Alternative titles: Petridophyta
The bryophytes are a group of non-vascular plants that exist on land. Terrestrial refers to their growing on land, whereas nonvascular signifies they lack food and water bearing channels. There are three main categories of this group: mosses (Bryophyta), liverworts (Marchantiophyta), and hornworts (Anthocerotophyta).
These incredible bryophyte plants can be found all over the world, including Antarctica and the Arctic! Despite their preference for more humid climates and inability to tolerate extreme arid conditions, these plants can also be found in moist pockets of the desert and near the intertidal zone. Bryophytes are essential to the health of ecosystems wherever they grow. They play a key role in soil formation, maintaining soil moisture, and nutrient cycling.
Bryophytes have two attributes in common - they are terrestrial and they do not have vascular systems. What exactly does this mean? The bryophytes represent one of three groups of plants, including the tracheophytes, the thallophytes, and the bryophytes.
There are a few other characteristics that are common to all bryophytes besides being terrestrial and nonvascular.
By using the chlorophyll in their chloroplasts, bryophytes are photosynthetic plants that produce their own food. In contrast, the way they absorb minerals and water differs greatly from that of their vascular cousins.
Bryophytes, which lack roots and stems, absorb water and minerals directly through their leaves. Their overlapping leaves and abundant rhizoids increase the efficiency of absorption. These nonvascular plants, however, have no means of retaining moisture. The tendency of an organism’s water content to change with its environment is called poikilohydric.
Poikilohydric organisms require water to grow. Therefore, if there is no moisture in their environment, they will enter a state called desiccation. Surprisingly, if a bryophyte is in a desiccated state, it can still survive. Water will rehydrate the plant and enable it to begin growing again once the ecosystem is re-hydrated.
Bryophytes have a life cycle consisting of two distinct stages or a succession of generations, each with a different shape. These are the sporophyte and gametophyte generations, each stage being named for what they produce. The sporophyte generation, for example, produces spores that develop into new plants. Meanwhile, the gametophyte generation consists of the adult plants that produce haploid cells that fuse together to make viable spores.
Initially, a spore germinates to form a protonema, which is like a seedling in a flowering plant. Upon reaching the mature gametophyte stage, the plant moves to a more elaborate, leafy state.
The gametophyte can produce either male or female sex organs, with some species being able to produce both. Archegonium are female organs that contain a single egg. Antheridiums, or male organs, are full of cells that produce sperm. In the mature archegonia, sperm are attracted to the egg by a fluid, and the antheridia release the sperm. Sporophytes begin to develop following fertilization.
In turn, the fertilized zygote will develop into a diploid sporophyte. During its maturation, however, it is dependent on the gametophyte for nutrition. As the sporophyte develops, it becomes a long stalk, topped by a sporangium. The rupture of this structure allows for the dispersal of spores, and the cycle begins again.
Thallophytes were the first plants on earth. This water-bound, floating algae is often just a single cell thick, and it absorbs water and nutrients directly into its cells. About 400-450 million years ago, plants began to move onto land, leading to the evolution of embryophytes. Two important traits of green algae emerged at different times: a waxy cuticle and a gametangia. In both cases, the development protects the plants from drying out too soon and allows them to live in terrestrial ecosystems.
Initially, these thallophyte plants needed to stay relatively close to water. Having a whole new world to expand into, new, drought-tolerant species began to evolve. Among the descendants of these early embryophytes are liverworts, hornworts, and mosses today.
While all three groups are bryophytes, they belong to different phyla. Molecules are classified as Bryophyta, liverworts as Marchantiophyta, and hornworts as Anthocerotophyta.
A moss is often described as being soft and fluffy because its leaves and stems look distinct. Sporophytes develop at the end of a long stem (called a seta) that is topped by spore-producing capsules. The leaves of mosses are spirally arranged along the stem, with a vein running through the middle of every leaf.
Interestingly, moss sporophytes have stomata on the sporangium. The process of transpiration is facilitated by stomata, which are typically located on the leaves and stems of tracheophytes. According to one theory, stomata in bryophytes are responsible for helping the sporangium dry out and prepare for spore release.
Moss is not the same thing as lichen, which is a combination of at least two organisms - fungi and algae.
Moss are divided into two types based on their growth habits: acrocarps and pleurocarps. Acrocarps have vertical stems, producing sporangia at the ends. Pleurocarps, on the other hand, have a creeping, horizontal growth pattern. Sporophytes extend laterally from the stem sides.
Moss lives in a variety of habitats around the world. Moss is prevalent in rainforests, wetlands, and alpine ecosystems, but you can also find it nestled in brick walls and in sidewalk cracks in urban and suburban areas. A few species can even live in arctic and desert climates.
Among the bryophytes, moses have the greatest diversity, with around 15,000 species. Moss species are further divided into four categories within the Bryophyta phylum:
Even though some types of moss are more common than others, all four kinds can be found around the world. Comparatively speaking, temperate broadleaf forests, boreal forests, and tundra tend to have the highest diversity of mosses.
Mosses have the ability to reproduce asexually, which usually happens via vegetative reproduction. This occurs when part of the plant is separated and becomes its own individual. Breeding can also be accomplished through 'brood bodies', which are special groups of cells capable of developing into plants.
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.
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.
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!
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.
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.
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.
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.
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.
Showing preference for warmer climates, hornworts are usually found in tropical and warm temperate ecosystems.
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.
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.
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