Skip to main content

What is Lichen:

Map Lichen:

What Is Lichen:

I was driven by the question, What is Lichen, to research this amazing organism. The more I read the more intrigued I became. This is a freaky relationship between two separate classifications of organisms that bond and form a symbiotic relationship that works across most surfaces of the world.

A Great Macro-photography hub on Lichen By Homesteadbound:

Crustose Lichens:


What Is Lichen:

Lichen is a type of fungus and is not a plant at all. Lichen is also a relationship that exists both on a chemical level and on a biological level. Lichen is a family of fungus that are certainly complex and yet simple organisms. We have all seen lichen though we may not have known what it is. Many people refer to lichen as moss and that is not a correct analogy. Moss is part of the plant kingdom while lichen belongs to the fungus family which are not plants.

Lichen is a type of fungus but not quite in the same sense that a mushroom is a fungi. The difference is slight but lichen are symbiotic where other funa are parasitic. I would say that the best way to define lichen is by its relationships. This is an organism that develops a symbiotic relationship with another plant that is capable of a photosynthetic reaction. The second plant is usually a form of green algae, though not always. The symbiotic relationship between lichen and its host cell is something right out of the depths pf sci-fi.

Lichen on Tree Branch:


The Crux of Symbiosis:

The family of fungus/fungi are not capable of producing chlorophyll and as such can not create their own food source. An even more pronounced differentiation between fungi and plants is that a fungus does not have vascular structures with in its tissue. Plants use veins to transport water and nutrients from the roots to the cellular structures within the plant. Fungi have flesh that is sponge like. They absorb food from the air and from absorption. This is why lichen fungus forms a symbiotic relationship with an algae cell. The fungus receive photosynthesis from the algae and the algae receives the benefit of an extended environment. Algae are quite remarkable on their own. They are capable of living in salt water or freshwater but when combined with a lichen they can survive almost anywhere here on earth. Lichen live in the hardest regions of the earth. Their habitat range from the polar tundra to the hottest desert. They are often the first form of life in any sterile environment. Because of the symbiotic relationship they share with algae, they can live on or in environments that have no standard substance, such as, cement walls, tombstones, or icy tundra.

There are arguments that the relationship between lichen and algae is not purely symbiotic. This is primarily because humans associate these types of relationships with free will. In nature, free will has not yet been defined. Does the algae choose this relationship or it is just part of the natural order of things? For the sake of argument, I think it can be assumed that the lichen chooses this type of relationship by mechanism of survival. The lichen certainly does not fatally harm the algae and the two live together quiet peacefully. The algae reproduces as pure algae and then is absorbed by a lichen and thus the lichen is able to grow.

Crustose Lichen on A Black Walnut Tree Trunk:


The Biology of Lichen:

To begin a discussion on lichen biology requires that we first look at the general biology of plants and the differences between lichen and plants. Structurally, plants are more like humans then are lichen. Though even that statement is a stretch of the imagination, there is still truth in those words. Humans consumer food as a means of producing energy. That food is broken down and then distributed through the human body by the circulatory systems of heart,veins and arteries. Plants use a root system as a mouth and though the root system of plants is not anything like the mouth on a human the function remains the same... an intake port for nutrients. Nutrients that plants absorb are distributed by a system of veins (xylem) to the cellular structure of the plant. Plants, unlike humans or lichen, also have the capacity to produce chlorophyll which is a form of energy that is produced in the plants cells. Plants utilize sunlight to make chlorophyll and as long as there is sunlight and water, plants can feed. Lichen have no such vascular structures and they are not capable of making chlorophyll. Their "root" system is only used for anchoring the lichen to its environment. The flesh of lichen is sponge like and does not have a waxy surface or cuticle like plants do. This is the reason behind the symbiotic relationship that lichen forms with algae cells...As a means of surviving.

The physical structure of lichen is much like a lasagna with layers of lichen fungi and algae. Together the two make up what we know as lichen. Lichen is very beautiful and amazing in its own right.

