Bryophytes (Mosses, Liverworts, and Hornworts)

Bryophytes describe a group of plants that are both terrestrial and nonvascular. To clarify, ‘terrestrial’ means they grow on land, and nonvascular refers to their lack of food and water-conducting channels. This group is further divided into three main categories: mosses (Bryophyta), liverworts (Marchantiophyta), and hornworts (Anthocerotophyta).

These incredible plants can be found all over the world – including Antarctica and the Arctic! While they grow most abundantly in more humid climates and can’t tolerate extremely arid environments, they can be found in moist pockets of the desert or near the intertidal zone on the coastlines. Wherever they are in the world, bryophytes are integral to ecosystem health. They play a key role in soil formation, maintaining soil moisture, and nutrient cycling.

What Is a Bryophyte?

All bryophytes have two things in common – they are terrestrial and nonvascular. But what does this mean? To put bryophytes in perspective, they constitute one of three groups that all plants fall into including tracheophytes, thallophytes, and bryophytes. 

• Tracheophytes include vascular plants that have lignin and vascular tissue. Lignin is a polymer that gives plants structure, allowing plants to grow tall. Xylem and phloem act as the vascular system that transports water and nutrients throughout the plant. Additionally, tracheophytes are terrestrials or land plants. Examples of tracheophytes include trees, ferns, grasses, and flowering plants.
• On the other end of the spectrum, there are thallophytes, which, while also nonvascular, require a fully aquatic life to survive. These include algae, seaweeds, kelp, and diatoms. 
• In between these two are the bryophytes, which share terrestrial ecosystems with tracheophytes, and share the lack of vascular vessels with thallophytes.

Characteristics of Bryophytes

Besides the two main delineations of being terrestrial and nonvascular, there are a few other characteristics that most bryophytes have in common.

• Small Stature: Without the lignin that evolved with vascular plants, bryophytes are ground-ridden. They don’t have the structural ability to grow upwards. Therefore, most bryophytes create a spreading carpet-like appearance, covering rocks, logs, and the ground.
• Rhizoids: These are nearly imperceptible hairlike organs that serve to anchor the plant in place. Though they can be mistaken for roots, remember that bryophytes are nonvascular. Rhizoids do not conduct nutrients or water. 

Bryophyte Nutrition

As photosynthetic plants, bryophytes make their own food through the activity of chlorophyll in their chloroplasts. However, their methods of absorbing minerals and water are starkly different from their vascular cousins. 

Water-Absorbing Leaves

Lacking roots and stems, bryophytes absorb water and minerals directly through their leaves. To increase the efficiency of absorption, they have developed a couple of physical characteristics such as overlapping leaves and abundant rhizoids. On the other hand, these nonvascular plants have no way of retaining that moisture. The tendency of an organism’s water content to change with its environment is called poikilohydric.  

Growth Cycles Tied to Water Availability

Poikilohydric organisms need water to grow. Consequently, if their environment doesn’t have moisture available, they will go into a water-stressed state called desiccation. If a bryophyte is in a desiccated state, it can survive! Once water is reintroduced to the ecosystem, the plant will rehydrate and can begin its growth cycles once again. 

Life Cycle and Reproduction

The bryophyte life cycle involves two distinct stages or an alternation of generations, each with a different physical form. These are the sporophyte and gametophyte generations. Each of these stages is named for what they produce. For example, the sporophyte generation produces the spores that develop into new plants. On the other hand, the gametophyte generation is the adult plant that produces the gametes, or haploid cells, that fuse together to form the viable spore.

Spore Develops into the Gametophyte

First, a spore germinates to produce the protonema which is analogous to a seedling in a flowering plant. The small, threadlike protonema develops into the adult gametophyte stage which is characterized as the more elaborate, leafy stage of the plant’s life cycle. 

Gametophyte Releases Gametes

Depending on the plant, the gametophyte can produce male or female sex organs, with some species producing both. The female organ is called the archegonium and contains a single egg. The antheridium, or male organ, contains many cells that develop into sperm. When mature, the archegonia releases a fluid that attracts the sperm to the egg, and the antheridia will release the sperm. Once fertilization occurs, the sporophyte begins to develop.

