Bryophyte: the story on HearLore

Bryophyte

In the year 2013, scientists in Canada achieved a feat that defied the laws of time and biology by reviving a 400-year-old bryophyte specimen that had been frozen in a retreating glacier. This tiny, unassuming plant, known as a moss, had lain dormant in the ice for four centuries, yet it retained the capacity to photosynthesize and grow once thawed in a laboratory setting. This remarkable resilience is not an isolated anomaly but a defining characteristic of bryophytes, a group of terrestrial plants that includes mosses, liverworts, and hornworts. These organisms lack the complex vascular systems found in trees and flowers, meaning they have no true roots, stems, or leaves to transport water and nutrients over long distances. Instead, they rely on a direct, intimate relationship with their immediate environment to survive, absorbing moisture and nutrients directly through their surfaces. Despite their small size and apparent fragility, bryophytes are among the most successful plants on Earth, having colonized habitats ranging from the scorching heat of deserts to the freezing winds of the Arctic. They exist in a world where vascular plants cannot grow, thriving on bare rock and nutrient-poor soil where other life forms would perish. The term bryophyte itself, derived from the Greek word for moss, was first suggested by the botanist Braun in 1864, but it was Wilhelm Schimper who, in 1879, formalized the classification to include all three major groups of non-vascular land plants. For over a century, these plants were viewed as a simple, primitive link between algae and the complex vascular plants that dominate the modern landscape, but recent genetic studies have revealed a far more complex and ancient evolutionary history.

The Gametophyte Dominance

The life cycle of a bryophyte operates on a principle that is the exact inverse of the plants most people are familiar with, such as oak trees or rose bushes. In the vast majority of land plants, the dominant, long-lived stage is the diploid sporophyte, the structure that produces spores and eventually flowers or cones. In bryophytes, however, the dominant stage is the haploid gametophyte, the green, leafy, or thalloid plant that we typically recognize as the moss itself. This gametophyte is the primary, photosynthetic body of the organism, living for years or even decades, while the sporophyte is a fleeting, dependent appendage. The sporophyte, which is diploid and unbranched, never detaches from the gametophyte; it remains attached and nutritionally dependent on its parent for its entire existence. It consists of a stalk called a seta and a single capsule known as a sporangium, which produces haploid spores through meiosis. These spores are dispersed by wind, and if they land in a suitable environment, they germinate to form a new gametophyte. This cycle of alternation of generations is fundamental to all land plants, but the dominance of the gametophyte in bryophytes suggests a primitive state of plant evolution. The sporophyte in bryophytes is always unbranched and produces only a single sporangium, a stark contrast to the branched sporophytes of vascular plants that bear multiple sporangia. In liverworts, the sporophyte elongates almost exclusively through cell expansion rather than cell division, whereas in mosses, the meristem is located between the capsule and the stalk, pushing cells downward to elongate the stalk and elevate the capsule. Hornworts, meanwhile, possess a meristem at the base of the sporophyte that pushes the body upwards, allowing for continuous growth. This unique developmental strategy highlights the diversity within the group, even as they share the fundamental constraint of lacking vascular tissue.

Common questions

What is a bryophyte?

A bryophyte is a terrestrial plant that lacks vascular tissue and includes mosses, liverworts, and hornworts. These organisms have no true roots, stems, or leaves to transport water and nutrients over long distances. They rely on a direct relationship with their immediate environment to survive by absorbing moisture and nutrients directly through their surfaces.

When was the term bryophyte first suggested?

The term bryophyte was first suggested by the botanist Braun in 1864. Wilhelm Schimper formalized the classification to include all three major groups of non-vascular land plants in 1879. This classification covers mosses, liverworts, and hornworts as distinct lineages of plants.

How do bryophytes reproduce?

Bryophytes require a thin layer of water on the surface of the plant to enable the movement of flagellated sperm from the antheridia to the archegonia. Unlike seed plants, they use water to transport sperm to the egg rather than pollen. Some species are monoicous with organs on the same gametophyte, while others are dioicous with organs on separate plants.

What happened to the 400-year-old bryophyte specimen in 2013?

Scientists in Canada achieved a feat in 2013 by reviving a 400-year-old bryophyte specimen that had been frozen in a retreating glacier. The moss had lain dormant in the ice for four centuries yet retained the capacity to photosynthesize and grow once thawed in a laboratory setting. This event demonstrated the remarkable resilience of bryophytes to survive for centuries in frozen conditions.

Why are bryophytes important to ecosystems?

Bryophytes play a critical role in shaping environments by helping to improve water retention and air space within soil. They serve as bioindicators that reveal the health of an ecosystem by indicating the presence of heavy metals, air pollution, and UV-B radiation. Some species produce natural pesticides and antifeedants to protect themselves from being eaten by slugs and mice.

How was Sphagnum moss used during World War I?

During World War I, Sphagnum moss was widely used to dress wounds because it could absorb large amounts of blood and pus while preventing infection. The antibiotic properties and ability to retain water of Sphagnum moss made it a useful packaging material for vegetables, flowers, and bulbs. However, the moss can harbor Sporothrix schenckii, a fungus that can cause infection.

