On Lichens

Topics: Table of Contents
fructiose podentia

above and below

The lichen thallus is a vegetative plant body of remarkable complexity composed of two organisms. The fungal component (mycobiont) is an Ascomycete or Basidiomycete that has succeeded in establishing a symbiotic relationship with green algae or blue-green algae (phycobiont). When these diverse components combine, the resulting thallus forms a discrete, long-lived organism that appears to be a distinct plant, bearing no resemblance to either non-lichenized fungi or algae. The fungal and algal elements can be recognized only when the thallus is sectioned and examined under a microscope. Here, the algae show up as the dark spots:


Concepts of Symbiosis
Parasitism. A parasite is a heterotrophic organism that is able to establish a nutritional relationship with a host, a relationship that is favorable to itself but usually unfavorable to the host. …Schwendener envisioned the fungus in lichens to be parasitic on the enslaved algae and described this condition as helotism. [helot = neither slave nor free; from the name for a class of serfs in ancient Sparta.]

[To understand lichen, some scientists posit that fungi must join in the relationship because they are able to live off of dead algae and therefore would be guaranteed a food supply. The scientists who (in the absence of other data) believe this hypothesis is a more plausible form of symbiosis than the others clearly believe that symbiosis is usually not mutually advantageous, but rather advantageous for one and harmless for the other. For more on this, see below.]

Mutualism [or symbiosis]. The term mutualism describes a mutually beneficial relationship where one or both components may be dependent on the association for survival. Reinke in 1872 had called this union consortism, and a lichen is commonly referred to as a consortium. …An important feature of biotrophism is that one member of the pair cannot grow alone, but is dependent on the other not only for nutrients but also for specific growth factors (Hale, pp. 67-68).

Hale seems to support the symbiosis hypothesis:

69 The advantages that accrue to the algae seem fairly obvious. They gain mechanical protection from the elements by being tightly enveloped by hyphae and should also benefit from somewhat improved water relations and resistance to desiccation, although there is no proof that free-living algae cannot withstand dry periods of up to 70 years. The opaque hyphae protect the algae from high irradiance, since there is a considerable reduction of light within the thallus.…

69 Aside from mechanical protection, the algae may improve their nutritional regimen by an exchange of metabolites with the fungus…. Being heterotrophic organisms, the fungi may be able to excrete as yet unidentified substances that promote photosynthesis in algae. The algae cannot help but benefit from the minerals that are accumulated in the lichen thallus in high concentrations, and apparently released during the rewetting phase.

71 [on advantages for the fungus] …Even when lichens grow on organic substrates such as humus or bark, the transport of organic nutrients to the thalus appears to be very limited and certainly insufficient to sustain the fungus. Vitamin deficiencies, widespread in fungi, are satisfied by thelae. Where the phycobiont is blue-green, nitrogen fixation provides additional sources of organic nitrogen to the fungus. …A lichenized fungus has a much greater life span than other fungi.… Being adapted to stressed pioneer environments, the symbiont thallus is actually highly sensitive to microclimatic changes which can easily upset the symbiotic balance. The fungus is wholly dependent on symbiosis with algae to form sexual fructifications, and for lichens which no longer have need for sexual reproduction, the dependence on clonal or vegetative reproduction largely eliminates gene exchange and hybridization. Even in species that reproduce only with spores the fungus must come into contact with a suitable alga to achieve resynthesis. …The sheer number of lichenized acomycetes, some 16,000-18,000 species, bespeaks the enormous success of symbiosis …

13 The most highly specialized endolithic lichens occur in Antarctica. Here a few species of Buellia and Lecidea occur below the rock surface, but form stratified, more or less continuous tissues between the rock crystals…

