Lichens are not plants. They are "composite organisms" made up of two, or maybe three or four, completely different kinds of organisms. It's as if you combined an animal such as a dog with a plant such as an oak, maybe with a fungus thrown in as well, and ended up with something completely different from animals, plants and fungi.
Every lichen species is part fungus. Usually the other species is a photosynthesizing alga, but sometimes it can be a photosynthesizing bacterium known as a cyanobacterium. Sometimes all three kinds of organisms are found in one lichen. The above drawing gives an idea of what fungal hyphae wrapping around alga cells might look like at the microscopic level.
In this amazing association the fungus benefits from the algae because fungi, having no chlorophyll, can't photosynthesize their own food. A lichen's fungal part is thus "fed" by its photosynthesizing algal part. The alga and/or cyanobacterium benefit from the association because the fungus is better able to find, soak up, and retain water and nutrients than they. Also, the fungus provides the resulting lichen shape, and the reproductive structures. This kind of relationship between two or more organisms, where all organisms benefit, is known as mutualism.
The main body of a lichen is called a thallus.
At the left you see the British Soldier Lichen, Cladonia cristatella. It's only about ¼-inch high (6 mm). In this common lichen the red spore-producing reproductive structures are clearly visible. The lichen's name, Cladonia cristatella, is actually the name of the fungus. The alga species in the lichen is known as Trebouxia erici. However, it's customary to name a lichen after its fungal part, so the whole lichen is known as Cladonia cristatella. British Soldiers are usually found on decaying wood, soil, mossy logs, tree bases, and stumps. They help break down old wood and put nutrients back into the soil where they can be used by plants. Lichens also take nitrogen from the air and put it into the soil so plants can use it.
Lichens are very sensitive to air pollution, so if your town has dirty air your backyard may not have many lichens to study. Moreover, unless you know what you're looking for, you can be staring right at a healthy lichen and not even know it. On the other hand, the picture at the right is a scanning I made of a lichen-covered twig fallen from a big Pecan tree above where I live (only a little magnified) so obviously there's some interesting stuff here for those of us living where air pollution isn't so bad. Let's start organizing our thoughts about the matter...
Traditionally three broad categories of lichen have been recognized: crustose (crusty), foliose (leafy),and; fruticose (shrubby). Nowadays sometimes other forms are recognized.
For example, very simple lichens for which fruiting bodies have never been observed, looking like no more than powdery patches, are known as leprose lichens. Sometimes crustose lichens develop blister-like "squamules," where part of the plant body, or thallus, lifts off the substrate on which the lichens grow. Such lichens can be squamulose lichens. There are also filamentous and gelatinous types.
However, for our introductory, backyard needs, we can stick with the traditional three lichen categories. Click on the following links to see some examples of each kind:
Ecologically, lichens are important because they often occupy niches that, at least sometime during the season, are so dry, or hot, or sterile, that nothing else will grow there. For example, often the only plant growing on a bare rock will be a crustose lichen.
That crustose lichen will be patiently collecting around and beneath itself tiny amounts of moisture, and mineral and organic fragments. When freezing temperatures come, the lichen's collected water will expand as it forms ice and maybe this expanding action will pry off a few more mineral particles from the rock below the lichen, thus making more soil. The water itself is a bit acidic, plus humic acids from the organic matter collected by the lichen will also be acidic, so these acids will likewise eat away at the stone.
Over a period of perhaps many years, even centuries, the lichen gathers an extremely thin and fragile hint of a soil around it. As the lichen grows the soil-producing processes speeds up and takes place over an ever-larger area.. Eventually other more complex plants, perhaps a foliose or fruticose lichen, or mosses or ferns, or even some form of flowering plant, may take root in the modest soil and replace the crustose lichen.
Thus crustose lichens on bare rock often begin a succession of communities, as described on one of our ecology pages. And when your heel dislodges a patch of lichen from a rock, you may be undoing the patient work of centuries...
Certain lichens live on leaves, sometimes as parasites. These special leaf-living lichens are known as foliicolous lichens (not foliose). You might enjoy downloading a free, well-illustrated field guide to foliicolous lichens, in PDF format, presented by the Field Museum of Chicago.
Lichens reproduce in two ways:
On the Web, take a look at these sites:
Lichens of North
Oregon State University's "Fun with Lichens"
Introduction to Lichens (UC Berkeley)
American Bryological & Lichenological Society
Arizona State University: Lichen Herbarium
Arizona State's Page of Many Lichen Web-page Links
North American Lichen Checklist
Lichens of Ireland/ Biology of Lichens
Search Recent Lichen Literature
More lichen books: