Foliose Lichen Descriptive Essay

Lichen collection, preservation and identification methods for beginners

Introduction

The word ‘lichen' has a Greek origin, which denotes the superficial growth on the bark of olive trees. Theophrastus, the father of botany, introduced the term ‘lichen' and this group of plants to the world.

Lichen is a combination of two organisms, an alga and a fungus, living together in symbiotic association. The algal component in the lichen is called phycobiont or photobiont while fungus as mycobiont. The phycobiont and the mycobiont loose their original identity during the association and the resulting entity (lichen) behave as a single organism, both morphologically and physiologically. Hence the lichen is called as a composite organism. In lichen thallus (body) the mycobiont predominates with 90% of the thallus volume and provides shape, structure and colour to the lichen with partial contribution from algae. Whatever visible from out side in a lichen thallus is fungal part, that holds algal cell inside. Hence the lichens are placed in the Kingdom – Mycota (Fungi). The fungi present in lichens are called as lichenized fungi. Among the 20,000 lichen species known in the world 95% belongs to the Ascomycetes group of fungi while Bacidiomycetes and Deuteriomycetes groups are represented by only 3% and 2% of species respectively.  

Lichens can grow in diverse climatic conditions and on diverse substrates. The lichens that are growing on tree trunk and bark are called corticolous lichens, twig inhabiting ones are ramicolous, on wood - legnicolous, on rocks and boulders – saxicolous (epilithic), on moss - muscicolous, on soil - terricolous and on evergreen leaves – foliicolous (epiphyllous). In general any lichen growing on other plant is called as epiphytic . The lichens can grow on underwater rocks, but not freely in water or on ice. The lichens are widely distributed in almost all the phytogeographical regions of the world. Sufficient moisture, light and altitude, unpolluted air and undisturbed, perennial substratum often favour the growth and abundance of lichens.

Grouping lichens:

            By their appearance the lichens can be grouped into three main categories of growth forms,

•  Crustose lichens: The thallus in crustose lichen is closely attached to the substratum without leaving any free margin. The thallus usually lacks lower cortex and rhizines (root like structure). Such lichens are collected along with their substratum for the detailed study.

•  Foliose lichens: They are also called as leafy lichens. The thallus in this case is loosely attached to the substratum at least at the margin. Such lichens are collected by scraping them from the substratum.

•  Fruticose lichens: Here the lichen thallus is attached to the substratum at one point and remaining major portion is either growing erect or hanging. The lichen usually appears as small shrub or bush and easy to collect with hand.

 

There are few intermediate categories of growth forms such as,

•  Leprose lichens: The leprose lichen is powdery or granular and does not form smooth thallus.

•  Placodioid lichens: In this case the lichen thallus is closely attached to the substratum at centre and lobate or free at the margin, but lack rhizines.

•  Squamulose lichens: Here the lichen thallus is in the form of minute lobes, having dorsiventral differentiation. The rhizines may be present or absent. This is a form intermediate between crustose and foliose.

•  Dimorphic lichens: In case of dimorphic lichens single thallus has the characters of both foliose/ squamulose and fruticose lichens. The squamules are the primary thallus, which bears erect body of fruticose lichen, the secondary thallus.

The above-mentioned growth forms of lichens can be arranged according to their imaginary evolution; Leprose (pioneer) → Crustose → Placodioid → Squamulose → Foliose → Dimorphic → Fruticose (latest). The leprose, crustose, few placodioid and squamulose lichens are generally called as microlichens, because of their smaller size and require microscopic studies to identify. The foliose, dimorphic and fruticose lichens on the other hand are called as macrolichens . The macrolichens have comparatively larger thallus and a hand-lens, dissection or stereozoom microscope is sufficient to identify them.

The crustose, foliose, squamulose, placodioid and sometimes, dimorphic forms of lichens usually grow in a circular and centripetal manner. The rough and uneven shapes of the substratum may change the shapes of the lichen colony. The leprose lichens form irregular patches of thallus on the substratum. The fruticose lichens of smaller size usually grow erect while larger ones hang from the substratum with their growing point located at the tips.

Differentiating lichens from other groups of plants

The non-lichenized fungi, algae, moss, liverworts (bryophytes) are the plants, which grow on rocks, bark and on soil and can be confused for lichens to the beginners. However, lichens can easily be differentiated from these plants in the field. The lichens are never greener as algae, liverworts and mosses. Foliose lichens in the moist places or in wet condition may look greener, but have thick, leathery thallus while liverworts have non-leathery and slimy thallus. The dimorphic forms of lichens such as Cladonia may confuse with the leaf liverworts and mosses. However, leaf liverworts and mosses have dense small leaf like structures throughout the central axis of the plant, while in case of dimorphic lichens the squamules of semicircular shape usually present at the base of the central axis or sparse throughout. Algal mat are usually found in water-flooded habitat and slimy. The beginners may confuse the dried algal mat on rocks and bark for lichens. By spraying some water on these mat one can make out whether it is algal mat or lichen.

