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Liverwort

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Liverworts are a unique group of non-vascular land plants. They belong to the division Marchantiophyta. Many scientists also refer to them as hepatics. These plants are essential to understanding how early land plants evolved. They are part of a group called bryophytes, which includes mosses and hornworts. There are approximately 9,000 known species of liverworts globally.

Lunularia cruciata
Lunularia cruciata

They play vital roles in various ecosystems by growing on rocks, trees, and soil.

Most liverworts follow a gametophyte-dominant life cycle. This means the main plant body is haploid. A haploid cell carries only a single set of genetic information. This is very different from most animals and vascular plants. In those larger plants, the visible body is diploid, meaning it has two sets of genetic information. The liverwort life cycle begins when a haploid spore germinates. It first grows into a transitory structure called a protonema. This protonema can be a mass of filaments or a flattened thallus. Eventually, it develops into the mature gametophore, which bears the reproductive organs.

There are two primary physical forms of liverworts. The first type is known as thallose liverworts. These consist of a prostrate, flattened, and often ribbon-like structure called a thallus. The second type is known as leafy liverworts or scale liverworts. These produce flattened stems with overlapping scales or leaves. These leaves often grow in two or more ranks. In many species, the middle rank of leaves looks different from the outer ranks.

Liverwort life cycle
Liverwort life cycle

Some species even show dichotomous branching patterns.

Distinguishing liverworts from mosses requires looking at specific biological details. One major difference is the presence of rhizoids. In liverworts, these anchoring structures are only one cell thick. Liverworts also possess unique, membrane-bound oil bodies. These bodies contain isoprenoids and have a high refractive index. Most other plants have lipid droplets, but they are not enclosed in membranes like these. Furthermore, leafy liverworts often lack a costa, which is a midrib found in many mosses. They may also have leaves arranged in three distinct ranks.

Reproduction in liverworts involves both sexual and asexual methods. For sexual reproduction, the plant produces male and female organs. Male organs are called antheridia, and they produce sperm. These are often protected by a layer of cells called the perigonium. Female organs are called archegonia, which are protected by the perichaetum. The sperm are biflagellate, meaning they have two tail-like flagellae for swimming. They require a thin film of water to reach the egg.

Liverwort life cycle
Liverwort life cycle

In 2008, Japanese researchers discovered a surprising mechanism. Some species can fire sperm-containing water up to 15 cm into the air. This allows fertilization even when plants are more than a meter apart.

After fertilization, a diploid sporophyte develops within the archegonium. This sporophyte has three parts: a foot, a capsule, and a seta. The foot anchors the plant and receives nutrients from the mother plant. The seta is a stalk that pushes the capsule into the air. Inside the capsule, cells undergo meiosis to create new haploid spores. Some capsules also contain elater cells. These are spring-like cells that help scatter the spores when the capsule bursts. The sporophyte is usually very short-lived compared to other plants.

Liverworts also utilize asexual reproduction to spread quickly. Many thallose species, such as Marchantia polymorpha, produce small discs called gemmae. These gemmae grow in shallow cups on the plant body. When raindrops splash into these cups, they can launch the gemmae up to 120 cm away. This is the primary way these plants spread through greenhouses. In some species like Riccia, the plant spreads when older parts of the thallus die and younger tips become new individuals.

Lunularia cruciata
Lunularia cruciata

These plants are found in almost every habitat on Earth, except for the sea. They are most common in moist, tropical areas with moderate to deep shade. However, some species have adapted to live in the Arctic or even deserts. Some liverworts also live in a symbiotic relationship with fungi. Thalloid species often host glomeromycete fungi. Leafy liverworts typically associate with basidiomycete fungi from the genus Serendipita. These connections help the plants survive in diverse environments across the globe.

Lunularia cruciata
Lunularia cruciata
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