For most purposes it is enough to be comfortable with the terms crustose, foliose, fruticose and squamulose as defined above. In general a particular species will show the same growth form, no matter where it grows. Occasionally, for some reason perhaps genetic, perhaps environmental , a species that is usually, say, crustose might grow in a fruticose form.
Such occasional, but dramatic, differences in growth form in the one species are well-known to many gardeners. A plant species that usually grows as a tree may be found growing in, say, a prostrate form.
Often such plant variants are highly valued horticulturally and propagated vegetatively to preserve the variant form and sold as cultivars of the species in question. You may come across the terms macro-lichen and micro-lichen. These are two more examples of usefully imprecise terms.
Roughly speaking a macro-lichen is one that is foliose or fruticose and the rest are micro-lichens. Note that this has nothing to do with size, despite the impression given by the prefixes macro and micro. A species that typically grows as a foliose form to say a centimetre diameter would be a macro-lichen whereas a crustose species that typically grows to over 10 centimetres in diameter would be a micro-lichen. In the Usnea photograph above you can see a prominent smooth, circular disk.
If you look at this photograph of the foliose lichen Paraparmelia lithophiloides , you'll see that much of it is grey to blackish but there are also a number of brown disks. In those disks, called apothecia , the fungal partner produces spores and the apothecia are part of the fungal reproduction process.
The bulk of each lichen that is, the branches in Usnea and the grey to blackish areas in Paraparmelia lithophiloides is called the thallus and is known as the vegetative part of the lichen. The thallus is composed of fungal and photobiont cells, so well united as to give the impression that you are looking at just one organism. In most lichens it is the thallus that is dominant and when talking about lichen growth forms it is always the thallus that is being described.
There'll be more about apothecia and other spore-producing structures a little further on. For the moment, let's concentrate on the thallus of Paraparmelia lithophiloides. This is a foliose lichen so it is more-or-less flat in form so let's see what the thallus looks like in cross-section. The upper surface is composed of compacted hyphae and this band of compacted hyphae is called a cortex. Below the cortex is a band of photobiont cells and below that is the medulla , an area of loosely arranged hyphae.
It is in the medulla that the fungus stores the nutrients it has "harvested" from the photobiont. Below the medulla is the lower surface of the thallus, composed of compacted hyphae and constituting another cortex.
From the lower cortex root-like bundles of hyphae, called rhizines , anchor the thallus to the substrate. You find this sort of structure in many foliose lichens.
The thallus of Paraparmelia lithophiloides has an upper cortex and a lower cortex and that is the norm in foliose lichens. On the other hand, a crustose lichen lacks a lower cortex. It is meaningless to talk of upper and lower sides in the branches of a fruticose lichen. In such lichens any cortex would constitute the outermost band of each branch, with the photobiont cells typically immediately inward from the cortex and the medulla occupying the central area within the branch.
While a cortex or two and rhizines are features you will find in a great many lichens, there are species that lack rhizines or are without a cortex. In species with rhizines the density of rhizines is variable between species. There are those species that have few and sparsely arranged rhizines while in others the rhizines can be quite dense.
When present a cortex may be anything from very scanty to very well developed, depending on the species. In most lichens the photobiont cells are arranged in one band but in a small number of genera the photobiont cells are scattered randomly throughout the thallus. Lichens may reproduce asexually or vegetatively by several methods. A fragment broken off from a lichen thallus may grow into a new thallus. This is a means of vegetative propagation, the new thallus being genetically identical to the thallus from which the fragment came.
Many lichens are brittle when dry and are therefore easily fragmented, for example by some animal stepping on a dry thallus.
Obviously fragmentation is especially easy with the foliose and crustose species. Fragmentation could be described as 'accidental' vegetative reproduction. There are also other, more specialized, means of vegetative reproduction. The surface of a thallus may show minute, powdery granules called soredia , each soredium consisting of a few photobiont cells surrounded by fungal filaments.
Also, the thallus may produce tiny, simple or branched spiny outgrowths called isidia , again a mixture of fungal and photobiont cells.
The isidia are easily broken and both they and the soredia are easily dispersed and contain everything needed to produce new thalli. There are species which produce neither soredia nor isidia, others produce both and yet others will produce only one of the two. Only the fungal partner reproduces sexually, with the spores often produced in a long-lived saucer-like structure called an apothecium , which is easily visible to the naked eye in many species.
Instead of apothecia various lichens produce their fungal spores in perithecia, a perithecium being a small, and typically black, hemispherical pustule within which the asci are produced. A group of lichens with striking spore producing structures are the so-called graphid lichens, which produce their fungal spores in apothecia that are elongated and narrow and are called lirellae.
