Moss is part of a plant family known as the Bryophytes. The species has existed for millions of years, making it a resilient, ancient plant.
You may have an infestation of mosses in your garden. Or you may like the aesthetic moss can have, creating a subtle woodland effect.
This article takes a look at the fundamental question: what is moss?
What type of plant is moss?
If someone were to ask me, “define moss”, I’d first of all Moss, or mosses to be more grammatically accurate, are non-vascular plants. All species of moss possess rhizoids instead of roots. This means moss is a rootless species, and doesn’t grow wood or flowers either. Instead of producing seeds for reproduction, which is common of many plant species, they produce spores.
This blog will refer to moss and mosses interchangeably, even though ‘mosses’ is the grammatically correct term. Why? Because mosses are miniature plants that grow together as single colonies. We assume that the carpet, clump or mat these colonies tend to form is one plant - hence the prevalence of the singular term ‘moss’.
Where can moss be found?
Moss can be found all over the world. The species has developed a certain resilience in the millions of years it had existed on this planet, therefore you can find it in all sorts of habitats, including:
- Woodland forests
- Alpine ecosystems
- Urban areas (driveways, walls and pavements)
One thing you’ll notice these environments have in common is their tendency to harbour dampness and moisture, which is what allows moss to thrive. While not impossible, moss tends not to fully flourish in dry, arid climates due to the lack of water.
Seasonality may affect the appearance of moss, too. Moss tends to peak during the winter due to the moist, wet conditions associated with the season. The condensed level of sunlight and lower temperatures also helps the species to thrive, as the warmer temperature and excess sunlight during the summer can dry moss out, rendering it dormant throughout the season.
One thing you’ll notice these environments have in common is their tendency to harbour dampness and moisture
The life cycle of moss
Mosses may be different to other land plants in that they are rootless and reproduce through spores rather than seeds, they do share a similar life cycle in that they have an alternation of generations. What are alternating generations? Simply put, mosses have two alternating generations:
- The gametophyte (the first generation)
- The sporophyte (the second generation.
The gametophyte has half the genetic material as the second generation, and forms when the sporophyte releases spores which start to divide. Sperm cells travel from one gametophyte to another through a thin film of water to fertilise an egg. Once fertilised, the egg will develop into a sporophyte, which then produces spores. The sporophyte relies on the gametophyte to survive, making the latter the dominant generation. It provides the sporophyte with water and nutrients. Although moss and other land plants share this biological model of alternating generations, the way the model works in moss is the complete opposite of the majority of other land plants.
Moss spores will develop a protonema when they germinate. A protonema is a chain of cells, which can resemble an algal colony or a fern prothallus. It forms in the very early stage of the moss life cycle, and paves the way for single or multiple stems to grow from it. The stems may develop leaves that form into leafy-stemmed plants. These plants render the protonemata as transient, which is what happens in most species, as they become the dominant growth form. However, it is possible for the protonema to become dominant over the leafy growth in a minority of moss species.
The stems and leaves part is known as gametophore, which combines with the protonema (despite it’s often transitory nature) to comprise the gametophyte.
Mosses are fairly complex plants. So while the plant’s gametophytes may possess stems and leaves, that’s a bit too simplistic of a way to explain it.
The stems on mosses tend to be quite short and weak, especially if they’re free-standing. The colours can range from green to brown. They tend to be green in their infancy, as this is when they have plenty of chlorophyll in the cells. Chlorophyll is a pigment that gives plants their green colouring.
There are exceptions to the rule when it comes to mosses and their typically short and weak stems. These occur in the Dawsoniaceae and Polytrichaceae species, where the stems are the stark opposite to the norm - firm and upright. Since the plants can become quite tall, with stems growing to over 60 centimetres in some cases, it’s not uncommon for them to get mistaken for herbaceous flowering plants.
The two growth forms of moss: tufty and trailing
The two growth forms of moss plants - tufty and trailing.
Mosses with a tufty growth form have upright stems. A single erect stem usually grows from each plant, and this may produce branches but not always. These branches are what can give the moss plant its tufty appearance.
In a trailing growth form, the moss has (as the name suggests) trailing stems. These stems tend to cling to the substrate and some species they may drape from the branches, almost forming a veil of moss. The branches of trailing mosses will usually grow along the substrate, but there are some instances where the species deviates and instead produce short branches.
These branches originate from cells in the stem. Most are simple outgrowths. In Sphagnum moss, which is a protected species in the United Kingdom under the Wildlife and Countryside Act, 1981, branches develop in fascicles. Fascicles are vascular tissues that supply the plant with nutrients. Some branches in a fascicle are short and stout, whereas others are willowy and overhanging.