47 The morphogenesis of lichen thalli is one of the last unexplored areas in lichenology. Essentially, nothing is known of factors which enable or direct an amorphous mass of fungi and algae to form the highly differentiated, stable thallus. There are a few studies which trace developmental stages on the basis of a sequence of thalli in different stages in nature. In a recent study, Jahns utilized the scanning electron microscope to examine the initial stages of lichens on datable pine needles and terminal twigs of oak and ash. …Thin unbranched hyphae, presumed to originate from lichen spores, meander over the algal mat and form irregular masses with the algae which become recognizable as stratified thalli by the end of the second year. (See to right.)

crustose lichen, top view and cut through center with algae showing up as a dark band. Note the "feet" on the bottom attaching lichen to rock.
above: foliose lichen, top view and cut section through middle, with "feet" to attach to rock surface

lichen civitas


cities and highways and subways

(above) Hyphae presumed to originate from germinated lichen spores growing among clumps of green algae on a pine needle (47)

Lichens as Pioneers
Lichens have a well deserved reputation as pioneers in plant succession. They occur in virtually every pioneer terrestrial habitat from Arctic and Antarctic to tropical areas and in many deserts where they are able to form long-lived and stable communities. It is their adaptation to xeric environments that has enabled the majority of lichens to dominate in habitats where competition from other plants is very slight. This adaptability is closely correlated with the photophilous (light-loving) characteristics of most lichens. (84)
a closeup:
Medical Uses
During the Middle Ages lichens figured prominently in the herbals used by medical practitioners. According to the Doctrine of Signatures, Lobaria pulmonaria could be used to treat pulmonary diseases because of its superficial resemblance to lung tissue. Similarly, Parmelia sulcata, a foliose lichen with sulcate markings on the surface, vaguely resembling the surface of the brain, was eagerly sought as a remedy for cranial maladies. A widely accepted prescription for treating rabies called for a half-ounce of powdered Peltigera, a common foliose lichen growing on soil, mixed with two drachmas of black pepper. This mixture was taken on four consecutive days in a half-pint of warm milk. When Linnaeus named this particular lichen in 1753, he called it Lichen caninus, the Dog Lichen (135).

[The medieval doctrine of signatures held that a plant that visually resembled an external or internal body part might be able to be used to treat diseases of that part, due to the “correspondence” between the two in their external form, their “signature.” An assumption governing this belief was that the true identity of an object was revealed by its exterior since that exterior shape was ordained by God.]

[for more on the doctrine of signatures, See also Michel Foucault, The Order of Things, where he contrasts this medieval notion of identity with later more “inward” definitions stressing structure and then genetics. In contemporary biology, the Doctrine of Signatures has been transformed to the Doctrine of the Spiral Helix of Genetic Codes.]

fructiose podentia

above and below

Or Does Lichen Follow
a Colonial/Imperial Model?

“As the result of my researches, the lichens are not simple plants, not individuals in the ordinary sense of the word; they are, rather, colonies, which consist of hundreds of thousands of individuals, of which, however, one alone plays the master, while the rest, forever imprisoned, prepare the nutriment for themselves and their master. This master is a fungus of the class Ascomycetes, a parasite which is accustomed to live upon others’ work. Its slaves are green algae, which it has sought out, or indeed caught hold of, and compelled into its service. It surrounds them, as a spider its prey, with a fibrous net of narrow meshes, which is gradually converted into an impenetrable covering, but while the spider sucks its prey and leaves it dead, the fungus incites the algae found in its net to more rapid activity, even to more vigorous increase....”
—from Simon Schewendener's book on lichen published in German in Basel, Switzerland, 1869.

1. The English translation of the above has syntax that makes for curious logic: “As a result of my researches, ...the lichens are colonies,” rather than something like, “My research has discovered that the lichens are colonies....” Is the translation strictly accurate here? If so, there’s a hidden desire to do more than merely describe the world, a desire not unlike that attributed to the fungus.