Lirellae look like short scribbles on the thallus and the term graphid is derived from the classical Greek word for 'writing'. Spores or vegetative propagules may be dispersed by various agents. Fungal spores are quite small and it is easy to understand that, once ejected into the air, they could be easily carried away by even the slightest of breezes.
Obviously water is another potential dispersal agent, and animals are a third. For example, migratory birds may pick up vegetative propagules inadvertently and carry them considerable distances. Various distribution patterns do show themselves. There are endemic Australian species, Australasian species, Gondwanan species, bi-polar species, virtually cosmopolitan species and numerous other patterns.
Some of the widespread species are undoubtedly naturally widespread while others will have been dispersed unintentionally by humans. There are various organisms which, though not lichens, might be mistaken for lichens. Sometimes it is only the beginner in lichen studies who would be confused but at other times even an experienced lichenologist would need to examine a specimen microscopically to be sure.
There are several skin conditions which include the word lichen in their names and some examples are: lichen planopilaris, lichen planus, lichen ruber, lichen sclerosus and lichen simplex chronicus. The symptoms may include one or more of inflammation, itchiness, lesions, rashes or thickened skin and those medical conditions have no connection with the lichens of this website other than the name, The English word lichen is derived from a classical Greek word which already had a two-fold meaning, one denoting organisms growing on trees and the other a pustular skin disorder.
In the distant past the English word moss and the equivalent words in some other European languages were used in a very general sense to denote a variety of non-flowering organisms. Lichen-forming fungi mostly belong to a group called the ascomycetes.
But in , Spribille and his colleague Veera Tuovinen, of Uppsala University, found that the largest and most species-rich group of lichens harbored a second fungus, from a very different group called Cyphobasidium. For many, it was a game-changing discovery. To him, it seemed more that the lichens he studied have three core partners. Look on the bark of conifers in the Pacific Northwest, and you will quickly spot wolf lichens—tennis-ball green and highly branched, like some discarded alien nervous system.
When Tuovinen looked at these under a microscope, she found a group of fungal cells that were neither ascos nor cyphos. Wolf lichens, it turns out, contain yet another fungus, known as Tremella. Over the years, other lichenologists have detected Tremella in wolf lichens, but only ever in three specimens, and only in the context of abnormal swollen structures called galls. It seems to make extremely close contact with the algae, hinting at some kind of intimate relationship.
Tuovinen analyzed more than specimens of wolf lichens from the U. Wolf lichens are among the most intensively studied of all lichens, so how could such a ubiquitous component have been largely missed? And she knew to do that only after finding those genes amid wolf lichen DNA.
Earlier genetic studies, she says, might have missed them because they had specifically focused on the genes of the ascos. The alternative is that Tremella is a core part of the lichen. But she argues that lichenologists have too readily downplayed such organisms. Read: The ex-anarchist construction worker who became a world-renowned scientist. The role of the photobiont in lichens is clear — to provide carbon in the form of simple sugars. These sugars are used by the fungi to maintain physiological functions, to grow, and reproduce.
However, in the case of lichens with both green algae and cyanobacteria, the lichen gets an added nutrient input from the cyanobacteria in the form of fixed nitrogen. Although lichens can probably access inorganic nitrogen from the atmosphere directly, it can be a limiting nutrient, so having an internal source can be an advantage especially in heavily leached environments.
Only about species of photobionts are commonly found across all known lichens, representing 4 main genera. The vast majority of photobionts are from the genus Trebouxia , followed by Trentopohlia both Chlorophyta , Nostoc and Scytonema both Cyanobacteria.
Most green-algal photobionts are unicellular green forms, but small colonial types and filamentous algae occur as well.
Within the lichen thallus, most photobionts have a different morphology than they would when grown in isolation, so few photobionts can be reliably identified using traditional microscopic methods. Instead, it is best to rely on culturing studies, and more often, on molecular methods, as many different strains have very similar morphology.
The jelly-lichens are one exception, where the chain-of-pearls structure of Nostoc is very clear under the microscope. For example, the same fungal species will use different photobionts in different ecological settings, even within similar geographic areas. Some individual lichens even contain more than a single photobiont strain, a situation that has been better explored in other symbiotic systems. In corals and other marine reef organisms, the animals can regulate the populations of their varying photobionts to maximize photosynthetic output according to environmental variation.
Skip to main content Skip to search. The British Lichen Society toggle. Main menu. You are here. Search form Search. What is a Lichen? Which fungi form lichens? Many unrelated and very different fungi form lichens, including mushroom-forming fungi, and especially cup-fungi.
Lichen photobionts are the green algae or cyanobacteria that provide the simple sugars to their fungal partners. Lichen fungi specialise on particular photobionts. Typically they only associate with a small group of related species, though they may associate flexibly with different photobionts according to their environmental situation.
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