In some moss species, such as Polytrichum and Dawsonia, the plants tend to have just single stems and branching doesn’t occur. There are also the dendroid mosses, which have a cluster or branches that sit upon a vertical stem. Dendroid actually means ‘tree-like’, which is derived from the appearance and structure of this moss species. Dendroid mosses are also nicknamed ‘umbrella mosses’ sometimes, for the reasons just mentioned.
Mosses with upright stems often cause the plants to grow in close proximity to each other, forming colonies which form the mat or cushion appearance of moss we see spread on surfaces. In these formations it can be difficult to identify the individual plants unless you scrutinise the plants, prising them apart individually, to study their composition. Species such as Bryum argenteum exhibit this growing pattern, where they grow packed very tightly together to form a colony.
When moss forms to produce the soft, cushion-like material commonly found in gardens and woodland areas, the cushion is actually made up of dead material, That’s because during the stems’ growth, the leaves lower down the stem, which are older and thus further along in the life cycle, die off. This results in a thin green layer existing on the top of the sheet of brown, dead material. This brown dead material consists of a mix of dead leaves, rhizoids and stems, as well as any other organic matter acquired by the plants that comprise this moss cushion. These cushions tend to thrive in moist areas, where the stems are able to grow and thrive. More stems growing results in more dead material and brown colouration, which in turn creates more of the green cushion that sits on top.
Moss doesn’t always grow in these cushion-like formations. Some simple-stemmed species grow individually, rather than in clusters. Instead of forming tight-knit colonies, these species poke up from the soil, almost like green shoots.
What are rhizoids?
As mentioned at the beginning of this article; all moss species have rhizoids. Rhizoids aren’t roots per se, but they do serve a similar purpose by providing the moss plant with an element of anchorage and stability. However, they are completely opposite to true roots as they don;t possess absorptive qualities. Rhizoids tend to be branched and are always multi-celled.
Rhizoids appear at the protonemal stage in moss. They develop at the foundation of stems in tufty moss species, or on the stems themselves in trailing moss species. While all species of moss have rhizoids, some species will develop them more than others.
Whereas roots absorb and conduct nutrients internally to sustain and nourish the rest of the plant, rhizoids don’t possess this capability. However, an extensive group of rhizoids can conduct water through a process known as capillary action. This happens when a group of rhizoids are formed tightly together, and wound together almost to form a rope structure. The composition of these strands make capillary action, which moves the water without external forces such as gravity, possible.
Mosses can look quite different in dry and wet conditions. Dry moss plants typically feature folded leaves or ones which curl around the stems. Wet moss plants, on the other hand, will have leaves that unravel or straighten in wet and moist conditions. There are exceptions to the rules in some species, as their leaves can remain curled around the stem even in extremely wet or moist conditions.
Moss leaves tend to be small, typically measuring half a millimetre to three millimetres long. While they can affect the moss’s appearance depending on the climate, they’re always directly attached to the stem, meaning they don’t have short stalks. Most moss leaves tend to be one cell thick. This makes them translucent. The leaves are often thickened along their long central axes, which is called a nerve or costa. In some moss species, such as Leucobryum and Sphagnum, the leaves are different in that they are composed of several cells. The leaves will usually taper to the tip, which can happen suddenly or gradually. And in some cases, the tip may continue to extend. It appears hair-like, thus is termed a hairpoint.
The bases of moss leaves can differ depending on species. Some can be narrow at mid-leaf, whereas others can be wide. Their width can be long or short, too. There’s no definitive size range. The one thing all moss leaves do usually , or close to smooth, margins. These margins can also be toothed, but not like the wider toothed leaves you find on leafy liverworts, which are a similar species to mosses belonging to the bryophytes.
The leaves on moss can differ on the various parts of the plant. Many leaves in trailing species will develop on the upright branches and not the creeping stems. Regardless of whether the leaves are of the trailing or tufty type, those that surround the plant’s organs responsible for producing eggs and sperm are different from the rest of the leaves on the plant.
Where do mosses produce eggs and sperm?
The eggs and sperms produced by moss develop on the gametophyte, hence why they’re also known as male and female gametes. The gametophyte contains reproductive structures called antheridia and archegonia, which serve as incubators for the fertilised egg which then develops into a sporophyte. The spores then become a cog in the wider sexual reproduction cycle.
What are moss sporophytes?
Moss sporophytes are essentially capsules that contain the spores used for reproduction. They are found on top of a stalk, also called a seta.
The sporophytes in most mosses will feature a definitive mouth at the end of the stalk, or the part which connects the capsule to the stem. This mouth acts a passageway to release the spores, when they are ready to leave the capsule and get carried away to produce a new moss plant.A small minority of moss species, such as Andreaea, don’t have these mouths or openings. They tend to exist in sub-alpine and alpine areas, both in the tropics and the polar regions. Instead of the spores being released from mouths, they exit the capsules through slits in the side. In one species, like Archidium, the sporophytes erupt completely spontaneously, as they possess neither slits in their side or mouths at the end of their stalks.
The different ways in which the spores exit a capsule, either from the opening mouth, slits in the side, or spontaneous eruption, naturally plays a vital role in how the spores are successfully released and therefore able to keep the reproduction cycle going.
What is the purpose of moss?
While it might not look the most aesthetically pleasing in your garden, mosses do play an important role in the ecosystem.
For instance, mosses are crucial on a microcosmic level to help insects and other invertebrates survive by providing them with a source of nutrients as well as a cosy habitat. Moss also plays a vital role for ecosystems around the world by filtering and retaining water, firming the ground and absorbing harmful CO2 from the atmosphere.
Moss has even found its way into human ingenuity, as it has been used in the food packing process, as a method of insulating houses, and within the florist trade. There has been an instance where moss has been used as fuel in the Northern Hemisphere, as a result of peat formed from partially decomposed Sphagum moss.
Ecological uses of mosses
1. Help in the fight against air pollution
Mosses can be effective indicators of c02 emissions that pollute the air. They can also signal if an ecosystem has been damaged or harmed by acid rain.
2. Prevent erosion
Mosses retain moisture, which means they effectively retain it. This helps to stabilise soil susceptible to being washed or blown away, and thus reduce erosion.
3. Indicate water pollution
Similar in the way they can help to indicate the presence of air pollution, mosses can do the same with water. They can also be used to treat wastewater thanks to their moisture retention capabilities.
4. Provide nitrogen for arctic ecosystems
This is a bit more of a niche use of the species, but mosses can help arctic and subarctic ecosystems by providing them with nitrogen. In temperate climates and regions, this is a role usually carried out by plants of the leguminous species.
How do mosses absorb nutrients?
Moss absorbs nutrients to sustain its growth in different ways. Some types take advantage of their natural absorbent surfaces by retaining water that flows over them or rains down upon them. Other mosses may gather nutrients directly from the soil that surrounds them. This usually happens in the Polytrichum species, which harbours the soil and transports it to their growing tips in order to nourish them.
How do mosses reproduce and spread?
Moss can spread and reproduce in numerous ways. Much like it relies on moisture in order to grow, moss also relies on moister to sexually reproduce.
Moss reproduces through spores, which are the equivalent to a flowering plant’s seed. Like the species which the spores come from and end up producing, spores are more primitive than flowering plants’ seed as they are single-celled. Spores inhabit a brown capsule which rests on the seta, and as they ripen they get released from this capsule, landing on areas with sufficient moisture for them to grow. The young moss that grows from these recently released spores will look like an unkempt mop of branching green hairs, from which buds will begin to form, allowing stalks and leaves to grow.
There are male and female mosses. The male mosses can have cups directly on top of them which is what produces the sperm, and thus assigns them their male biology. Female mosses have eggs that nestle in between their leaves. The sperm crucially relies on water or moisture in order to fertilise the eggs, as when they reach maturity they need to swim to the eggs to successfully fertilise them. Once the egg is fertilised, it produces the brown capsule, and the reproduction process is repeated. Moss will reproduce more rapidly and quicker in moist conditions, so it’s understandable how it can spread and multiply so quickly.
Another way in which mosses reproduce is through fragmentation, where parts of the moss break off and get carried by water or wind to an area where it can produce a new plant, moisture permitting. There’s also such thing as asexual reproduction in mosses where it releases shoots from the previous year’s plants. This typically happens during the spring; the season renowned for being the time of birth and new life.
How many species of moss are there?
Moss is part of the bryophyte species, which also consists of liverworts and hornworts. Over 20,000 different species of bryophyte have been identified on the planet, and it’s thought approximately 10,000 of these are types of mosses. It’s not surprising, really, since the resilient non-vascular plant is found in diverse habitats around the globe - from tropical rainforests, to cold arctic climates or dry desert landscapes.
What are the types of moss?
Of the 10,000 and more types of mosses on the planet, there are three main types: peat moss, granite moss and true moss.
Commonly formed in cold alpine areas - in the mountains or even the arctic - granite mosses form on rocks in dark red-brown or black patches. Unlike true mosses, where the capsules open at the teeth (or peristome), capsules in granite mosses emerge from the longitudinal slits.
Peat mosses are usually found in acidic bogs. They fall in the sphagnopsida classification of mosses, and have a particularly distinct appearance as they’re typically found with bright green or deep red/purple colouring. The branches at their top are packed tightly together, and then their longer branches are encased by dead cells that are enveloped by green or dark red living cells. Although some cells are dead, they can still absorb water - with the incredible ability to absorb enough that weighs 20 times more than its dry weight.
In the bryopsida classification of mosses, we have true mosses. These mosses have one of two of the growth characteristics discussed earlier in this article. One is tufty (acrocarpous) and the other is trailing (pleurocarpous).