LICHENS drawing of lichen structure showing fungus hyphae around algal cells

LICHEN STRUCTURE

Lichens are not plants. They are composite organisms made up of cells of photosynthesizing algae and/or cyanobacteria held together by cobwebby filaments of various species of fungus, plus they're "jam-packed full of bacteria," as lichen researcher Toby Spribille says. Within lichens some bacteria provide defense, while others make vitamins and hormones. The above sketch gives a general idea of how threadlike fungal hyphae wrap around algal cells. The photomicrograph below shows how it really is.

algal and fungal cells in Umbilicaria lichen; image courtesy of Jon Houseman, via Wikimedia Commons Photomicrograph of cells in Umbilicaria lichen; A=Fungal layer (upper cortex), B=Medulla (algal cells), C=Algal cells; scale bar=0.02mm; image courtesy of Jon Houseman, via Wikimedia Commons

Despite the lichen's amazing structure and lack of recognition by the general public, they're abundant and important for the maintenance of the planetary biosphere.

In Merlin Sheldrake's much-read Entangled Life (Random House, 2020), we read that lichens encrust as much as 8% of the Earth's surface, which is more than that covered by tropical rainforests.

Golden-eye Lichen, TELOSCHISTES CHRYSOPHTHALMUS

At the right, in the pretty Golden-eye Lichen, Teloschistes chrysophthalmus, as is the case with other lichen species, the fungus part of the body provides form to the lichen and enables reproduction. The golden-topped, umbrella-like features are apothecia, which are fungal in nature, and release fungal spores. When a spore is released and finds a new home, if environmental conditions are good for germination, and if the appropriate species of algal cells or cells of cyanobacteria are nearby, a new Golden-eye Lichen may form. In exchange for the fungus's gifts of form and sexual reproduction, the algal and/or cyanobacteria cells photosynthesize food for the whole lichen body. It's a classic symbiotic relationship -- two or more different organisms in a mutually beneficial relationship.

KINDS OF LICHEN:

Wolf Moss, LETHARIA VULPINA

Lichenologist Walter Fertig at Arizona State University's Lichen Herbarium, on a page about lichens, tells us that possibly 17-30% of all fungus species are capable of becoming lichens. Since it's believed that the number of fungus species may be over 1.5 million, there may be at least 250,000 lichen species. One of those species is shown at the left. That's Wolf Moss, Letharia vulpina-- a lichen despite its name.

CRUSTOSE
(crusty)

FOLIOSE
(leafy)

FRUTICOSE
(shrubby)

Traditionally, the vast realm of lichen species has been divided into the three broad categories listed at the left, where the links lead to examples and information about each type. Nowadays other forms also are recognized but for our backyard needs, these three provide a good starting point.

Calcareous Rimmed Lichen, ASPICILIA CALCAREA, cracks in thallus

Both lichen species shown above -- the Golden-eye and the Wolf Moss -- are fruticose lichens, as their branching, bushy form suggests. The Calcareous Rimmed Lichen, Aspicilia calcarea, at the right, looking like a crust, is a crustose type.

LICHEN ECOLOGY:

three crustose lichens on a Mesquite twig

Ecologically, lichens often occupy niches that, at least sometime during the season, are so dry, or hot, or sterile, that little else will grow there. At the left you see an example, with maybe three crustose species on a Mesquite tree twig in hot, semi-arid southwestern Texas. Just imagine how most of the time very little water and nutrients are available on that twig.

Black Pit Lichen, VERRUCARIA NIGRESCENS

An even more hostile environment is that of naked rock exposed to intense dryness, temperature extremes and sunlight, such as is shown at the right. There the Black Pit Lichen, Verrucaria nigrescens, somehow thrives on a sun-blasted, wind-lashed, exposed limestone rock also in very hot, semi-arid southwestern Texas.

Black Pit Lichen, VERRUCARIA NIGRESCENS, microscopic view

Though water and wind erosion deform rocks and sometimes pit them, the above Black Pit Lichen contributes to the pit-making by secreting acids that dissolve the cement holding together the rock on which it grows. At the left, a much magnified image shows the Black Pit Lichen's thallus separated into individual warty bodies, which issue root-like appendages atop the naked rock. The pitting produced by this lichen surely has taken centuries to occur.

Powdery Twig Lichen, RAMALINA POLLINARIA

If a lichen species survives where other forms of life are unable to, then that species has evolved special adaptations. For example, many fruticose lichens, like the Powdery Twig Lichen, Ramalina pollinaria, at the right, not only are much branched but also bristle with slender projections. By increasing surface area exposed to the air, these branches and projections take water from the air when it's foggy or when dew covers things in early morning.

Crustose lichens on bare rock often begin a succession of communities, as described on one of our ecology pages. When your foot dislodges a patch of lichen from a rock, you may be undoing the patient work of centuries.

foliicolous lichen on leaf in the Yucatan, Mexico

Certain lichens, like the unidentified ones at the left, 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 are very vulnerable to pollution, as seen in the picture below:

Lichen death on roof

The roof's dark part is produced by lichens. The clear part lacks lichens. Probably air pollution from the chimney or mineral leakage from the metal chimney caused the lichens to die. "Mineral leakage" from the chimney cap may be the main cause because nowadays "zinc strips" and galvanized flashing on roofs are marketed as relatively safe and inexpensive ways to "... effectively kill or retard the growth of mosses and fungi and appear to have effect up to 15 feet below the zinc flashing along the length of the flashing," as one ad for zinc flashing says.

LICHEN REPRODUCTION:

Lichens reproduce in two ways: sexually and asexually.

apothecia on Eastern Speckled Shield Lichen, PUNCTELIA BOLLIANA

Sexual reproduction is handled by the lichen's fungal part. The great majority of lichen species belong to the fungus phylum Ascomycota, in which spores are produced in a bowl-like apothecium such as the ones at the right, of the Eastern Speckled Shield Lichen, Punctelia bolliana. If the spores land in a suitable environment in which to germinate, and if the fungal hyphae encounter the right species of alga and/or cyanobacteria, the hyphae will grow around the cells and a new lichen will begin developing. These are big IFS, with the consequence that many lichen species reproduce much more asexually than they do sexually.

Asexual, or vegetative, reproduction, therefore, is so important to lichens that among the various species it's accomplished in various ways.

The technical literature mentions yet other kinds of asexual lichen reproduction, but they're hard to identify in the field and often there's debate about how they function. For example, there's schizidia, phyllidia, folioles and blastidia, and who knows what else?

MORE INFORMATION:

On the Web, check out these sites:

LichenPortal.Org
Lichens of North America
Wikipedia's Lichen Page
US Forest Service "Learn About Lichens" page
Introduction to Lichens (UC Berkeley)
Wikipedia's Lichens of the United States
Wikipedia's Lichens of Europe page
Glossary of Lichen Terms by British Lichen Society
Jim's field notes on lichens