A formed lichen consists of four layers. The basal layer (E in diagram below) has a single purpose, to attach the lichen to its environment. A basal attachment may be in one of two forms. Rhizines are threads or chains of lichen cells that hold the lichen to a surface through many different contact points. A holdfast is the second type of lichen basal attachment that is is much like a pedestal or a pillar. Neither basal structure contains venous structures nor do they have any purpose other then to hold the lichen in place. The lichen cortex (A/D in Diagram below) is the outer layer of the cellular structure of the lichen thallus. A thallus is a group of cells that form sheets or strings which make up the body of fungi or alga type plants. A thallus is a simple cellular form without a higher function. For the sake of this article, the Thallus should be considers the flesh of the lichen. The cortex of a lichen is the outer layer of cells that in humans would be much like our skin or on a tree may be considered the bark. The third layer is the algae (alga) layer (B in diagram below). Most lichens utilize green algae as their host or partner. Though sometimes lichens may use a blue-green algae or even cyanobacteria. In some cases a lichen may use both the algae and the cyanobacteria. Cyanobacteria are a type (phylum) of bacteria that utilize photosynthesis to produce energy. The fourth structure of a lichen is the medulla (C in diagram below) which is what makes up the inside of a lichen structure. The medulla are cellular structures that have thin cell walls and are formed in long threads. They give the lichen a soft cottony center.

Cross Section of A Lichen:

Electron Microscope View of A Green Algae Cell

Cell Diagram of Green Algae:

I added the diagram to this cell photo.

I added the diagram to this cell photo.

Inside the Symbiotic Relationship:

Because the fungi does not produce chlorophyll and because they do not have a vascular system to move nutrients from cell to cell, the fungi that forms lichen bonds with an algae cell. The relationship is formed around the mutual cooperation and survival. It is this relationship between these two organisms that interested me the most. This is a plant (algae) and non-plant (fungus) which have bonded in order to heighten their chances of survival. Each is capable of surviving without the other; however, when bonded they form a single organism which we call Lichen.

The relationship is symbiotic because neither organism harms the other. This is quite different then parasitic relationships in which one organism feeds off the other until the hosting organism is dead or the vital resource to the parasite is consumed. A parasitic relationship is often seen when studying insects. An example would be a parasitic wasp that lays her eggs on a living caterpillar. As the eggs from the wasp hatch, the larva begin to feed on the living caterpillar until and usually after the caterpillar is dead or until the wasps mature enough to undergo a metamorphic change into an adult wasp.

In the world of all living organism there are few if any other relationships that exist between the lichen fungus cell and the green algae cell. There are of course symbiotic (of sorts) relationships that exist such as the relationship between the ant and the aphid. The difference between these two symbiotic relationships is that the ant remains an ant and the aphid remains an aphid. With lichen the two cells become one organism. The benefits and the power of this relationship is amazing and the results are an organism that is complex, but capable of surviving in every landmass on earth. To make this statement clearer... If one earthly organism could colonize the moon it would be lichen. Yes that is a big if... but considering all of the other organisms including man, that answer would be none! This is the power behind Lichen.

Scroll to Continue

An Unknown Lichen Exhibiting Color:

The Three Main Types Of Lichen:

The lichen family consists of three main types of lichen. Within those "types" are a great many morphed species. The morphing occurs when more then one kind of algae is used as a host. Environmental elements can also cause lichen to morph.

The first type of lichen is called Foliose Lichen. This is best identified as a lichen that has two very discernible sides or a top and a bottom. The Foliose Lichen may have many characteristics and physical attributes such as curly lettuce type foliage. Foliose Lichen can also be flat or it can be very textured with a surface that is made up of ridges and bumps

The second type of lichen is called Fruiticose Lichen. These are more of the long hairy type lichens such as Old Man's Beard. Old Man's Beard looks much like spanish moss but moss and lichen are not even closely related. Some Fruiticose Lichen also have cups and look like small fungus.

The third type of lichen is called Crustose Lichen, and is much as the name describes, It forms a type of crust. These lichen like to cover a surface. These are most likely the lichen you would see on old rocks and newer stone walls. They can be very brightly hued and can be quite beautiful to look at both from afar and up close.

Foliose Lichen:

Old Man's Beard Fruticose Lichen:

Crustose Lichen:

Lichen are Amazing:

They survive on almost every part of the earths surface including Antarctica. They live in the hottest desserts, on the highest mountains, and the coldest areas of earth. They live more places then do ants and that alone is amazing. How do they survive such diverse climates and habitats? There survival is linked directly to the unique relationship between the fungi and the algae. Both organisms give something to the relationship. The algae provides photosynthesis. The fungus provides a state of stasis that allows both organisms to survive harsh and dire environments. Because fungus do not have vascular structures they rely on moisture and wind to provide them with sustenance. The outer layer of the fungal thallus is very thick. It is capable of preserving the liquid that is found in the inner cells. As long as the algae has liquid it can continue the process of photosynthesis. If the lichen reaches a state where it is too hydrated to provide the algae with water, both organism enter a state where they merely are. This is called stasis. There they wait until water or liquid comes into contact with the fungi's thallus or outer cortex. The liquid is then absorbed and normal life continues. Lichen are able to survive such extremes because they store food and supplies in