During the last week, I posted some articles about different Lichens. So, it's a good time to take a closer look on some basic facts about these very interesting organisms. I did this already in my post about the Field trip to “Ruhr University” in Bochum, Western Germany. However, this was over a year and it was also only a short introduction about these highly complex topic. On the other side, to tell you everything about lichens would be a huge task which I cannot master in only one Article. So, this time, I want to confine myself about only two aspects of the Lichenology: anatomy and propagation.
1) Basic Taxonomy
Lichen aren't plants but fungi. They belong to the Division of Ascomycota (Schlauchpilze in German). This is one of the largest Divisions within this kingdom; many other well-known species like the mildews (Aspergillus and Penicillium) belong to this division.
Within the Ascomycota, the majority (but not all) of Lichens belong to the taxonomical class Lecanoromycetes . Another lichen-ized class within the Ascomycota are e. g. the Lichnomycetes,
Overall, the Lichens could be itemized into following pedigree.
So, lichens are fungi. The term “lichen” refers more particularly to a non-taxonomic group of fungi from the Ascomycota, which live in a symbiosis with different algae (or sometimes Cyanobacteria). This symbiosis allows lichens, which have normally no chlorophyll (because they are fungi), to make photosynthesis and opens a new source for nutrients for them.
It should be said, that the fungus (the Mycosymbionr) benefits much more from the symbiosis than the algae (the Photosymbiont). However, the Photosymbiont also benefits from the protection of the lichen. The fungus protects them e. g. befor dehydration.
As fungi, each lichen has a characteristic body of vegetative tissue, which is a complex network of hyphae. In Lichenology, this network is called Thallus. Each Thallus consists of several layers, which may vary in strength and thickness (depending on the species). These layers are:
- Upper cortex: The upper cortex is the first, real layer of the Thallus. It's a thin and dense network of hyphae, which are in direct contact to the environment of the lichen. It protects the lower layers.
- Algae cortex: this is a much looser network of hyphae, which is also the home of the symbiotic algae. They are locatet in the gaps between the hyphae. Thereby, this layer is also the place were the lichen gets its nutrients from the algae. This exchange happens in different ways and varies from species to species. For example, some lichen simply grow around the algae and take their nutrients passive, while other ones entwines the symbiont completely. Finally there are also lichens, which penetrate into the algae per Haustoria.
- Marrow: this is the thickest layer of the lichen. It's a loose network of hyphae without any algae.
- Lower cortex: The lower cortex lies directly on the substrate. Its anatomy is similar to the upper cortex (a dense network of hyphae). The main function of the lower cortex is to connect the lichen with the substrate (rocks, cobblestones or bark). For this purpose, there are long, thread-like outgrowths, with which the lichen climps to the substrate. In Lichenology, these extensions are called Rhizines, because they remind roots. Rhizines can also spring from the marrow.
Depending on the growth of the Thallus, lichens are also subdivided into different non-taxonomic categories. The main types are:
- Crusty Lichen: This kind of lichen has a flat, crusty Thallus, which lies directly on the substrate. In Lichenology, such a Thallus is called Aerole. Crusty lichens are very common on rocks and stones but also grows on the bark of trees. The Thallus doesn't need to grow contiguously and small, grainy pieces are also possible.
- Foliose lichen: The Thallus is flat but more leaf-like and lies more loose on the subtrate. Foliose lichen may vary greatly in form and shape and can grow on many, different substrates like leaves, barks and stones.
- Branched lichen: The Thallus of a branched lichen is subdivided into a primary and secondary Thallus. The primary Thallus is the same as in the foliose lichen. On it, the secondary Thallus grows as small branches. These branches are also called Podetium.
- Gelatinous lichen: This special type results from the symbiosis between the fungi and cyanobacteria. Cyanobacteria have the property to swell, if they become wet. Thereby, the Thallus has a gelatinous habitus and often a dark violet color.
a gelatinous lichen; note the dark color of theCyanobacteria
It's important to say, that this is only a short extract from the anatomy of the lichens. It's a high and very complex topic, which is almost impossible to tell completely in a simple blog post. Anyway, I hope I could give you an overview about the basic and most important terms. Now, it's time to take a look at the propagation of the Lichens.
