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FAQ
Frequently Asked Questions (if anyone actually asked any)

These pages will be used to provide various notes and explanations.
I intend that the FAQ will expand into an illustrated glossary, but this will take time.


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Lichen asexual dispersal
 

squamules, isidia, soredia, blastidia, phyllidia, schizidia, conidia


Additional types of asexual dispersal are described in Smith et al. (2009) (page 25, fig. 6), adapted from Hawksworth & Hill (1984). A useful summary was also provided by Ryan et al. (2002).


Lichen sexual reproduction: there is a problem

The great majority of lichens belong to the Ascomycota, in which the sexual reproductive unit is the ascospore. This is fine; ascospores are generally thick-walled and are tough, resilient units of dispersal and there is a realistic chance of a small number of the ascospores arriving at suitable sites for germination. However, lichen fungi require a suitable algal component (phycobiont) in order to survive and to form a lichen. Ascospores disperse only the fungus.

Consequently, one of two things must happen:

  1. the spore could fortuitously land in a place where the alga already exists in a free state.
     
  2. the spore could land on another lichen, the fungus then taking over the phycobiont from the existing lichen. This is a common phenomenon; e.g. species of Diploschistes, a common genus of crustose lichens on boulders, mostly grow initially as parasites of other lichens. The lichen Rhizocarpon viridiatrum begins as a parasite on Aspicilia caesiocinerea.

Rhizocarpon viridiatrum invading Aspicilia caesiocinerea
 
 
 
Parasitism by Rhizocarpon viridiatrum,
(Nenthead, Cumberland, August 2012).
 
R. viridiatrum (yellow) is invading and taking over a thallus of Aspicilia caesiocinerea (grey), presumably retaining the phycobiont. Unparasitised A.  caesiocinerea is to be seen at the right.

If, instead, the lichen can produce units of dispersal that contain both fungus and alga, this will obviously allow and speed up colonisation of new sites. Consequently, lichens have evolved various methods of transmitting the alga along with the fungus.


 

Units of asexual dispersal

 
1.   Squamules

The lichen primary thallus may be composed of squamules (squamulose lichens are more fully described here), or squamules may be produced from the surfaces of secondary structures such as podetia. Either way, squamules are easily broken away and may be distributed by animal activity, trampling or wind, before genenerating to produce new thalli. It is likely that this is a major mode of spread in Cladonia species, particularly at existing sites.

 
 
 
Squamulose podetia in Cladonia bellidiflora,
(Aberdeenshire, 2008).
 
The podetia are thickly clothed in squamules, any one of which might break away and, if lodging in a suitable site, may regenerate to produce a new thallus.
Cladonia bellidiflora

It should be noted that in Cladonia, squamules in some species are sorediate, soredia being produced from the exposed medulla on the underside, or from the margins. Presumably, these give a greater chance of longer-range dispersal.

Stereocaulon vesuvianum phyllocladia
 
 
 
Stereocaulon vesuvianum, (Gleniffer Braes, Renfrewshire, April 2011).
 
Phyllocladia, produced from the surfaces of pseudopodetia in Stereocaulon species, presumably have a similar disperal role to that of squamules.


 
2.   Isidia

Numerous minute outgrowths produce peg-like, coral-like or globose structures. Each of these is an isidium. Isidia contain algal cells as well as fungal tissue, i.e. the photobiontic layer also extends up into the isidia, and eventually they are liable to break away and be dispersed, leaving tiny scars on the thallus surface. With the cortex providing a tough outer surface, this package of fungus plus algal cells is a durable unit of dispersal.

 
 
 
Development of isidia,
a vertical section of the thallus shows outgrowths from the upper cortex. Tissue from the medulla, notably including the photobiontic layer, grows into these outgrowths to form cortex-bounded, peg-like or coralloid isidia, which can then break off and be dispersed. Each isidium contains both fungus and alga. (From Smith, 1921).
 
 
 
Isidia in Melanelixia glabratula,
(Paisley, Renfrewshire, 2012)
 
Cylindrical and branched isidia project from the lobe surfaces, and readily break away. The shape of the isidia is of value in identifying M. glabratula and similar species of Melanelixia and Melanohalea.
 
This is a common species, but apothecia are infrequent, suggesting that most dispersal is by the isidia.


 

 
3.   Soredia

Like isidia, soredia are dispersal packages of both fungus and alga. Unlike isidia, a soredium is not enclosed in a protective outer cortical layer. Soredia can be produced in much greater numbers, and generally are like powder, but probably are shorter-lived. On substrates such as damp bark, they can germinate quickly and are a highly efficient medium of at least local dispersal.

Soredia are produced from exposed medulla tissue. They may be produced from lichen surfaces lacking a cortex, e.g. uncorticated areas of a Cladonia podetium, but commonly they originate from a defined structure, a soralium.

 
 
 
Development of soredia from a soralium,
a vertical section of the thallus shows a break in the upper cortex, exposing the medulla. Tissue from the photobiontic layer is differentiating into rounded, powdery granules, each consisting of algal cells wrapped in fungal hyphae. (From Smith, 1921).
 
 
 
Soredia in Evernia prunastri,
(Slapton, Devon, 2011).
 
Soredia are produced from irregular soralia, principally along the lobe margins.
Evernia prunastri, soredia

Soralia vary in form and location, the character often being of identification value. On lobes of foliose species they may be laminar (produced from the lobe upper surface), marginal or terminal. They may be pretty much level with the lobe or crust surface, markedly convex, or in some species they are characteristically excavate (somewhat hollowed). They may be irregular or delimited, sometimes disc-like, or produced from fissures in the lobe surfaces or margins.

 
 
 
Disc-like, marginal soralia in Ramalina farinacea,
(Glen Tilt, Perthshire, 2008).
 