The non-lichenized fungi are the most confusing ones with crustose lichens in the field. Such fungus usually forms patches with loosely woven hyphae, which will be evident under hand lens. The lichens on the other hand form smooth and circular patches. The fungi are usually whitish in colour and lichens are greyish, off white, yellowish, yellowish-green or sometimes bright yellow or yellow orange in colour. The lichen thallus usually bears cup like structures called apothecia (Fig. 8), or bulged, globular or immersed pitcher like structures called perithecia (Fig. 9) as sexual reproductive organs. The finger-like projections called isidia (Fig. 10) or granular, powder-like structures called soredia (Fig. 11) are common vegetative organs. Some crustose lichens belonging to family Graphidaceae bear worm like structures, which are nothing but the modified apothecia, and are called as lirellate apothecia (Fig. 12). While collecting lichens it is necessary to look for such structures with the help of hand lens. When a lichen thallus does not have any such structures, it is difficult to differentiate it from fungus.

Upper row - Fig 8, 9
Middle row - Fig 10, 11
Last row - Fig 12

In any case, it is experienced that a beginner normally collects fungus and other plants in place of lichens. Such specimens are identified by taking a thin section of the thallus and studying under microscope. If the section contains both fungal tissue and algal cells, the specimen is lichen otherwise it is something else. In India basidiolichens (looking like mushroom) are rare or absent.

Collection and preservation:

Both the micro and macrolichens are visible to naked eye in the field. However, a hand lens, preferably of 10x magnification, is necessary to examine the fine structure of the thallus while collecting. A sharp, flat edged chisel (1 to 2 inch wide edge) and a hammer (1 kg weight) are the tools required for collecting lichens from the bark. A pointed or stout flat edged chisel can be used to collect lichens growing on rocks. Polythene packets (smaller (6 x 12 inch) and bigger sized), rubber bands, labeling stickers, a field book, notebook, pen, pencil, plant press, knife, secateur (twig cutter), hand lens, old news papers or blotters, (nylon) ropes, collection bags, herbarium packets are the other necessary items required during a lichen collection trip. An altimeter, Global Positioning System (GPS), camera and few other instruments can be carried as per the objectives of the study (Fig. 13). The field book is different from the notebook in having printed columns for entering required data, such as date, locality, altitude, collector's name and remarks. Every page of field book has serial number. The numbers are also printed several times, one below the other on the free (right) side of the filed book. These numbers are for placing along with the specimen in the field ( Fig. 14).

The lichens are usually collected along with their substratum irrespectively of their growth form. Only the lichens that are very loosely attached to substratum are scraped out and collected. The corticolous lichens growing on tree trunk at reachable height (up to 2 – 3 m from ground) are usually collected and canopy lichens can be found fallen on ground. Special tree climbing methods can be adopted for studying the canopy lichens. Superficial bark should be removed with the help of chisel or knife in order to avoid damage to the trees. The ramicolous lichens are collected by cutting twig with secateur. In case of saxicolous lichens smaller pieces of the rocks should be collected in order to avoid over weight. The lichens on the edges or crevices of rock are collected by breaking the rock. Sufficient amount of specimens (at least 2 thallus or patch of 5-10 cm) should be collected, as the material will be consumed for chemical analysis (TLC) and microscopic study. The bulk collection also helps to designate it as various types (Holotypes, Isotype, Partype) in case it is new taxa. Further, it will also be convenient to distribute it to other herbaria as exsiccates or voucher specimens. However, unnecessary or repeated collection of same material should be avoided in order to conserve this group of plants. For beginner different lichen specimens can look same or specimens of same species may look different. Till one gains experience, samples looking different on careful observation can be collected. The collected lichen samples are transferred to the polythene packets, labeled and closed with the help of rubber bands. Several such packets are then transferred to larger polythene or collection bags. Or one can also keep the collected material in newspaper or blotter packets. It will be better if the different specimens are kept in different packets to avoid mixture. Otherwise, all the collections from a single tree can be kept together, or even collections from several same species of tree in a study area can also be put together in bigger polythene bag. The lichen specimens should not be kept in polythene packets for longer duration as it gets spoiled due to fungal attack when wet or changes in colour as it dries. While collecting the lichens the field data required should be noted in the field book and its respective number is cut and put in the packet along with the specimen. After returning from field all the specimens should be transferred to newspaper or blotter packets along with their labels for drying. The lichen specimens on wet barks should be kept in plant press and tied tightly. Otherwise the bark gets curled up as it dries, makes uncomfortable to preserve in herbarium packets and also gives shabby look. Much dried and curled specimens can be stretched using water and by spreading on blotters. The specimens can be dried under sun. If the specimen-processing place is moist, damp or if it is winter or rainy season, the materials can be dried with the help of heater or hot-air oven. If insects are seen in a collection they should be killed either by drying the specimens openly in hot sun or by placing sealed specimen polythene in deep freezer (- 20°C) for three days.

The lichen herbarium packets should be made up of thick, white or brown hand made acid free paper . The paper sheet of dimension 13.5x11.5 inches is folded lengthwise twice and then side ways to produce the packets of dimension 7x5 inches with upper flap of 3.5 inches to stick the label (Fig. 15). The herbarium label should contain the information of name and family of the lichen (which can be written after the identification), detailed locality and altitude from where it has been collected, date of collection, a reference number, collectors name and notes on its substratum and any other interesting observation. After the identification name of the person who identified (determined) the specimen can also be mentioned. The label should be written legibly with black, waterproof, permanent ink pen and not with ballpoint or gel pen. Alternatively A4 sized paper can also be used for making herbarium packets. If it is used, the dimensions of the packet change slightly. Using A4 papers has the advantage of saving time, avoiding messy gum, ink and shabby look due to bad handwriting by printing the details on the paper. A label template can be designed in the word processor for this purpose. If all the information regarding a species is available in computer, it is even possible to programme PC to print large number of label within a short period of time. The black laser print should be taken and not inkjet prints.