3) Propagation & Reproduction
a) Sexual reproduction
Because they are fungi, the reproduction of lichens goes mainly about spores. Because lichens belongs to the Ascomycota, it's important to understand the basic reproduction of this class of fungi. The Acomycota have a very complex reproduction cycle which combines asexual and sexual propagation. They spend the most of their lifetime in the haploid phase and have only one set of chromosomes, what is typical for many lower organisms (especially fungi).
Xanthoria parietina - with Apothecia
However, sexual reproduction is analog to animals or plants. It depends on the encounter of two different but mutually compatible hyphae. This compatibility is similar to our genders, but the way how the hyphae recognize a compatible partner is highly complex and nearly unclear.
At the Ascomycota, it's not unusual for a fungi to concentrate many compatible cells a single segment of its Thallus. This structure is also called Ascogonium, because it reminds to the Archegonia of higher plants.
The cells of two compatible hyphae fused with their cytoplasma. This isn't the same as the fusion of the egg and sperm, because the cores of the cells didn't fused. In Cytology, the fusion of the cytoplasma is called Plasmogamy.
After the Plasmogamy, the cells starts to duplicity their chromosomes in high numbers. Meanwhile, the Ascogonium starts to build up a dense network of hyphae, which is called Apothecium. At the lichens, an Apothecium has normally a cup like structure, while the tissue of an Apothecium with the fertile spores is called Hymenium.
Shematic drawing of an Apothecium
a(m) = asci with pre-meitotic spores;
a(r) = asci with ripe spores
But the Apothecium isn't the only reproduction tissue. Some lichens didn't form such a cup-like structure and the Asci are simply embedded within the Thallus. In Mycology, these structures are known as Perithecium. In this case, the spores are simply released per pores.
Shematic drawing of a Perithecium
During the duplication of their cores, the two hyphae have a high exchange of genetic material. Per Meiosis, the new combined genes are packed in spores (Ascospores), which are now the primary gene carrier. At the Ascomycota, these spores are created in long, tube like structures, which are called Asci (sing. Ascus; Latin for “tube”). These Asci are an unique feature of the Ascomycota and the reason for the name.
Shematic drawing of an Ascus
a = ascus, as = ascospore, h = hyphae
When the spores become ripe, they are released from the asci, fly away and the life-cycle starts again.
As you can see, the reproduction cycle is very complex but I hope, that I was able to decipher it a little bit for you. Just keep in mind, that the sexual way goes per spores and that these spores are created in the Ascus, which is a unique feature of the Ascomycota and the lichens.
b) vegetative reproduction
Last but not least, we look at the vegetative reproduction. In contrast to animals and plants, the sexual way isn't the dominant way of propagation at the fungi; just only one way. Asexual propagation one the other side is very common. In the most ways, fungi simple widespread their spores without any mating or genetic recombination. In this case, a fast spreading is more important than genetic variety.
As fungi, lichens have also developed some ways of vegetative propagation. Two of the most important ways, which I have also mentioned before, are the Sorals and the Isidia.
- Sorals: Sorals are simply small cracks on the surface of the Thallus. Through these cracks, small packages from the inner tissue can be released to the environment. In Lichenology, such a package from a soral is called Soredium. Basically, a Soredium is a Photosymbiont, which is wrapped by hyphae. A Soredium is spread out similar to a spore (through water or wind) and starts to grow on the new place, where it lands.Shematic drawing of a soral
l(ub) = upper layer, sr = soral, s = soredium
l(a) = algae layer, l(m) = marrowShematic drawing of an Sorediumh = hyphae; s = symbiont
- Isidium: An Isidium is simply a piece of the Thallus, which has broken off and grows now independent from the rest of the lichen. The principle is similar to the off-shoots of the higher plants.
So, I hope, that I was able to give you and actual and basic knowledge about the anatomy and propagation of the lichens. As you can see, it's a very high and complex topic, but if you understand some basics, you also understand, that these inconspicuous organisms, which you can find everywhere outside, a very interesting. It's not always necessary to travel to exotic places, if you want to see some fascinating species.
Spring dawns soon and with it this year's vegetation period. So, the lichens (and mosses) were also a good winter topic. In the next weeks, I will introduce some other species before the higher plants will return.