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Lichens (2018): Home

What is a Lichen?

Lichens are the plant-like organisms you find growing on trees, walls, and sidewalks, in speckles or clumps. They may be white, brown, orange, green, or black and take one of three forms: foliose (leafy), crustose (flaky, like chipped paint), and fruticose (branching, like coral or a shrub). They grow all over the world in every type of habitat, from desert to Arctic tundra. But what are they?  Lichens aren’t plants at all. They are composite organisms of fungi and algae that live together in a mutually beneficial relationship. Algae provide nutrients to the fungus, and the fungus protects the algae by sheltering it from light and locking in moisture.

This species of lichen is fruticose, with a branched, coral-like structure. It is dried and mounted on paper for preservation in a herbarium, a collection of dried plants and fungus used for study. The labels on this voucher reveal the history of this particular specimen and the many herbaria through which it passed before it was donated to NYBG’s Steere Herbarium


Herbarium specimen of Letharia columbiana, 1907

William and Lynda Steere Herbarium


Christian Andreas Besemann (1760–1818)

Tab. IX: Parmelia, Borrera

Hand-colored engraving

In Lichenographia universalis by Erik Acharius

Gottingae: apud I. F. Danckwerts, 1810

[Reproduction of original from the LuEsther T. Mertz Library]


These 19th-century diagrams illustrate the internal structure of several lichens. Cross-sections show how algae is enveloped by a fungus. The fungus bears sexual spores (reproductive parts) inside the cup-shaped growths, similar to those seen in the dried specimen nearby.


This is an example of a leafy, or foliose, lichen. Though it resembles a leafy plant, lichens are not plants and do not have real leaves. This lichen gets its green color from the algae that sandwiches its fungus core.

Cetrelia chicitae

Photograph by James C. Lendemer


History of Lichenology

Erik Acharius (1757–1819), a Swedish botanist, is considered the father of lichenology, the study of lichens. Acharius was the first to study lichens seriously and categorize them into genera and species, giving them proper Latin names. This book, published in 1814, was the last of his major works and includes descriptions of all lichen species known to Europeans at that time.

Johan Gustaf Ruckman (1780–1862)

E. Acharius


In Synopsis methodica lichenum by Erik Acharius

Lundae: Litteris et Sumptibus Svanborg, 1814


Carl Frederik Akrell (1779–1868)

Tab. V

Hand-colored engraving

In Methodus qua omnes detectos Lichenes by Erik Acharius

Stockholmiae: impensis F.D.D. Ulrich, typis C.F. Marquard, 1803

[Reproduction of original from the LuEsther T. Mertz Library]


Acharius’ findings form the cornerstone of lichenology. In his lifetime he collected more than 5,500 lichen specimens and categorized them into more than 3,000 species. The hand-painted illustrations on this page depict foliose (leafy) lichens with red cups.

Lichens and other fungi are interspersed in this spread. Historically scientists believed that they were plants, so mycology (the study of fungus) and lichenology were branches of botany (the study of plants). Today we know that fungi are more closely related to animals than plants, and we categorize them separately from all other forms of life. Still, mycology and lichenology remain associated with botany and NYBG collects and studies fungus and lichens as well as plants.


This folio is taken from a 19th-century German textbook about the natural history of the plant kingdom, including 601 illustrations. The book’s crowded layout, with some plants overlapping one another, is unusual for an educational text.

Gotthilf Heinrich von Schubert (1780–1860)

Plate LII

Colored lithograph

In Naturgeschichte des Pflanzenreichs in Bilder, edited by Christian Ferdinand Friedrich Hochstetter (1787–1860)

Stuttgart: Schreiber & Schill, 1854

[Reproduction of original from the LuEsther T. Mertz Library]

Lichen Dyes

Lichens have been used in medicine, food, and even liquor and cosmetics. More commonly, lichens are used for dying fabrics. There are two types of lichen dye: boiling water dyes, which produce yellows and browns; and fermentation dyes, which produce reds and purples. Unfortunately many lichens are slow-growing and rare, so cannot be ethically harvested on a large scale.

This guide to Swedish lichens and their uses includes 24 colored charts depicting the dyes that can be extracted from each species. The author, Johan Peter Westring (1753–1833), was the physician to the king of Sweden as well as a botanist and lichenologist. Not all lichens can produce dye, but one species may yield a variety of colors.

Carl Wilhelm Fröberg Venus (1770–1851)

Mjolklaf (Lichen lacteus)

Hand-colored engraving

In Svenska lafvarnas färghistoria by Johan Peter Westring

Stockholm: tryckt hos Carl Delén, 1805

[Reproduction of original from the LuEsther T. Mertz Library]


The other items exhibited in this case are listed in the Readings Related to Lichens. 

Lichen Studies Today

Contemporary lichenologists continue to identify and describe new lichens. They also apply lichenology to other scientific fields, including studies of the environment. Lichens are bioindicators. This means that their health is a good indicator of the general state of the environment. Lichens absorb pollutants, and are even used in air-quality studies to determine the types and levels of pollution in an area. A shift in lichen health or the absence of lichens in a region—a “lichen desert”—indicates the decline of their ecosystem. Abundant lichens indicate higher air quality.

Lichens are small organisms that often go unnoticed, but field guides demonstrate their diversity and unusual beauty. Lichen handbooks for laypeople have been published all over the world, from Romania to Japan to the Bronx.

James C. Lendemer, Ph.D., is NYBG’s resident lichenologist. His most recent publication is this forthcoming guide to the lichens of Delmarva, the East Coast peninsula that comprises Delaware and parts of Maryland and Virginia. Pictured on the cover is Lobaria quercizans, a leafy lichen that has disappeared from much of the eastern United States in the last century.


James C. Lendemer and Nastassja Noell

Delmarva Lichens: An Illustrated Manual

Memoirs of the Torrey Botanical Society Vol. 28

Torrey Botanical Society, 2018



The other items exhibited in this case are listed in the Readings Related to Lichens. 

Display documentation

Photograph of library rare book room

Rare Book Room Window: Panel 1

Photograph of library rare book room

Rare Book Room Window: Panel 2

Photograph of library rare book room

Rare Book Room Window: Panel 3

Photograph of library rare book room

Rare Book Room Window: Panel 4

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Plant Information

Readings Related to Lichens and the exhibit


This exhibit was created by Samantha D'Acunto and Dr. James C.Lendemer with editorial contributions from Susan Fraser, Joanna Groarke and Victoria Lewis. Kelsey Miller worked to design and install the physical component of this display in the LuEsther T. Mertz Library.

Images courtesy of Marlon Co and Dr. James C.Lendemer. Yarn dyed with common rock tripe courtesy of Zachary Muscavitch.

Thank you to everyone who worked on this exhibit.