In this case, the soralia along the angles of the flattened branches can readily be mistaken for apothecia.
Ramalina farinacea, soralia


 

 
4.   Blastidia

Blastidia are not easily distinguished from soredia and in practical terms can be considered much the same. Technically, blastidia bud from other blastidia and originally from the thallus edge or surface, and so might be in clusters or short chains, and they may be partly corticate. As they may aggregate together, they tend to be less powdery.

In the small species of Caloplaca in the "C. citrina group" (genus Flavoplaca as separated by Arup et al., 2013), some species are sorediate, the soredia perhaps initially produced at the areolar edges and later extending across the areole surfaces. Other species are considered to produce blastidia instead, again often initially at the areolar edges. The blastidia are less powdery but there is little to distinguish them from soredia by normal microscopic examination. Fortunately, the distinction is rarely critical in identification.

Caloplaca limonia
 
 
 
Caloplaca limonia, (Great Cumbrae, Clyde Isles, 2012).
 
C. limonia characteristically has the areolar surfaces clothed in relatively large, somewhat angular propagules that are considered to be blastidia, or a mixture of blastidia and soredia. In the closely related C. austrocitrina, the propagules are smaller and powdery and considered to be soredia.

A variety of other small, granular propagules may be seen in different lichens. In Cladonia, the distinction between soredia and larger, perhaps partly corticate granules on podetial surfaces can be of identification importance.


 

 
4.   Phyllidia and folioles

Phyllidia are small, leaf-like or scale-like outgrowths from an always (?) foliose thallus, constricted at the point of attachment and so readily detached and dispersed by wind or animals. They are corticate on both surfaces, so differing from squamules. Folioles (or lobules) are much the same thing, though the term is generally used for what appear more like miniature lobes that detach a little less readily. Casual observation of folioles in a local colony of Parmelia ernstiae confirms that while they are abundantly produced early in the year, they are largely gone by early summer, supporting their evident rôle as dispersal agents. Phyllidia and folioles can be considered as leaf-like, flattened isidia.

 
 
 
Phyllidia in Degelia ligulata,
(Isle of Muck, 2012).
 
Phyllidia are abundantly produced along the older lobe margins and break away, potentially to regenerate to produce a new thallus. In related species, globose isidia fulfil the same function. [Some published descriptions of D. ligulata use the term "schizidia" rather than "phyllidia", but this is incorrect.]
Degelia ligulata
 
 
 
Folioles in Parmelia ernstiae,
(Lochwinnoch, Renfrewshire, 2010).
 
This species, a close relative of P. saxatilis, produces abundant folioles on the lobe margins early in the year (along with clusters of isidia on the lobe surfaces). Later in the year, this same thallus was almost devoid of folioles.
Parmelia ernstiae


 

 
5.   Schizidia

Schizidia are small, scale-like portions of upper cortex, along with some underlying tissue (photobiontic layer and medulla), that break away from a thallus surface. The term is over-used (see above) and true examples are evidently few, but include Fulgensia species.

Fulgensia fulgens
 
 
 
Fulgensia fulgens ('Scrambled-egg Lichen'),
(Stackpole, Pembrokeshire, 2009).
 
Schizidia have flaked from the thallus surface, leaving white areas of exposed medulla.

Ryan et al. (2002) regard the disc-like granules, with corticate upper surfaces, produced within the cups of Cladonia pyxidata, as schizidia.


 

 
6.   Conidia

Conidia are asexual fungal spores, commonly thin-walled and usually functioning more as a means of rapid, local reproduction than of longer range dispersal. Fungi in which a sexual stage is very rare, or apparently non-existent, may have thicker-walled, more resilient conidia, but those of lichenised fungi are typically of the thin-walled and presumably short-lived type.

The rôle of conidia in lichens is much more a matter for debate. While apparently often simply for asexual reproduction, it appears that at least some may also or alternatively function as male gametes in the sexual process. Conidia are commonly produced from within pycnidia, closed, flask-like structures, often visible as black dots set in the lichen thallus.

While necessarily included here as units of asexual dispersal, conidia cannot carry the phycobiont and so are not units for reproduction and dispersal of the complete lichen association. A good review of lichen conidia, their variation and the structures that produce them has been provided by Bungartz (2002).


 

References
    •   
Arup, U., Søchting, U., & Frödén, P. (2013). A new taxonomy of the family Teloschistaceae. Nordic Journal of Botany 13: 16–83.
          
    •   
Bungartz, F. (2002). Morphology and anatomy of conidia-producing structures, in Nash III, T.H., Ryan, B.D., Gries, C., & Bungartz, F. (eds.) Lichen flora of the Greater Sonoran Desert Region, vol. 1, Arizona State University, Tempe, pp. 35–40.
          
    •   
Hawksworth, D.L., & Hill, D.J. (1984). The lichen-forming fungi, Blackie, Glasgow.
          
    •   
Ryan, B.D., Bungartz, F., & Nash III, T.H. (2002). Morphology and anatomy of the lichen thallus, in Nash III, T.H., Ryan, B.D., Gries, C., & Bungartz, F. (eds.) Lichen flora of the Greater Sonoran Desert Region, vol. 1, Arizona State University, Tempe, pp. 8–23.
          
    •   
Smith, A.L. (1921). Lichens, Cambridge University Press, Cambridge.
          
    •   
Smith, C.W., Aptroot, A., Coppins, B.J., Fletcher, A., Gilbert, O.L., James, P.W., & Wolseley, P.A. (eds.) (2009). The lichens of Great Britain and Ireland, British Lichen Society, London.
          


 

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Photographs and text © A.J. Silverside
March 2014
Line illustrations from out of copyright source, as noted.