Lichens Are Awsome, And So Can You

    Before I post about any lichens in particular, I want to put a sort of "lichen basics" guide out there to give people a point of reference. Lichens are a fascinating set of organisms that live virtually everywhere and are oft overlooked because of their small size and low profile. There are many advantages to enjoying learning about lichens:

1. They live on every rock, tree, and forest floor in every habitat during every season. Lichens are remarkably resilient, adapted to living on bare rock surfaces that bake in full summer heat before freezing in winter cold, year after year. 
2. Many lichens are readily identifiable to the genus level by their outward appearance with a hand lens, even if plenty do require a microscope to identify on the species level. 
3. Lichens provide a range of fantastic colors and shapes not seen in plants or fungi separately.
4. Many lichens have a cosmopolitan distribution, with favorites such as Rhizocarpon geographicum growing in the Rockies as well as the Himalayas. This means that a travel enthusiast can be comforted in the knowledge that their favorite simple symbionts are always keeping them company. 

Virtually any rock surface in Colorado. Many rocks are so covered in lichens that 

their original color is not apparent. This was Pike's Peak granite, which is a rosy-pink 

color, covered in a plethora of grey and green lichens. 

     The terminology surrounding lichen identification is not particularly hard to learn, but it is idiosyncratic to the point of being arcane. On top of this, many lichens lack a colloquial name as nobody but lichenologists has payed them much heed over the years. This makes lichen literature tricky to access, as the written word describing the lichens can actually become an impediment to understanding them. My hope with this post is to lay out the basics of what someone needs to appreciate the stunted, crusty world of lichens in a casual way. 

Lichen Basics 

      First off, it is good to establish that lichens are neither a plant nor a fungus, but a sort of combination of both. The bulk of the organism that is visible to the naked eye is the fungal body of the lichen, typically an ascomycete. The fungal symbiont lives in relationship with a photosymbiont (a symbiotic organism capable of photosynthesis), which is either an alga or a cyanobacterium, or rarely both (1). The anatomy within the lichen where these organisms interact is microscopic, and a good description can be found on this Wikipedia page. A quick field test for which photosymbiont you are dealing with is to simply wet the lichen. This renders the upper layers of the lichen's cortex transparent, allowing the color of the photosymbiont to show through. A light to forest green indicates an alga (see below), while darker, almost black coloration indicates a cyanobacterium. This test does not work on lichens with strongly pigmented coloration. Note that a lichen's binomial (scientific) name is always the name of the host fungal organism, which may live in symbiosis with a variety of algae or bacteria. 

Xanthoria fallax, a common lichen on tree bark in urban areas. Note the difference between the dry sample (left) 

and the same sample when wet (right). The light green color indicates an algal symbiont for this species. The 

black marks in the left image are millimeters.

    The relationship between the fungal body, which provides habitat, and the photosymbiont, which provides food, allows lichens to colonize environments such as desert rock faces which would be lethal to virtually any other living thing. Part of lichen's ability to colonize these surfaces apparently involves their ability to dry completely, and then revive again once water becomes available (2). 

Lichen Morphology 

     Lichens are broadly classified by the characteristics of their fungal body, or thallus (yes, with a "th"). There are several categories of lichen body based on macroscopic features, but for the purposes of getting by it is sufficient to know about crustose, foliose, and fruticose lichens. 

Crustose Lichens 

     A crustose lichen's thallus is tightly attached to the surface it is growing on, which is frequently stone. Crustose lichens tend to be fairly simple, with three main structures that are of interest (see illustration below).

A typical crustose lichen. Note the circular growth pattern of the thallus. 

1. Apothecia: (singular apothecium) reproductive structures of lichens where spores are formed and disseminated. On crustose lichens they may be a variety of shapes, but are typically cup or wart-shaped. In many cases they are easy to pick out from the thallus surface because they are a different color, different shape, or both. 
2. Thallus: the "body" of the lichen where photosynthesis and growth occur. In a crustose lichen the thallus may be warty (as above), wavy, or grainy, but it is always fused to the substrate it is growing on. 
3. Margin: the growing edge of the lichen, where the fungus has gotten ahead of the photosymbiont. This structure is not present in all or even most crustose lichens, but it is a handy fieldmark when you can find it. It is frequently a different color from the thallus, but not always. The margin tends to be flatter than the thallus, and less textured.  

Lecidea tessellata, a familiar crustose lichen seen here growing on Mount Evans next to an 

unidentified foliose lichen. Note the black apothecia and gray, tile-like thallus. 

Lecidea atrobrunnea (brown and black) and Rhizocarpon geographicum (yellow-green with black spots)

growing on quartz. These species tend to grow together. Note the well-developed black margins of the thalli.

A violently orange crustose lichen, probably Xanthoria elegans, growing on gravely soil 

at near 14,000ft on Mount Evans. The apothecia in this species are much more disc-shaped 

and are so numerous that the thallus is hardly visible in this photo. 

Foliose Lichens 

     Foliose lichens are so-named because their thalli grow in creeping leaf-like structures over the surfaces they cover. The thallus tends to be attached either at a central point, referred to as an umbilicus, or to have several small attachments at the base of each "leaf" as the lichen grows. See illustration below: 

A composite illustration of features of foliose lichens. This illustration is not based 

on a particular species and shows features that may occur.

1. Apothecia: serve the same function in foliose lichens as in crustose. In foliose lichens the more cup-shaped apothecia seem to predominate, although the more wart or bump-shaped ones are also found. Note the rounded edges present in the image above. These are referred to as the margin of the apothecia, and the margin's presence or absence, shape, and color are all useful field marks. 
2. Thallus: again, the body of the lichen. The curling "leaves" that spread from the center of the lichen are referred to as lobes. 
3. Rhizines: root-like structures that a lichen may use to anchor itself to a surface. To my knowledge rhizines only serve as anchors, and do not perform nutrient uptake as they might in a plant. Rhizines are actually bundles of mycelium, so the analogy to roots is somewhat strained. 
4. Isidia and Soredia: sporulating structures that make the thallus look crusty or dusty. I've included them here together because they are difficult to differentiate without a decent amount of magnification, so number 4 in the illustration could reasonably be either. Isidea are small, horn-like protrusions from the thallus surface that contain photobionts and may break off to start new lichens (3). Soredia are hyphae-jacketed photobionts that erupt from the thallus in various places. A site where this is happening in a cluster is called a soralium. The goal of both structures is reproduction.
5. Pycnidia and Parithecia: sunken sporulating structures each of which consists of an urn-like structure in the thallus which releases spores through a small opening. There are some functional and structural differences between the two, but these are not really visible without a microscope. To the casual lichen lover, both structures appear as sunken black pores studding the surface of the thallus.   

Rhizoplaca chrysoleuca, a common foliose lichen. Note the lobed 

thalli and the bright orange apothecia with gray margins.

Vulpicida pinastri, sporting bright yellow soredia. At the scale of this  photograph it would be 

difficult to distinguish soredia from isidia. The tentacle-like lichen in the background is in the genus Cladonia. 

Fruticose Lichens 

  
     I opted not to do an illustration for fruticose lichens, as they come in a bewildering array of shapes. Fruticose lichens maintain a three-dimensional structure by wrapping the medulla in cortex the whole way around. This allows for the growth of bizarre cup shapes, horns, filaments, and all kinds of other variations. Fruticose lichens may have any of the structures present in foliose lichens, but their main identifying trait tends to be the overall shape of the thallus. I have included some pictures below to help give an idea of the diversity of fruticose lichens. 

Stereocaulon tomentosum growing on the damp forest floor near Pike's Peak. Note the 

upright habit. The maroon blobs are apothecia. Large patches of this lichen can 

look like unseasonable frost from far away.

Cladonia pyxidata growing on a bed of moss. The orange coloration is not typical. The 

primary thallus is the small, foliose structure at the base, while the much more salient 

secondary thallus is the fruticose cup-shaped structures. The grainy texturing on the cups 

is from masses of isidia. 

Small tufts of a lichen in the genus Usnea, likely U. lapponica,  adorn deadwood on Mount Evans. Usnea 

species all look quite similar and may be found draped from deadwood on conifer trees throughout the state.

 A higher magnification of the thallus reveals the studded thallus and branching habit. I have not been able to figure out the function of the black spots, which are neither apothecia nor pycnidia. 

Bryoria fuscescens, a.k.a. horsehair lichen, growing with somethingorother foliose 

on dead wood. In the interest of science, a photo of someone wearing B. fuscescens as a 

beard may be found here. 

A magnification of  B. fuscescens. The white disc-like structures which I 

originally mistook for apothecia are actually soralia, with the white 

medulla showing through where the cortex has ruptured. 

Other Morphological Subtypes 

     The subtypes I have chosen not to address are relatively uncommon compared to the main three types listed above. These can be helpful, but I have found that at least in Colorado it is mostly possible to get by without them. Squamulose lichens have thalli that are composed of interlinking scales (squamules) which, like crustose lichens, lack a lower cortex (4). However, as in foliose lichens, the scales may lift above the surface the thallus is growing on. Leprose lichens lack the layered cell structure present in other lichens, and the thallus simply consists of powdery-looking granules. There are even more subtypes listed on the page I have linked to and in other sources. 

Going Out into the Wild World of Lichens 

     The first step in appreciating lichens is simply to recognize them in your environment, which may be enough for most people. Next time you are on a walk in nature, slow down by a tree or a rock surface and look closely at the great variety of shapes and colors blossoming on a tiny scale in front of you. If you want to try to identify actual genera or species, I recommend picking up a simple hand lens and an illustrated guide. 
     Unfortunately, good guides for the budding (or sporulating) amateur lichenologist are hard to come by. A Rocky Mountain Lichen Primer, by Corbridge and Weber, is a good guide for Rocky Mountain lichens which was written with an amateur in mind. This is probably good, since this is the only compiled guide to lichens in the Rocky Mountains in particular that I have been able to find. The Consortium of North American Lichen Herbaria (CNALH) has online hosting of impressively exhaustive lists of lichen species present in different locations around Colorado, complete with photos and descriptive information, which may be accessed here. 

     Finally, in a fit of graduate school-induced, hyperfocused, work-avoiding behavior, I curated information and photographs for about 150 lichen species based on this list of lichens seen in Rocky Mountain National Park. This was based on the assumption that the lichen flora of RMNP would be fairly representative of the lichens that live in the state at large. The list, which should have mostly correct citations for all print sources, may be viewed and downloaded here in .pdf format (large file warning). I did not take the time to cite all my photo sources: all photos are intellectual property of whomever took them originally. Please do not sue a poor amateur lichen enthusiast, thank you. All species are organized in alphabetical order by binomial name with photos on the first slide and information on the second. The backgrounds of the slides are colored and categorized by color to reflect my perception of the lichen's thallus color to improve ease of browsing. I apologize if the colors seem off: there are many variations within lichens between beige, brown, tan, gray, grey, green, green-gray etc. which can easily be interpreted one way or another.  

    I hope you have enjoyed learning about the bizarre world of lichens. I will reference this post periodically as I post specifically about given lichen species, and I encourage those of you who are interested to come on back and review your terminology here. If you take a few minutes to do so, you are taking your first steps into a smaller world! Thanks for reading. 

References

1. Corbridge, J. N., & Weber, W. A. (1998). A Colorado Lichen Primer. Niwot, CO: University Press of Colorado.
2. Lichens: Life History and Ecology (n.d.) Retrieved from http://www.ucmp.berkeley.edu/fungi/lichens/lichenlh.html
3. The Difference Between Isidea and Soredia (n.d.) Retrieved from http://www.plantscience4u.com/2014/07/difference-between-isidia-and-soredia.html#.VjAv3berTIU
4. Images of British Lichens (n.d.). Retrieved from http://www.lichens.lastdragon.org/faq/lichenthallustypes.html#squamulose