Fungi are a diverse group of organisms that are in their own kingdom Fungi , separate from plants. Fungi do not contain chlorophyll or any other means of producing their own food so they rely on other organisms for nutrition. Fungi are widely known for their role in the decomposition of organic matter. They are also necessary for the survival of the ecosystem around them, such as partnering with plants and trees for nutrients and survival. Lichens are another such partnership for fungi to gain nutrients from another organism.
The algal partner photosynthesizes and provides food for the fungus, so it can grow and spread. Sclerotia veratri , a cup fungus. These types of fungi are the most common fungal partner in lichen biology. Photo by Chris Wagner, U. Forest Service. Algae are in another kingdom Protista separate from plants and fungi. There are several types of algae: green, brown, red, gold. They can survive in salt water and in freshwater on their own, and in any environment when part of a lichen relationship.
Although cyanobacteria are called blue-green algae, they are actually bacteria, and are part of the bacteria kingdom, Monera. The "blue" in the common name refers to the fact that they need to live in water, and "green algae" refers to their photosynthetic abilities, like green algae. Peltigera britannica , dog-pelt lichen.
Notice the bright green surface that is green algae showing through. Look closely and you will see dark spots. Those spots are pockets of cyanobacteria. Photo by Karen Dillman, U. Isn't lichen that mossy stuff on rocks and trees? When people think of lichens, many of them think of them as a kind of moss. That could not be further from the truth.
Although moss and lichens are both called non-vascular plants, only mosses are plants. Mosses are included in a group of non-vascular plants called bryophytes. Mosses are believed to be the ancestors of the plants we see today, like trees, flowers, and ferns. Lichens, on the other hand, are not similar in any way to mosses or other members of the plant kingdom.
Cladina arbuscula , a lichen, is also known as reindeer moss. For example, Lobaria pulmonaria is normally in a shaded environment, yet when it grows in an exposed environment, the color is different, usually darker, and browner. Different species that adapt to brighter, hotter environments are generally more pigmented. This could be a mechanism of the fungus to protect the algae from getting too much light and burning out.
Umbilicaria phaea , rock tripe, is a rock lichen that commonly occurs on exposed surfaces. Lichens need homes too! Every lichen lives on top of something else. The surface of that "something else" is called a substrate. Just about anything that holds still long enough for a lichen to attach to and grow is a suitable substrate. Trees, rocks, soil, houses, tombstones, cars, old farm equipment and more can be substrates. The most common natural substrates are trees, rocks, and soil. Rocks are a natural substrate for lichens, as well as moss and ferns.
What kind of growth form are these lichens? Group of boulders with crustose lichens. Having lichens growing on your rocks, trees and ground around your property is a good thing. That means the air you breathe in is healthy and clean. Although lichens can cause some damage to buildings and man-made structures, it is a very slow process and does not endanger those substrates.
Soil is another important substrate for lichens. It provides moisture, nutrients, space to grow, and depending on the location, shelter as well. One unique habitat lichens can colonize is dune systems. If stable for a long enough time, shifting sands can be "held down" by soil crusts, allowing other communities to establish themselves over the top. Soil crusts consist of cyanobacteria, mosses, and lichens. Be careful, though. Once these soil crusts are disturbed, they do not come back for many years and the process has to start over again.
The shifting sands themselves pose a risk by blowing over the crust communities and covering them up, preventing light from getting to the organisms underneath and killing them. Letharia vulpina , wolf lichen, on tree bark. The rough surface and stability of the substrate lends itself to lichen colonization. Breadcrumb Home Celebrating Wildflowers. The thalli consist of more or less circular disks, rather than of erect lobes.
The discoid thalli, shown here , are from two to eight millimetres in diameter and, though different in gross morphology, match the usual forms of this species in both chemistry and micro-morphology. Typical forms of Siphula coriacea were found elsewhere in the same general area. All of these expressions may be called usefully imprecise descriptive terms.
They are useful in the same way that expressions such as shrub and tree are useful when talking about plants. They are imprecise in that sometimes it may be difficult to place a particular specimen in a particular growth form 'pigeon-hole'. Similarly, occasionally you may wonder which of shrub or tree is the better term to describe a particular plant.
What of the byssoid lichens? Logically you could argue that a byssoid growth form is three dimensional as is a clump of cotton wool and so a byssoid lichen is really just a very delicate fruticose lichen. Some lichenologists consider squamulose and placodioid forms as simply variants of crustose.
As you can see there is a variety of terms more than listed above and some debate over the boundaries between them. It is useful to be aware of these issues, since different books or websites may use some terms in slightly different senses, but there is no point in getting bogged down in terminology. 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'.
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