David Beeson, 28th February 2022
About 450 million years ago, in the Silurian era, plants invaded the land. With water and land / air being such different habitats evolution had to throw up some divergent life forms to survive there. It would take millions of years for the complete colonisation of land. The first true land plants were the bryophytes, including the mosses and liverworts, although the algae were the link.
After the bryophytes had established themselves the sequence was: horsetails, club mosses and ferns, to gymnosperms (conifers) and finally the angiosperms, the flowering plants.
Everyone has encountered osmosis at school. Osmosis is all about gradients, so let me elaborate.
Place a car at the top of a hill, take off the brake and it will naturally run down the elevation GRADIENT. High to low.
Open a strong perfume bottle and the aroma will spread out from high to low along a perfume GRADIENT.
Breathe in some air into your lungs. The oxygen is in higher concentration in the lungs than the blood, so it flows down an oxygen GRADIENT into your blood, while carbon dioxide flows along its GRADIENT out of the blood and into the lungs.
In biology gradients are everywhere and are critically important. If an organism cannot use a gradient it has to employ energy to pump the material, and that costs. Gradients are free.
Osmosis is about water gradients. Less water outside and water flows in along a water gradient.
Imagine some algae in a transitory pool of salty seawater. The alga has many chemicals inside its cells, so has less water than the sea. Water flows in along a gradient – osmosis. The pool dries out in the sunlight, now the water gradient changes, and the alga can lose water by osmosis. Death … unless it has some clever trick.
The trick is to have a waxy, non-cellular outer layer, called the cuticle, that inhibits water movement, and to have a cell membrane that can actively control it. (The cell wall is very permeable to water, indeed water flows through it from one cell to the next and that is an important function.)
In a wet / dry environment evolution forced the survival of algae with thicker, more efficient waxy cuticles … and some rockpool algae still have a cuticle layer.
There were other problems to solve in moving to a terrestrial existence. There is 280 times more carbon dioxide in water than oxygen. That’s good for photosynthesis, which obviously needs it. In the air there are 570 oxygen molecules to every one of carbon dioxide, which also moves more slowly in cells. So, land plants have a problem with carbon dioxide supply. A waxy cuticle only antagonises this problem. The solution appears to be in having a waxy layer on the upper surface and (closable) holes on a plant’s lower surface – stomata.
Eventually, with increasing size plants would need a transport system (Phloem and xylem), supporting mechanisms (woody cells) and roots to seek out and supply water and nutrients. And, of course, some sexual mechanisms that did not need gametes swimming from plant to plant.
The Bryophytes.
Mosses.
We are all familiar, in the UK, with the green mosses that coat trees, fences, roofs and the ground in damp locations. Yet few people understand their structure and life cycle. A life cycle called ALTERNATION OF GENERATIONS … and this is one that occurs in all land plants, including trees. Essentially, plants switch between two forms in their life cycle – gametophyte and sporophyte.
But, first, let us look at a typical moss plant.
Depending on the time of the year you look at one, you should see a stem covered in minute green leaves (the gametophyte) AND a non-green addition to the top, shown as the sporophyte below.
This plant has no true roots, no phloem or xylem*, no lignin to make wood, a very limited cuticle and such thin leaves that stomata are not present (but maybe on the sporophyte). Mosses can lose water easily. As such, mosses live almost exclusively in damp conditions and are small plants. Their gametes still need water to complete their sexual reproduction.
*In Polytrichum there are some dead cells in the stem centre that can carry water upwards, yet it is not xylem.
The photosynthetic plant is the gametophyte, and surprisingly it has only one set of chromosomes. (We have two sets, as do most organisms we meet daily.) As you will have guessed, the gametophyte can produce gametes – male (motile) and female (non-motile). If fertilization occurs the resulting zygote will grow into a sporophyte, which is parasitic on the gametophyte, has two sets of chromosomes and is non-photosynthetic. Eventually, the sporophyte produces and releases spores that can germinate into a new green gametophyte. (For biologists, the spores are produced via meiosis and the gametes by mitosis. Not what you might expect. A great exam question, so watch out!)
The sexual cycle alternates between gametophyte and sporophyte – Alternation of Generations.
Sphagnum is a moss genus, there are many different types of sphagnums. In many ways, they are quite distinct. They dwell in waterlogged situations, under acidic conditions, especially in floating rafts, in raised bogs and they die and fail to decompose in anaerobic, acidic conditions to make Sphagnum peat … that holds a huge carbon store.
(If you ever only have sphagnum and salty water, and need drinking water … by passing the water through the moss it acts as an ion-exchange mechanism, and the salt is held by the moss and the water come out drinkable. Sodium and hydrogen atoms are exchanged.)
The sphagnum gametophyte can hold twenty times its own mass of water, ensuring a very wet bog even if the water table is well below the surface. Also, its structure contains many large, dead cells alternating with photosynthetic cells and that is where the water is stored. Squeezing pushes water from these dead cells but leaves the living cells intact. And sphagnum’s outer shell of leaves can channel water upwards without any need for an internal transport system.
Many sphagnums are coloured, with cells containing the red anthocyanin pigment.
The spore-producing sporophytes are not easily spotted in sphagnums, but keep your eye out for one as you slowly sink into a bog in June or July. You will die happy, perhaps.
Liverworts
Most of the liverworts I encounter are flat, green structures seen clinging to a very wet surface such as a bridge’s support at water level. Look also alongside waterfalls or where springs emerge from soil. You may encounter them growing in nursery potted plants.
This is the gametophyte, and the structure is quite distinct from the mosses, although some more leafy ones occur clinging to the surface.
The reproductive structures are quite distinctive.
Bryophytes are not everyone’s favourite plants but they have their devotees. As a group they probably evolved from green algae about 430 million years ago, so have survived well and have even colonised tall trees in wet environments. Yet, lacking effective water transport and supporting tissues they remain close to their substrate and depend on water in which their male gametes can swim. They are poorly adapted to life on land. The next groups, including the ferns, can survive in drier locations and are bigger.
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