Sporophyte Releases Spores

The fertilized zygote will then develop into the diploid sporophyte. However, while it matures, it is dependent on the gametophyte for nutrients. Once developed, the sporophyte grows into a long stalk, topped with a sporangium. This structure eventually ruptures to allow for the dispersal of spores, and begins the cycle over again.

Bryophyte Evolution & Classification

Thallophytes were the first plants. Often just a single cell thick, these water-bound, floating algae absorbed the water and nutrients directly into their cells. Then, somewhere between 400-450 million years ago, embryophytes evolved when plants moved onto land. This likely happened on several distinct occasions when a type of green algae developed two important traits: a waxy cuticle and gametangia. Both these developments protect the plant from drying out too quickly and allow them to move into terrestrial ecosystems. 

At first, these new plants needed to stay relatively close to water. But with a whole new world to expand in, new, more drought-tolerant species began evolving. Descendants of these early embryophytes include the liverworts, hornworts, and mosses in the world today. 

While all three groups are considered to be bryophytes, there are three distinct phyla. Mosses are deemed Bryophyta, liverworts called Marchantiophyta, and hornworts under the group Anthocerotophyta.

Mosses (Bryophyta)

 With the appearance of having distinct stems and many tiny leaves, moss often looks very soft and fluffy. The sporophytes develop on the end of a long stem (called a seta) topped with a spore-bearing capsule. Mosses have leaves that are spirally arranged along the stem and a ‘vein’ that runs along the middle of each leaf.

Interestingly, moss sporophytes have stomata on the sporangium. Stomata are typically found on the leaves and stems of tracheophytes to assist in the process of transpiration. While the function of stomata in bryophytes is not totally clear, one theory is that the stomata help the sporangium dry out and prepare to release the spores.

Moss should not be confused with lichen which is a combination of at least two different organisms – fungus & alga.

Growth Pattern

Moss typically are divided into two categories depending on their growth habits: acrocarps and pleurocarps. Acrocarps have stems that grow vertically, producing sporangium at the ends of the main stems. On the other hand, pleurocarps demonstrate a more creeping, horizontal growth pattern. The sporophytes extend laterally from the sides of the stems.

Habitat

Moss can be found around the world in a variety of habitats. Common in rainforests, wetlands, and alpine ecosystems, moss can also be found in urban and suburban areas, nestled in sidewalk cracks or creeping up brick buildings. Additionally, some species can even be found in arctic and desert climates.

Distribution and Diversity

Mosses include the greatest species diversity of bryophytes with around 15,000 different species. Within the Bryophyta phylum, there are generally four primary classes that moss species are further delineated into: 

• Sphagnopsida – peat or sphagnum moss
• Andreaopsida – rock and lantern moss
• Polytrichopsida – nematodontous moss
• Bryopsida – arthrodontous moss

While some moss types are more common than others, all four classes of moss can be found worldwide. However, areas with temperate broadleaf forests, boreal forests, and tundra tend to have the highest diversity of moss, even compared to tropical rainforests.

Life Cycle Differences

Moss are able to reproduce asexually, usually by means of vegetative reproduction. This occurs when part of the plant is separated and becomes its own individual. Some species can also reproduce using ‘brood bodies’ which are a specialized group of cells that can grow into adult plants.

Examples of Moss

• Common Haircap Moss (Polytrichum commune) is found throughout the western hemisphere. This acrocarpus species extends upward with light green spiked leaves giving the appearance of a star when looked at from above.
• Pincusion Moss (Leaucobryum glaucum) is also an acrocarpus species, but with its intensely dwarf structure, it doesn’t get much height. Instead, it grows in thick clumps to form domes of green. 

Liverworts

Compared to moss, liverworts grow much closer to the ground, hugging the surface with large, flat rubbery leaves. Their sporophyte form appears as either a rubbery, green ‘flower’ or like a globe on a stem, depending on the growth pattern.