Swimming Sperm and Ancient Roots

The reproductive strategy of bryophytes reveals a deep connection to their aquatic ancestors, as these plants still require a thin layer of water on the surface of the plant to enable the movement of flagellated sperm. Unlike seed plants, which use pollen to transport sperm to the egg, bryophytes must rely on water to allow their sperm to swim from the antheridia, the male gametangia, to the archegonia, the female gametangia. This requirement for water has shaped the evolution of these plants, limiting their distribution to moist habitats, although some species have adapted to survive in drier environments through specialized structures. The arrangement of these reproductive organs varies significantly among species, leading to a complex system of sexual classification. Some bryophytes are monoicous, meaning that the antheridia and archegonia occur on the same gametophyte, making the plant hermaphroditic. Others are dioicous, where the male and female organs are found on separate plants, making the individual plant unisexual. Within the monoicous category, the arrangement can be autoicous, with organs on different shoots, or synoicous, where they are clustered together in a common structure. The term dioicous, derived from the Greek for two houses, contrasts with monoicous, meaning one house, and these terms refer to the gametophyte sexuality of bryophytes rather than the sporophyte sexuality of seed plants. In some species, such as the liverwort Marchantia, elaborate structures called gametangiophores are created to bear the gametangia, sometimes at the tips of shoots or hidden under thalli. Arthropods can even assist in the transfer of sperm, a rare instance of animal involvement in the reproductive cycle of these plants. This reliance on water for fertilization is a trait shared with ferns and lycophytes, suggesting that the transition from water to land was a gradual process that left these plants with one foot in the aquatic past and one on the terrestrial present.

The Debate Over Plant Families

For decades, the classification of bryophytes has been a subject of intense scientific debate, with the group shifting between being considered a natural, monophyletic group and a paraphyletic assemblage of unrelated lineages. In 2005, a study supported the traditional view that bryophytes formed a single group, but by 2010, a broad consensus had emerged among systematists that bryophytes as a whole were not a natural group. This view was based on nucleic acid sequences, which suggested that the three lineages of bryophytes had diverged independently. However, a 2014 study challenged this conclusion, arguing that previous phylogenies were subject to composition biases and that phylogenies based on amino acid sequences suggested that bryophytes are monophyletic after all. Since then, almost all phylogenetics studies based on nuclear and chloroplastic sequences have concluded that the bryophytes form a monophyletic group, although phylogenies based on mitochondrial sequences continue to fail to support this view. The three bryophyte clades are the Marchantiophyta, commonly known as liverworts, the Bryophyta, which consists of mosses, and the Anthocerotophyta, or hornworts. Some researchers have proposed that these clades should be de-ranked to the classes Marchantiopsida, Bryopsida, and Anthocerotopsida, respectively. There is now strong evidence that the liverworts and mosses belong to a monophyletic clade called Setaphyta, while hornworts are sometimes considered the sister group to vascular plants. This debate has profound implications for our understanding of plant evolution, as it suggests that the complex sporophyte of living vascular plants might have evolved independently of the simpler unbranched sporophyte present in bryophytes. If the monophyletic view is correct, it implies that stomata evolved only once in plant evolution, before being subsequently lost in the liverworts. The traditional morphological distinction, based on the lack of vascular structure, is now seen as problematic, as some of the earliest-diverging non-bryophytes, such as the horneophytes, did not have true vascular tissue, and many mosses have well-developed water-conducting vessels.

The Commercial and Historical Legacy

The practical applications of bryophytes extend far beyond their ecological roles, with these plants serving as vital resources for human civilization throughout history. Peat, a fuel produced from dried bryophytes, typically Sphagnum, has been used for centuries as a source of energy and a soil conditioner. The antibiotic properties and ability to retain water of Sphagnum moss made it a useful packaging material for vegetables, flowers, and bulbs, and its antiseptic properties led to its use as a surgical dressing in World War I. During the conflict, Sphagnum moss was widely used to dress wounds, as it could absorb large amounts of blood and pus while preventing infection. However, the use of Sphagnum is not without risks, as the moss can harbor Sporothrix schenckii, a fungus that can cause infection, although it is not clear if the contamination originates in an environmental bog reservoir or during processing for its use as plant packing or filling material. The commercial value of bryophytes is not limited to their use as fuel and medical supplies; they are also used in the production of natural pesticides and antifeedants. The liverwort Plagiochila produces a chemical that is poisonous to mice, and other species produce chemicals that protect them from being eaten by slugs. These natural compounds have been studied for their potential use in agriculture and medicine, offering a sustainable alternative to synthetic chemicals. The ability of bryophytes to survive in extreme environments has also made them valuable for scientific research, as they provide insights into the migration of plants from aquatic environments to land. The 400-year-old bryophyte specimen revived in Canada in 2013 is a testament to the resilience of these plants, which can survive for centuries in frozen conditions and then be brought back to life in a laboratory setting. This ability to survive in extreme conditions has made bryophytes a subject of interest for researchers studying the limits of life and the potential for survival in harsh environments.