Imagine you and your friends are exploring a forest and stumble upon a creek bed. You notice a green, reptilian-looking plant covering the edges of the creek. Suddenly, you hear one of your friends shout out, "Thallose liverwort!" and then "Conocephalum salebrosum!" Sadly, your friend hasn't found a supernatural plant, though the name sounds straight out of a magical textbook! But they have instead recognized a well-known liverwort species!
Liverworts, along with hornworts and mosses, are types of non-vascular land plants known as bryophytes!
- First, we will look at the definition of bryophyte.
- Then, we will explore some examples of bryophytes.
- Next, we will dive into the life cycle of bryophytes.
- Lastly, we will look at the reproduction of bryophytes.
Bryophytes Definition
So, we know that liverworts, hornworts, and mosses are types of bryophytes, but what exactly is a bryophyte? Its definition is shown below.
Bryophytes are non-vascular, seedless land plants found on every continent in the world. They are often referred to as the amphibian of the plant kingdom as they need to be close to both soil and water to survive.
Let's further break down what this definition reveals about bryophytesbefore diving into some examples.
From this definition, we learn three important characteristics of bryophytes:
Unlike most plants, bryophytes are non-vascular, meaning they lack the vascular tissues (xylem and phloem) and true roots necessary to transport water and minerals. To survive, bryophytes need their environment to provide them with water they can absorb.
This is why you find them by creeks and in other moist, damp locations, such as the forest floor!
Again, unlike many plants with seeds and flowers, bryophytes are seedless and flowerless. But wait, how do bryophytes reproduce if they don't produce seeds? By using spores!
We will explore this more when discussing their reproductive cycle so keep reading.
Lastly, we will learn that they are land plants. Because they depend on water, bryophytes have adapted to have a waxy cuticle and root-like structures (rhizoids) to live successfully on land. The waxy cuticle helps prevent water loss through evaporation, and the rhizoids are hair filaments that anchor bryophytes and allow growth on places such as the surfaces of rocks.
Even though all bryophytes share the above characteristics, there is a lot of diversity in the different phyla of bryophytes. Let’s dive into some common examples!
Bryophytes Examples
Bryophytes are thought to have evolved around 400 million years ago, with over 20,000 species currently described. As we've already covered, there are three distinct phyla of bryophytes: liverworts, hornworts, and mosses.
We will briefly discuss each one!
Liverworts
Liverworts, scientifically known as Phylum Marchantiophyta, are usually found in moist, tropical habitats. One of their distinguishing features from other bryophytes is the presence of oil bodies, which are lipid-filled membrane-bound structures. Though the purpose of these oil bodies is not definitively known, it allows for easier identification of liverworts.
There are thought to be between 5,000-7,000 species that are either classified as leafy liverworts or thalloid liverworts:
An example of a liverwort species is the one your hypothetical friend found in the forest: C. salebrosum. It is commonly referred to as snakeskin liverwort, a type of thalloid liverwort.
Liverworts get their name from the fact that some resemble the shape of the human liver and were thought by early herbalists to bring relief from liver troubles!
Hornworts
Next up, we have hornworts (Anthocerotophyta)! Though they are similar in name to liverworts, there are some important differences.
Hornworts and liverworts look pretty similar in their gametophyte (sexual) phase, both having a flat, sheet-like thallus, but noticeably different in their sporophyte (asexual) phase. Hornwort sporophytes are erect, long, horn-like structures that have an area of noted, continuous growth from their base.
A couple of distinguishing aspects of hornworts are:
There are significantly fewer hornworts, with less than 500 known species.
Hornworts usually only have one chloroplast per cell and are the only land plant with pyrenoids - structures in the chloroplast that help in carbon dioxide fixation.
A common example of a hornwort is the field hornwort (Anthoceros agrestis), which can be found growing across North America and Europe.
Figure 2. Field hornwort.
Mosses
Finally, we will discuss the most diverse phylum, with over 15,000 species, that you are likely most familiar with, found in rainforests across the world and climbing along buildings in the middle of our cities: mosses (Bryophyta)!
Similar to liverworts, mosses have leaf-like structures; however, they are arranged spirally around a main stem-like structure.
Mosses are unique from other bryophytes in that their rhizoids are multicellular compared to the single-celled rhizoids of liverworts and hornworts. This makes the rhizoids of moss thicker. Moss also tends to grow taller than the other phylum of bryophytes.
A common and fun example of moss is peat moss (Sphagnum moss) which was used in WW1 to dress bandages due to its ability to absorb water.
Figure 3. Peat moss.