Growth patterns

Liverworts have two primary growth patterns: leafy and thalloid. Leafy liverworts constitute the majority of the phyla. Appearing similar to moss, their leaves are arranged in two rows (as opposed to spirals) and lack the center vein. The remainder of liverworts are thallus. Thalloid liverworts grow horizontally and have wide, flat leaves that are rubbery. Leafy liverworts more commonly sport the globe spore capsule while thalloid liverworts the flower-like capsule. 

Distribution and Diversity

Slightly less diverse than moss, there are between 6,000-8,000 species of liverwort. Two primary classes of liverworts are distinguished by the growth pattern. Thallus liverworts are grouped in the Marchtiopsida class, and leafy liverworts represent the Jungermanniopsida class. Jungermanniopsida includes upwards of 80% of liverworts with an enormous diversity in appearance and habitat preference.

While liverworts can be found worldwide, they are more common in the tropics. Like most bryophytes they are resilient and 25 species can even be found in Antarctica!

Habitat

Common in tropical habitats, the different classes of liverworts do display slightly different preferences for substrate. Thallus liverworts (Marchtiopsida) prefer moist soil or rocks while the leafy variety branch out onto tree trunks and damp wood.

Life Cycle Differences

While liverworts can follow the typical life cycle of bryophytes, most can reproduce asexually using a gemmae. Held in gemma cups, these disks of tissue are produced from the gametophyte. Rainfall distributes them into the ecosystem, and they begin developing into another gametophyte.

Examples of Liverworts

• Snakeskin Liverwort (Conocephalum salebrosum) has thallus that are reminiscent of large overlapping straps. Air pores cover the surface of the thallus, giving it the appearance of snakeskin. This species is common of moist soil throughout North America.
• Variable-leaved Crestwort (Lophocolea heterophylla) is a leafy liverwort that develops a carpet of leafy shoots. Greenish-yellow in color, the leaves can often have a slightly translucent appearance.

Hornworts

Hornworts look fairly similar to liverworts in the gametophyte stage, and they are generally blue-green in color. However, the sporophyte generation gives them their name. A distinctive long, thin, needle-like sporangium looks like a horn. These bryophytes differ from all other plants as each cell only contains a single large chloroplast.  Similar to moss, hornworts also have stomata on the sporophyte generation.

Growth Pattern

As gametophytes, hornworts are similar to a liverwort’s thallus growth form. Wrinkled or lobed sheets creep over the substrate. Depending on the genus, hornworts grow in either a branching or circular pattern.However, hornworts show no differentiation among cells throughout the gametophyte generation.

Distribution and Diversity 

The least diverse phyla of bryophytes, there are only about 100-300 species of hornworts. The largest genus of hornworts, Anthoceros, can be found worldwide, but many of the other genera can only be found in tropical ecosystems.

Habitat

Showing preference for warmer climates, hornworts are usually found in tropical and warm temperate ecosystems.

Examples of Hornworts

• Field Hornwort (Anthoceros agrestis) is a member of the largest genus of hornworts. The thallus looks like a thin dark green disk, and the sporophyte is a lighter green stalk.
• Bighorn Hornwort (Megaceros aenigmaticus) forms very thin dark green sheets on wet rocks. The thallus is lobed with toothed edges. This species rarely produces reproductive structures.

Ecological Impact 

Bryophytes act as the humble backbone to many ecosystems. Throughout their lifecycles, they play an important role in soil formation, maintaining soil moisture, and recycling nutrients. The presence and diversity of bryophytes in an ecosystem can inform the status of nutrient cycling.

Beyond their impact on general ecology, species of bryophytes have proven to be beneficial to humans. For example, peat or sphagnum moss is critical for gardeners trying to improve water retention in the soil. Because of its density, peat moss is also used for energy production and as an impressive substitute for cotton bandages during World War I. Famously, Scotch whiskey relies on sphagnum moss to create the characteristic smoky flavor.

The uses of bryophytes don’t stop there. In Sweden, a species of moss was discovered to remove arsenic from water. Even artists have begun using moss as a medium of inspiration.

Whether it be mosses, liverworts, or hornworts, all bryophytes have developed incredible adaptations to survive in all corners of the planet. Working to keep ecosystems healthy, these inconspicuous plants have been under our feet all along.