2. Schewendener has hints of pity for the “slaves,” but sides with the masters and seems to be foretelling Germany’s own upcoming experiment with colonialism in Africa and elsewhere after unification in the next decade after Schewendener's book was published in 1869. Germany felt it needed a colonial empire to prove the virility of its new nationhood competing with rival European states, especially Britain, France, Portugal, and Holland. S. is particularly drawn to the idea that empires seem an advanced form of capitalism, able to increase the productivity levels of its workers, “pushed to more visible increase....” There is also implicitly here the rationalization for such brutal production quotas: the workers benefit as well as the bosses.

3. “The lichen ... are colonies” is a revealing solecism. Strictly speaking, Schewendener’s colonial trope is describing neither the colonies abroad nor the empire’s “home” turf, but rather a complex in which both imperial center and slave colonial production share the same spaces. That is, “lichen” is created from both fungus and algae and thus is not a distant colony but the empire itself, the network of colony and imperial center. Yet the metaphorically-charged writing above, despite its fascination with the seemingly invincible power of the “masters,” perhaps also suggests a fear of being too closely dependent upon the colony, of even being overthrown by it “hundreds of individuals” vs. “one alone” who “plays” the master. Especially intriguing is the phrase “vigorous increase,” which means to connote only those activities enforced and controlled by the master but which inevitably comes to suggest energy threatening to the Master’s power (?).

4. “of the class Ascomycetes...”: for a hallucinatory moment, it seems as if Schewendener is describing a Greek or Roman tyrant and the classical city-state empires that were seen as precedents for European colonial ambitions many centuries later. A Thucydides of Mycobionts!

5. cf. Vernon Ahmadjian, who argues for parasitism of the funghi on the algae, “using” its photosynthetic properties for its own ends. Yet if this is what it is, it is a parasitism that does not kill the “host” but actually gives it more protection than it would have in its “natural” state. See Vernon Ahmadjian’s The Lichen Symbiosis (New York: John Wiley, 1993).

6. The Blob ('50s Hollywood movie) as wiggling with U.S. fear of colonization

different kinds of lichen algae
fungi (longer "fingers") first combining with round algae, then intertwining and rapidly multiplying and building up mass (thalli)

Lichens and Hummingbirds
Hummingbird nests: the main material is down (milkweed, etc.), laced together and fastened to a branch with spiderweb pieces. As a finishing (and camouflaging touch) they are often covered with lichen bits.

Delicate Strengths

Symbiotic Plant & Planet: Food for Thought
Traditionally, we’ve been taught to speak of competition between individuals and competition among species—survival of the fittest as the prime mover of evolution.

It’s surely crucial. But what if an edge toward survival (and thriving in one’s eco-niche) could be achieved by symbiosis, cooperation, exchange? Examples? (Think of individuals in groups within a single species, then think of individuals from different species finding a balance: lichen the best-known example of the latter.)

Could it be that symbiosis is just as important as competition in helping the code survive? Witness:

“Although Charles Darwin’s theory of evolution laid the foundations of modern biology, it did not tell the whole story. Most remarkably, The Origin of Species said very little about, of all things, the origins of species. Darwin and his modern successors have shown very convincingly how inherited variations are naturally selected, but they leave unanswered how variant organisms come to be in the first place.

“In Symbiotic Planet, renowned scientist Lynn Margulis shows that symbiosis, by which she simply means members of different species living in physical contact with each other, is crucial to the origins of evolutionary novelty. Ranging from bacteria, the smallest kinds of life, to the largest---the living Earth itself---Margulis explains the symbiotic origins of many of evolution’s most important innovations. The very cells we’re made of started as symbiotic unions of different kinds of bacteria. ...Dry land became forested only after symbioses of algae and funghi evolved into plants. ...Gaia, the largest finely tuned ecosystem of the Earth’s surface, is just symbiosis as seen from space....”

---from an ad for Lynn Margulis, Symbiotic Planet: A New View of Evolution (New York: Basic Books, 1999).

cell contents (above)
A Last Thought:

Why do most challenges to teaching evolution in schools come from communities that are ethnically very homogeneous

(usually almost all white)




Gnomon 1