In general, because they lack vascular tissues and the ability to transport water, bryophytes tend to be short, and many don't grow taller than 10 cm.
Now that you are a liverwort, hornwort, and moss expert, we can explore how these tiny, but diverse, plants have thrived by examining their life cycle. First, we will take a broad view to ensure we understand all the moving parts, and then look more specifically at the alternation of generation and reproduction.
Alternation of Generation in Bryophytes
Though bryophytes are different from other plants in many ways, their general life cycle is similar and can be summed up in three words: alternation of generation.
Alternation of generation, in this context, refers to alteration between a sexual phase (haploid gametophyte) and a nonsexual phase (diploid sporophyte) in the life cycle of bryophytes.
In case you need a review, haploid and diploid refer to the sets of chromosomes an organism has.
- Haploid = one set of chromosomes, half of diploid.
- Diploid = two sets of chromosomes, double of haploid.
Some important takes aways are:
Two haploid gametes fuse together to create the sporophytes which produce spores.
These spores develop into gametophytes which produce the gametes (male or female sex cells) that fuse together to create the spores and the cycle of alternation of generation.
Now that we have a general idea and the necessary vocabulary to discuss the alternation of generation, let's specifically see how this plays out in the life cycle of bryophytes.
Bryophytes Life Cycle
A unique aspect of the life cycle of bryophytes is that their gametophyte phase is dominant, meaning they spend most of their lives as gametophytes and a very limited time as sporophytes. Gametophytes are the part of the bryophytes you can easily see, such as the thallus and main body, while sporophytes are often smaller and dependent on the gametophytes for survival.
Don't worry if these terms are still intimidating. We are going to work through both the gametophyte and sporophyte generation of bryophytes together!
Sporophyte
So far, we know that the sporophyte generation is formed when two haploid gametes fuse together through meiosis to produce spores. But what does a sporophyte phase look like in bryophytes?
Sporophytes are composed of three basic structures:
Foot: anchors the sporophyte to the gametophyte.
Seta: stalk that supports the spore-bearing capsule.
Sporangium capsule: on top of the stalk where the haploid spores are actually produced.
Once spores are created, they are dispersed via the wind, and if they land in an appropriate environment, the spores will develop into gametophytes.
Gametophytes
Once the spores undergo mitosis, they will develop into gametophytes, which in turn produce gametes. Depending on the plant and species, it will have a female or male sex organ.
Archegonium is the female sex organ of bryophytes that contains a single egg.
Antheridium is the male sex organ of bryophytes that produces sperm.
Eventually, sexual reproduction occurs when a sperm fertilizes an egg, fusing them together to produce a spore which starts the cycle anew!
You may be wondering, "How exactly can this fertilization occur?" Well, I'm glad you asked, as the last thing we will quickly touch on is bryophyte reproduction!
Bryophytes reproduction
Picking up right where we left off, one way bryophytes reproduce is sexually, when a sperm fertilizes an egg. This can only occur when sperm swim through water from the antheridium to the archegonium and successfully reach and fertilize an egg. But can you guess another way they can reproduce? Asexually!
Asexual reproduction in bryophytes is also known as vegetative reproduction and occurs in two ways: fragmentation or through gemmae.
Fragmentation occurs when a plant fragment falls off and can grow into a new, separate, genetically identical plant.
For bryophytes, this occurs during the gametophyte generation, and, interestingly, many of them have weak areas that encourage fragmentation!
Gemmae are small, green, multicellular buds found inside gemma cups that grow on the thallus of bryophytes.
In a similar mechanism to fragmentation, gemmae can fall from the parent plant, germinate and develop into a new bryophyte.
Bryophytes - Key takeaways
- Bryophytes are non-vascular, seedless land plants consisting of three groups: liverworts, hornworts, and mosses.
- Liverworts can be split into two categories: leafy liverworts and thalloid liverworts.
- Hornworts can be distinguished by their singular chloroplasts and pyrenoids.
- Mosses are the most diverse phylum and have multicellular rhizoids.
- Bryophytes have a life cycle defined by alternating generations between gamete-producing gametophytes and spore-producing sporophytes.
References
- Figure 2: Field Hornwort (https://commons.wikimedia.org/wiki/File:Anthoceros_agrestis_060910b.jpg) by BerndH (https://commons.wikimedia.org/wiki/User:BerndH). Licensed by CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en).
- Figure 3: Peat Moss (https://commons.wikimedia.org/wiki/File:Common_green_peat_moss_(Orphan_Lk)_3.JPG) by Fungus Guy (https://commons.wikimedia.org/wiki/User:Fungus_Guy). Licensed by CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en).
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