No poisoning methods are available for lichen preservation. The sufficiently dried lichen specimen should be pasted on thick, hard paperboard of dimension 6.5x4.5 inches (little less than the total packet size) and then placed inside the herbarium packet. The board also should have the same reference number as on the label. In case of terricolous lichens gum (Quick Fix, Fevicol, paint) should be applied all around the soil cluster in good amount (without spoiling the lichen), so that the soil does not get powdered. Later on such specimens can be pasted on the board. Small or delicate samples can be kept within a paper packet pasted to the board, or in small boxes. A piece of tissue paper is kept over the lichen specimen pasted on the board while keeping it within the herbarium packet. A herbarium packet should have only the specimen belonging to single species and mixtures should be avoided. If the material is in large amount they can be distributed in two or more packets to avoid spoiling of specimen due to crushing or shabby look of the packet. However, all these packets should have same number, one packet with good specimen can be treated as original while others are marked as duplicate. Several herbarium packets are then kept upright in drawers or cupboard / hardboard boxes of dimensions approximately 8x5x12 inches (like shoe box, Fig. 16). The boxes are then placed in almirahs (Fig. 17).

The storage place of herbarium should be dry to prevent fungus growth on lichens. Booklice are the common insects which quietly damage packets and lichen specimen in short time during storage. However, booklice can be kept away by placing naphthalene balls in herbarium boxes and in almirah. The lichen specimens within the boxes as well as in almirah can be arranged in alphabetical order. As in flowering plants, no particular classification system is followed for lichen specimen arrangement.

Identification of lichens

The collected lichen specimens are initially segregated according to their growth form. Within the growth forms the specimens can be further grouped according to the type of their fruiting bodies (apothecia, perithecia, or sterile).

Before starting the examination of specimen for identification, one should have a tool or pencil box containing few items needed for handling the specimens. The simple but necessary items needed are razor or snapper blades, plastic-handled needles, pointed and flat-tipped forceps, injection syringes (2 ml capacity) or capillary tubes or glass rods, pencil, sharpener, eraser, small transparent scale, round brush of 0 – 1 size, Quick Fix, permanent ink pen, etc (Fig. 18). The syringes are used for keeping and applying chemical reagents during identification. One syringe will always have distilled water in it. The syringes are kept capped while not in use.

Lichens are identified by studying the morphology, anatomy and chemistry of the specimen. The micro and macrolichen keys of Awasthi (1988, 1991) are the important literature referred for identification of lichens. The beginner should have a glossary of technical terms in lichenology while identifying the lichens. Illustrated glossary or identifications manuals are rare for Indian lichens. A botany student or one with mycological background can follow the terminology and key. All the observations made on a specimen can be written on a piece of paper (description slip) and kept inside the herbarium packet to avoid repeated observation or section cutting. The description slip should contain the specimen number.

The morphological and anatomical characters to be observed in lichen specimens differ from one group of lichen to the other. However, some common characters to be noted are given below.

Morphology

The morphological characters of a lichen specimen is studied under dissection or stereomicroscope. Type of thallus (leprose, crustose, foliose, squamulose, dimorphic, fruticose), its shape (irregular, circular) and size is noted.

Upper surface : The colour of the thallus, texture (smooth, rough, warty), presence of finger like projections (isidia), granular powder in groups (soredia), fine powder (pruina), black dots (pycnidia, Fig. 19) and whitish decorticated areas (pseudocyphellae, Fig. 20) are to be noted in case of crustose and foliose lichens. The branching pattern, length and breadth of marginal lobes, presence of hair like structures (cilia, Fig. 21) in case of foliose lichens has to be noted.

The morphoology of fruiting bodies have to be studies separately. In case of apothecia, shape (rounded or stretched or lirellate apothecia), size, the mode of attachment (stalked or not), colour and texture of the apothecial margin and disc, presence or absence of powder (pruina, Fig. 22) on the disc, shape of the disc (convex or concave) are necessary characters to be observed. In case of lirellate apothecia the branching pattern and colour of slit can be noted. Sometimes apothecial disc become loose, powdery or hazy and such cups are usually held by long stalk. Such apothecia are called as mazaedium (Fig. 23). The colour of the mazaedium and length of the stalk can be noted.   In case of perithecia, its colour, shape, size, the position of the opening (apical or lateral), whether single or grouped are to be noted. Some crustose lichen does not form definite fruiting body. However, fruiting body structures group together and visible as bulged or flat structures with various shapes usually black, brown or grey coloured. Such structures are called unorganized ascocarp (Fig. 24) or fruiting body.

Lower surface: The lower surface of only foliose lichens can be seen, as it is absent in crustose lichens while fruticose lichens do not show dorsiventral differentiation. The colour of lower surface, presence of any pores, presence or absence of rihizines (root like structures, Fig. 25 (given above)), their colour, distribution, branching, abundance is to be noted.

Anatomy:

The anatomy of lichen thallus and fruiting bodies is examined under compound microscope with magnification 4 - 40x. The anatomy of the thallus is occasionally studied to see the thickness of various layers (upper cortex, algal layer, medulla, lower cortex), type of algae and their distribution (stratified – heteromerous or uniform – homeomerous) and to study the arrangement of fungal hyphae (vertical or horizontal) within the thallus (Fig. 26 (given above)) . The section of thallus can be cut with razor blade by keeping the fragment of thallus in potato or papaya pith. Microtome sections are very helpful but it is a time-consuming process. Just to check the type of algae present in the thallus one need not cut a section. By the colour of the thallus one can make out the type of alga (at least group) present. Lichen with blackish, bluish, slate grey thallus usually has blue green alga, while greyish, yellowish, brownish, greenish thallus has green alga. However, it is better to confirm the type of alga present by following an easy procedure. The algal layer of the lichen thallus is exposed by scraping the upper cortex with razor or snapper blade. Alga (which appears green, dark green, blue green, black) is picked up with blade or needle, transferred to the slide and examined. Sometimes in the field while differentiating lichens from fungi one can check for alga (green or dark green pigments) by exposing the upper cortex with blade.

The anatomical characters of fruiting bodies (ascocarp) are very important identification aids, especially in case of crustose lichens. The type of spores (simple, septate), their shape, size, number of spores per a spore-sac (ascus), colour of ascocarp wall, presence or absence of crystals, algal cells in the wall, colour of different layers of ascocarps are to be noted (Fig. 27).

A thin, hand section of ascocarp is taken with the help of blade while it is still attached to the thallus or substratum and by viewing through dissection or stereomicroscope. The ascocarp is made wet with a drop of water before cutting the section (Fig. 28). Few thin sections are enough to observe the necessary character. (The sections of crustose lichen thallus are also cut in the same way). In case of unorganized ascocarp, section can be taken in similar way or ascocarp is scraped with the help of blade and transferred to slide for observation. In case of mazaedium the disc is scraped and slide is prepared to see the type of spores.

The sections of thallus and ascocarps are mounted with plain water to observe the major characters. The lactophenol, cotton blue and other stains can be used to colour different tissue as per requirement. Semi-permanent slides are prepared by adding a drop of glycerol-water mixture (1:1) to the slide and by sealing the cover slip with gum (Quick Fix). For measuring the size of various structures the microscopic should be micrometrically calibrated (find the procedure in any practical biology book).

Chemistry:

Lichens produce around 600 secondary metabolites and these metabolites are popularly known as lichen substances. Out of the 600 lichens substances around 550 are unique to the lichens and are not available in any other plant groups. Most of these lichen substances act as an important character for identification of lichens (chemotaxonomy). The lichen substances are identified by performing colour spot test or thin layer chromatography (TLC) or by high performance liquid chromatography (HPLC). Standardized methodology is available (Walker and James 1995) for performing lichen chemistry.

Colour spot test: Three chemical reagents commonly used for colour spot test are aqueous potassium hydroxide (K), bleaching powder or aqueous solution of calcium hypochlorite (C) and aqueous solution of paraphenyldiamine (Pd). K-test is performed either on upper surface of thallus (cortex) or on medulla by exposing it with blade, or on both. A drop of K solution is taken with the help of syringe, capillary tube or with glass rod and then placed on the cortex or medulla and colour reaction is noted. Similarly, C and Pd test are performed and colour changes are recorded. KC-test is performed by applying K solution first and then applying C solution over earlier K solution drop immediately. The colour of the cortex or medulla changes due to presence of particular lichen substances in lichen thallus (Fig. 29).

TLC: Sometimes, many lichen substances are undetectable in colour spot test or it may not give proper result. In such cases TLC have to be performed. In TLC various lichen substance presents in a lichen thallus get separated as spots on plate. The spots are identified as different lichen substances by noting its colour and measuring the distance moved by it, with the help of TLC manuals (Fig. 30).

HPLC: Now-a-days usage of HPLC with reversed-phase columns, gradient elution, benzoic and solorinic acids as standard has become a common practice in abroad to identify lichens substances, which are unidentifiable in TLC. A retention index (RI), calculated from the elution time of the appropriate peak with reference to the standards, is used in identification (Fiege et al 1993).   RI values have been calculated for most of the lichen substances and are available. HPLC is quite an expensive method and also not possible to try on every collected specimen. Therefore, the materials to be chemically investigated are initially screened out with TLC and only few interesting or the specimens with complicated chemistry should be analyzed through HPLC.

Some lichens thallus emits florescence (yellowish, bluish) when observed under UV light due to the presence of lichen substance called lichexanthone, and it is an important taxonomical character for identification of species in few cases. Such lichens are examined by keeping them in a closed UV lamp chamber and exposing them to UV light of wavelengths 254 and 365 nm.

As per rule of International Code for Botanical Nomenclature (ICBN), after the identification of a species, a set of voucher specimens should be deposited in any recognized herbarium and it has to be mentioned in any publication arising out of the study. This will help other researchers to refer whenever needed.

Some important characters to be examined in a lichen thallus during identification are summarized in flow chart below (Fig.31).

Concluding note:

The identification is the primary step in any lichenological study. The lichens may appear as difficult plants to study but in reality they are an interesting and quite easy group. The lack of simplified, illustrated identification manuals is the main constrain faced by several interested Indian researchers. Such field-guides and handbooks are yet to be produced by Indian Lichenologists. For the time being, following are the most important literatures required for identification of Indian lichens,

•  Awasthi, D.D. 1988. A key to the macro lichens of India and Nepal. J. Hattoi Bot. Lab. 65: 207-302.
•  Awasthi, D.D. 1991. A key to the microlichens of India, Nepal and Sri Lanka. Biblioth. Lichnenol. 40, J. Cramer, Berlin, Stuttgart. Pp. 337.
•  Walker, F.J. & James, P.W. 1980. A revised guide to microchemical technique for the identification of lichen products. Bull. Brit. Lich. Soc. 46: 13-29 (Suppliment).
•  Fiege, G.B., Lumbsch, H.T., Huneck, S. & Elix, J.A. 1993. Identification of lichen substances by a standardized high-performance liquid chromatographic method. J. Chromatogr. 646: 417-427.

These literatures are quite voluminous and might not be available in all libraries. Consulting any laboratory engaged in lichenological research is the best method to get the required literature and guidance. The updated information for several groups of lichens is now available. The Lichenology Laboratory at National Botanical Research Institute, Lucknow, is ready to guide any one who is interested to take up lichens for their career or even as a hobby.

Legends for the figures:

Figure 1. Crustose lichen – Letrouitia domingensis (Pers.) Haf. & Bellem.
Figure 2. Foliose lichen – Heterodermia sp.
Figure 3. Fruticose lichen – Ramalina pacifica Asah.
Figure 4. Leprose lichen – Chrysothrix chlorina (Ach.) Laundon
Figure 5. Placodioid lichen – Lecanora valesiaca (Müll. Arg.) Stitzenb.
Figure 6. Squamulose lichen – Phyllopsora parvifolia (Pers.) Müll. Arg.
Figure 7. Dimorphic lichen – Cladonia coniocraea (Flörke) Sprengel
Figure 8. Apothecia
Figure 9. Perithecia
Figure 10. Isidia
Figure 11. Sorelia (Soredia in group)
Figure 12. Lirillae
Figure 13. Picture showing the items needed for lichen collection
Figure 14. Field book
Figure 15. Preparation of herbarium packet
Figure 16. Herbarium packets arranged in hardboard box
Figure 17. Arrangement of herbarium boxes in almirah
Figure 18. Pencil box with necessary items during identification
Figure 19. Pycinidia
Figure 20. Pseudocyphellae
Figure 21. Cilia
Figure 22. Apothecial disc with whitish pruina
Figure 23. Mazaedium
Figure 24. Unorganized ascocarp
Figure 25. Rhizines
Figure 26. Section of crustose lichen thallus
Figure 27. Section of apothecia
Figure 28. Cutting thin hand sections of apothecia
Figure 29. Performing K colour test on medulla. See the colour change also in cortex
Figure 30. A developed plate of thin layer chromatography (TLC) for lichens
Figure 31. Flow chart showing some taxonomically important characters to be examined during identification of lichens
Activity:

If you have gone through the article, try to answer the following questions. You can refer to the article in case of any doubt or to check whether your answers are correct or not.

1. Can you recognise the following?

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2. Can you make a list of the items in a tool box, which are needed in identifying a lichen?

3. Try to make a note of the anatomical and morphological features of lichens.

4. Where do you find lichens?

What is a lichen?

If you read no more than this page, you will gain a good basic understanding of lichens. Much of the rest of the website consists of expansions of the topics presented here and you can get to many of those expansions by clicking on the embedded links.

A lichen is not a single organism. Rather, it is a symbiosis between different organisms - a fungus and an alga or cyanobacterium. Cyanobacteria are sometimes still referred to as 'blue-green algae', though they are quite distinct from the algae. The non-fungal partner contains chlorophyll and is called the photobiont. The fungal partner may be referred to as the mycobiont. While most lichen partnerships consist of one mycobiont and one photobiont, that's not universal for there are lichens with more than one photobiont partner. When looked at microscopically, the fungal partner is seen to be composed of filamentous cells and each such filament is called a hypha. These hyphae grow by extension and may branch but keep a constant diameter. Amongst the photobionts there are those that are also filamentous in structure while others are composed of chains or clusters of more-or-less globose cells.

Given that they contain chlorophyll, algae and cyanobacteria can manufacture carbohydrates with the help of light via the process of photosynthesis. By contrast, fungi do not make their own carbohydrates. Every fungus needs existing organic matter from which to obtain carbon. In a lichen some of the carbohydrate produced by the photobiont is of course used by the photobiont but some is 'harvested' by the mycobiont.

Worldwide, over 20,000 species are known and over 3,000 are known from Australia. There has been scientific study of Australian lichens over the past two centuries, though with somewhat erratic progress during the first century and a half since European settlement. You can find out more via the HISTORY OF AUSTRALIAN LICHENOLOGY page.

Lichens can be found growing in almost all parts of the terrestrial world, from the ice-free polar areas to the tropics, from tropical rainforests to those desert areas free of mobile sand dunes. While generally terrestrial a few aquatic lichens are known. The surfaces (or substrates) on which lichens grow vary from the natural (such as soil, rock, wood, bone) to the man-made (bitumen, concrete, glass, canvas, metal - to give just a few examples).

Lichens possess structures not formed by either of the partners and produce chemicals usually absent when the fungus or the photobiont are cultivated separately and so lichens are more than a sum of their parts. In fact, lichens synthesize over 800 substances, many of them not found elsewhere in nature. Though the fungi that form lichens do not occur in nature as independent organisms, a number of the photobionts can be found in free-living forms. It is possible to separately culture the two partners in the laboratory but it is difficult to resynthesize the lichen. Success has been achieved when the fungus and photobiont are placed under stress (e.g. by reducing water and nutrient levels), suggesting that originally lichen partnerships formed to overcome adversity.

As well as having important ecological roles lichens have also been used by humans as food, medicine and for the dyeing of cloth. For example, traditionally lichens were used to produce the colours of Harris tweed. There's more about the subject on the LICHENS AND PEOPLE page.

Classification and identification

Lichens are classified with the fungi (being sometimes referred to as lichenized fungi). The fungi incorporated into lichens are largely ascomycetes, with very few basidiomycetes involved. If you are not familiar with the terms ascomycete or basidiomycete you will find brief explanations in the FUNGAL BASICS CASE STUDY. Though a number of lichen species can be readily identified in the field the precise identification of many lichens demands examination of their macroscopic and microscopic structures (such as reproductive structures, spores and cellular features) as well as chemical tests. Chemical reagents can be applied to the lichen tissues and the presence or absence of a colour change noted but such ‘spot’ tests are crude and chromatographic methods yield more precise analyses. The usefulness of the chemical tests lies in the fact that the chemical substances are often species specific.

There's more about these subjects on the CLASSIFICATION and the LICHEN CHEMISTRY pages.

Growth forms

Lichens show a variety of growth forms and there are terms used to name these forms. The following are three very commonly seen types:

Fruticose lichens are erect or pendulous and markedly three-dimensional. The genus Usnea (right) is an example.

Crustose lichens are markedly two dimensional and firmly attached to the substrate by their entire lower surfaces, making it impossible to see a crustose lichen's undersurface. A crustose lichen looks very much like a thin crust on the substrate. The bright orange Caloplaca is an example.

Foliose lichens could be thought of as halfway between crustose and fruticose. Though obviously three dimensional they grow in a more-or-less sheet-like form, but often with a lobed appearance. They are not attached by their entire lower surfaces to their substrates. Indeed, some foliose lichens are just centrally attached to their substrates with the rest loose, so making it possible to see both the lower and upper surfaces very easily. Xanthoparmelia substrigosa (below) is an example.

(click images to enlarge)

Those three growth forms will account for the majority of genera that most people are likely to see. It is also worth mentioning the concept of a squamulose lichen since the genus Cladonia is very widespread and often shows a squamulose growth form. But the species in the genus also produce upright fruticose structures called podetia - sometimes with the appearance of fairly simple stalks, sometimes flared at the apex and so presenting a somewhat trumpet-like form.

In this photo you can see a Cladonia colony growing on soil. There are numerous squamules on the soil but you can also see a number of the upright podetia with broader apices. Moreover, at the margins of some of the broader apices you can see additional podetia developing. Here is a closer view, showing an enlargement of part of the previous photo. You can also see that the podetia themselves also have flake-like squamules. Cladonia is not the only squamulose lichen genus, just a very commonly seen one. A colony of a squamulose lichen looks like a scattering of small flakes or scales on the substrate. A byssoid lichen has a somewhat wispy appearance, like cotton-wool teased out to some degree. Leprose lichens have a powdery or granular appearance. You can find combinations of growth forms in some lichens. For example, some species are crustose centrally but somewhat foliose at the margins. Just in case you're interested, such a lichen is called placodioid or placoid and Placopsis perrugosa is an example.

Morphological plasticity in Siphula coriacea

In most cases a species will always have the same gross morphology but a number of species are known to show some plasticity. This box gives one example.

Usually Siphula coriacea is characterised by erect, bluish grey lobes, shown here . The species is known from mainland Australia and New Zealand, in heathlands and grasslands. In moister, sheltered areas the lobes may be over a centimetre in length but in the drier rangelands of inland Australia the lobes are markedly shorter, often only a few millimetres in length. An unusual form has been found in Idalia National Park, western Queensland. The thalli consist of more or less circular disks, rather than of erect lobes. The discoid thalli, shown here , are from two to eight millimetres in diameter and, though different in gross morphology, match the usual forms of this species in both chemistry and micro-morphology. One lichenologist noted that the microhabitat in which he'd found the discoid form is possibly waterlogged after rain but otherwise dry and the discoid form could be a response to this MICROHABITAT. Typical forms of Siphula coriacea were found elsewhere in the same general area.

All of these expressions may be called usefully imprecise descriptive terms. They are useful in the same way that expressions such as shrub and tree are useful when talking about plants. They are imprecise in that sometimes it may be difficult to place a particular specimen in a particular growth form 'pigeon-hole'. Similarly, occasionally you may wonder which of shrub or tree is the better term to describe a particular plant. What of the byssoid lichens? Logically you could argue that a byssoid growth form is three dimensional (as is a clump of cotton wool) and so a byssoid lichen is really just a very delicate fruticose lichen. Some lichenologists consider squamulose and placodioid forms as simply variants of crustose. As you can see there is a variety of terms (more than listed above) and some debate over the boundaries between them. It is useful to be aware of these issues, since different books or websites may use some terms in slightly different senses, but there is no point in getting bogged down in terminology. For most purposes it is enough to be comfortable with the terms crustose, foliose, fruticose and squamulose as defined above.

In general a particular species will show the same growth form, no matter where it grows. Occasionally, for some reason (perhaps genetic, perhaps environmental), a species that is usually, say, crustose might grow in a fruticose form. Such occasional, but dramatic, differences in growth form in the one species are well-known to many gardeners. A plant species that usually grows as a tree may be found growing in, say, a prostrate form. Often such plant variants are highly valued horticulturally and propagated vegetatively to preserve the variant form and sold as cultivars of the species in question.

You may come across the terms macro-lichen and micro-lichen. These are two more examples of usefully imprecise terms. Roughly speaking a macro-lichen is one that is foliose or fruticose and the rest are micro-lichens. Note that this has nothing to do with size, despite the impression given by the prefixes macro and micro. A species that typically grows as a foliose form to say a centimetre diameter would be a macro-lichen whereas a crustose species that typically grows to over 10 centimetres in diameter would be a micro-lichen.

Thallus structure

In the Usnea photograph above you can see a prominent smooth, circular disk. If you look at this photograph of the foliose lichen Paraparmelia lithophiloides, you'll see that much of it is grey to blackish but there are also a number of brown disks. In those disks, called apothecia, the fungal partner produces spores and the apothecia are part of the fungal reproduction process. The bulk of each lichen (that is, the branches in Usnea and the grey to blackish areas in Paraparmelia lithophiloides) is called the thallus and is known as the vegetative part of the lichen. The thallus is composed of fungal and photobiont cells, so well united as to give the impression that you are looking at just one organism. In most lichens it is the thallus that is dominant and when talking about lichen growth forms it is always the thallus that is being described.

There'll be more about apothecia and other spore-producing structures a little further on. For the moment, let's concentrate on the thallus of Paraparmelia lithophiloides. This is a foliose lichen so it is more-or-less flat in form so let's see what the thallus looks like in cross-section. The upper surface is composed of compacted hyphae and this band of compacted hyphae is called a cortex. Below the cortex is a band of photobiont cells and below that is the medulla, an area of loosely arranged hyphae. It is in the medulla that the fungus stores the nutrients it has "harvested" from the photobiont. Below the medulla is the lower surface of the thallus, composed of compacted hyphae and constituting another cortex. From the lower cortex root-like bundles of hyphae, called rhizines, anchor the thallus to the substrate. You find this sort of structure in many foliose lichens.

The thallus of Paraparmelia lithophiloides has an upper cortex and a lower cortex and that is the norm in foliose lichens. On the other hand, a crustose lichen lacks a lower cortex. It is meaningless to talk of upper and lower sides in the branches of a fruticose lichen. In such lichens any cortex would constitute the outermost band of each branch, with the photobiont cells typically immediately inward from the cortex and the medulla occupying the central area within the branch.

While a cortex (or two) and rhizines are features you will find in a great many lichens, there are species that lack rhizines or are without a cortex. A clear example of a lichen without rhizines is Xanthoparmelia convoluta , a VAGRANT LICHEN, which sits loosely on the soil and may be moved about by wind or water. In species with rhizines the density of rhizines is variable between species. There are those species that have few and sparsely arranged rhizines while in others the rhizines can be quite dense. When present a cortex may be anything from very scanty to very well developed, depending on the species. In most lichens the photobiont cells are arranged in one band but in a small number of genera the photobiont cells are scattered randomly throughout the thallus.

You can find more details on the FORM AND STRUCTURE page.

Reproduction, dispersal & distribution

Lichens may reproduce asexually (or vegetatively) by several methods. A fragment broken off from a lichen thallus may grow into a new thallus. This is a means of vegetative propagation, the new thallus being genetically identical to the thallus from which the fragment came. Many lichens are brittle when dry and are therefore easily fragmented, for example by some animal stepping on a dry thallus. Obviously fragmentation is especially easy with the foliose and crustose species. Fragmentation could be described as 'accidental' vegetative reproduction. There are also other, more specialized, means of vegetative reproduction. The surface of a thallus may show minute, powdery granules (called soredia), each soredium consisting of a few photobiont cells surrounded by fungal filaments. Also, the thallus may produce tiny, simple or branched spiny outgrowths (called isidia), again a mixture of fungal and photobiont cells. The isidia are easily broken and both they and the soredia are easily dispersed and contain everything needed to produce new thalli. There are species which produce neither soredia nor isidia, others produce both and yet others will produce only one of the two.

Only the fungal partner reproduces sexually, with the spores often produced in a long-lived saucer-like structure called an apothecium, which is easily visible to the naked eye in many species. Instead of apothecia various lichens produce their fungal spores in perithecia, a perithecium being a small, and typically black, hemispherical pustule within which the asci are produced. A group of lichens with striking spore producing structures are the so-called graphid lichens, which produce their fungal spores in apothecia that are elongated and narrow and are called lirellae. Lirellae look like short scribbles on the thallus and the term graphid is derived from the classical Greek word for 'writing'.

You will find more details on the REPRODUCTIVE STRUCTURES page.

Spores or vegetative propagules may be dispersed by various agents. Fungal spores are quite small and it is easy to understand that, once ejected into the air, they could be easily carried away by even the slightest of breezes. Obviously water is another potential dispersal agent, and animals are a third. For example, migratory birds may pick up vegetative propagules inadvertently and carry them considerable distances.

Various distribution patterns do show themselves. There are endemic Australian species, Australasian species, Gondwanan species, bi-polar species, virtually cosmopolitan species and numerous other patterns. There's more on the subject of lichen distributions in the section on LICHEN BIOGEOGRAPHY. Some of the widespread species are undoubtedly naturally widespread while others will have been dispersed unintentionally by humans.

What's not a lichen

There are various organisms which, though not lichens, might be mistaken for lichens. Sometimes it is only the beginner in lichen studies who would be confused but at other times even an experienced lichenologist would need to examine a specimen microscopically to be sure. There's more about that on the WHAT'S NOT A LICHEN page.

There are several skin conditions which include the word lichen in their names and some examples are: lichen planopilaris, lichen planus, lichen ruber, lichen sclerosus and lichen simplex chronicus. The symptoms may include one or more of inflammation, itchiness, lesions, rashes or thickened skin and those medical conditions have no connection with the lichens of this website other than the name, The English word lichen is derived from a classical Greek word which already had a two-fold meaning, one denoting organisms growing on trees and the other a pustular skin disorder.

A lichen by any other name...

...would still be a lichen all the same.

In the distant past the English word moss and the equivalent words in some other European languages were used in a very general sense to denote a variety of non-flowering organisms. These organisms included mosses (in the modern sense) as well as lichens. As a result some lichens have English common names as mosses!


Gyrophora murina by James Sowerby
known as a 'Velvet Moss'

I've listed them below - first the common name and then the lichen species name.

  • Beard Moss - Usnea barbata
  • Canary moss - Parmotrema perlatum
  • Chalice Moss - Cladonia pyxidata
  • Iceland Moss - Cetraria islandica
  • Jaffna Moss - Alectoria sarmentosa
  • Reindeer Moss - Cladina rangifera, eaten by reindeer and caribou during winter
  • Velvet Moss - Umbilicaria grisea, once known by the name Gyrophora murina
  • White Moss - a term that has been used for a number of lichen species

Lichens are quite distinct from mosses. Lichens are classified with the fungi but mosses are plants and you can find out more about them on the Australian National Botanic Gardens' bryophyte website.

Ecology

Lichen ecology is a large subject. For the moment we’ll deal with various aspects by means of very brief summaries. All of these aspects (plus some others) are covered in more detail in the section on LICHEN ECOLOGY.

Nitrogen constitutes about 80% of the volume of the earth's atmosphere and is essential for life, yet the majority of organisms cannot make direct use of atmospheric nitrogen. Cyanobacteria are amongst the organisms that are able to make direct use of atmospheric nitrogen and such organisms are said to be able to fix atmospheric nitrogen. Hence, lichens with cyanobacterial photobionts fix atmospheric nitrogen. After fixation the nitrogen can become available to plants following the death and decay of the lichen thallus or through herbivore defecation after consumption of such lichens. Some nitrogen may be leached from the lichen and be trapped by other epiphytes (for eventual release through the same processes of death or consumption) or drain into the soil. Various studies have shown that lichens can be a significant source of nitrogen for plants.

Even when not nitrogen-fixing lichens can still contribute significantly to nutrient cycling. Lichens absorb mineral nutrients through their thalli. Think of forests where the trees are festooned with thick epiphytic lichen communities. The large surface area of such a dense epiphytic growth is a very effective means of trapping mist and rainfall (and the nutrients, such as ammonium nitrate, present in rain or mist). The NEGEV SNAIL CASE STUDY shows that even in arid areas lichens can be significant contributors of nutrients.

Some vertebrates eat lichens. The best known case of this is the reindeer and caribou of northern America and Eurasia. The fruticose species Cladina rangifera is eaten by those animals during winter when there is little in the way of vegetation. The animals still lose weight over winter but the lichen is essential for their winter survival.

Some lichens are very effective sand and soil binders and can help in dune stabilization and erosion control. In arid and sub-arid areas lichens, in association with bryophytes, can create extensive BIOLOGICAL SOIL CRUSTS on the soil and such crusts help maintain the underlying soil structure.

Lichen colonies provide niches for numerous invertebrates, often the very tiny invertebrates, which are then eaten by larger invertebrates which, in turn, are eaten by other creatures. Such lichen colonies are thus indirectly important in various food chains.

Lichens are amongst the first organisms to colonize barren surfaces (e.g. road cuttings, rock outcrops and volcanic ash) and prepare these areas for later plants by trapping moisture and windblown organic debris and then contributing to the organic deposits when they themselves die and decay.

Near the beginning of this page I mentioned that lichens can be found growing in almost all parts of the terrestrial world and that there are even a few aquatic species. There's more about this on the HABITATS page but when looking for lichens it's important to think not just of broad habitats such as rainforest, saltbush scrub and so on - but also MICRO-HABITATS. For example some rainforest lichens will grow only on the leaves of flowering plants while others will grow only on tree bark - two distinct micro-habitats in the rainforest macro-habitat! So, as you walk a few metres you may well be passing through a variety of microhabitats, each providing different growing conditions and each host to different species of lichens. Incidentally, when thinking of lichen habitats don't think you must go to exotic locations to see a good variety of species. There are many URBAN LICHENS, though the diversity in urban areas decreases as pollution increases.

Lichens and pollution

Lichens show a great ability to concentrate nutrients from very dilute sources and indiscriminately absorb many toxic substances from the atmosphere (e.g. sulphur dioxide, fluorides and heavy metals). Many lichen species are highly susceptible to air pollution, especially to pollution by sulphur dioxide. Most species of lichens found in areas containing sulphur dioxide show an increased concentration of sulphur in their thalli. The sulphur damages the chlorophyll and with a sufficiently high sulphur level the photobiont will die, thereby bringing about the death of the fungal component as it is incapable of surviving alone. As a general rule fruticose lichens are the least resistant and crustose lichens the most resistant to air pollution. Different species show varying levels of sensitivity to pollutants and by noting the species occurring in an area and their state of health it is possible to monitor pollutant levels. The idea of using lichens as pollution monitors was first made at least as early as 1859 and was more systematically developed in 1866 by the Finnish lichenologist W. Nylander as a result of his studies of the lichens near Paris. There's more on the POLLUTION AND LICHENS page.

 

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