discovered in Middle Devonian rock of Givetian age in New York. Just three years later, in 2010, five distinct types of fossilised liverwort spores were found in Argentina, pushing the evidence for liverworts back to the Middle Ordovician, around 470 million years ago. That date places liverworts among the very earliest colonisers of the terrestrial world. Epiphytic thalloid liverworts, the kind that grow on other plants, are known to have evolved during the Triassic through to the Cretaceous.

  These deep roots inform a significant debate about plant evolution. For a long time, liverworts were grouped together with mosses and hornworts in a single Division called Bryophyta, and within it the liverworts formed the class Hepaticae. Later, as scientists concluded that bryophytes as a whole were paraphyletic, liverworts were given their own division, Marchantiophyta. More recently, phylogenetic evidence has reopened the question, suggesting liverworts may belong to a monophyletic grouping alongside mosses and hornworts, which some researchers have proposed calling Bryophyta sensu lato. There is also strong evidence that liverworts and mosses alone form a distinct subclade named Setaphyta. One striking conclusion from these phylogenies is that the ancestral stomata, the pores used by most land plants to exchange gases, appear to have been lost somewhere in the liverwort lineage.

• 5. The Name and Its History

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  The word liverwort goes back to Old English and means, quite literally, liver plant. The belief driving that name was that liverworts could cure diseases of the liver, a conviction rooted in a historical idea known as the Doctrine of Signatures, which held that the shape or appearance of a plant revealed its medicinal use. Because some liverworts were thought to resemble the lobes of a liver, that supposed resemblance translated into a supposed cure.

  The Latin name hepatics, which scientists still use as a common alternative to liverworts, comes from the Latin word hēpaticus, meaning "belonging to the liver." A wholly unrelated flowering plant in the buttercup family Ranunculaceae, the genus Hepatica, was given a similar name for the same reason and has sometimes also been called liverwort in older texts, a source of persistent confusion. The divisional name Hepaticophyta, still seen in older classifications, is derived from this same Latin root and is frequently misspelled in textbooks as Hepatophyta.

  The formal scientific name of the division, Marchantiophyta, has a different origin entirely. Jean Marchant, a French botanist, named the genus Marchantia after his father, and subsequent classification built the divisional name around that genus as the most universally recognised member of the group.

• 6. Ecology and Quiet Importance

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  Liverworts are absent from the sea and from habitats with excessive dryness or high direct solar radiation. Everywhere else, they tend to appear wherever moisture accumulates, and they are most abundant in moist tropical areas both by number of individual plants and by species count. Desert species do exist and can tolerate periods of total desiccation; Arctic species have carved out a presence at the cold extreme of the range.

  Direct human use of liverworts is limited. Riccia fluitans, an aquatic thallose liverwort, is sold for aquarium use. Its thin, slender branches float on the water's surface and provide habitat for small invertebrates as well as for the fish that feed on them.

  The indirect influence of liverworts, however, reaches well beyond aquariums. They reduce erosion along streambanks, collect and retain water in tropical forests, and form soil crusts in deserts and polar regions that protect otherwise bare ground. In greenhouses, Marchantia polymorpha can become a serious weed, sometimes covering the entire surface of containers. Gemma dispersal, the mechanism by which small disc-shaped reproductive structures are scattered up to 120 centimetres by rain, is described as the primary means by which this species spreads through nurseries.

  Liverworts also maintain partnerships with fungi. Thalloid liverworts typically harbour glomeromycete fungi with arbuscular rootlets that resemble those found in vascular plants. Species in the Aneuraceae partner instead with basidiomycete fungi from the genus Tulasnella, while leafy liverworts tend to associate with basidiomycete fungi belonging to the genus Serendipita. The diversity and specificity of these fungal partnerships, running across different liverwort families to different fungal genera, points to symbiosis as a structural feature of liverwort ecology rather than an incidental arrangement.