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Some have been grown in laboratory culture, but they
grow very slowly and they lack the enzymes necessary
for degrading organic polymers, so they are, effectively,
constrained to live in a symbiosis. Only about 100 green
algae have been found associated with lichens. The most
common are single-celled algae of the genus Trebouxia,
found in most lichens of temperate and cool regions.
Trebouxiaspecies also seem to be ecologically special-
ized because they are found infrequently as free-living
species in nature. However, in Mediterranean and
tropical regions the green alga Trentepohlia(and other
genera) is often found instead of Trebouxia, and
Trentepoliacan often be found growing independently
in nature, so it is not as ecologically specialized as
Trebouxia.
About 10% of lichens have cyanobacteria as their
main or only photosynthetic partner. The most com-
mon examples in Northern Europe and Scandinavia
are lichens of the genus Peltigera, which have Nostocas
their photobiont. However, some lichens that contain
green algae can also have cyanobacteria in special
wart-like structures (cephalodia) on the lichen surface.
These structures are found in about 3– 4% of lichens
and their role is probably to exploit the nitrogen-
fixing abilities of the cyanobacteria.
The fact that lichens can be formed by more than
one type of fungus (Ascomycota or Basidiomycota)
and more than one type of photosynthetic partner
(green algae or cyanobacteria) suggests that this type

of symbiosis has evolved independently on several

occasions. It is impossible to trace the evolutionary his-

tory of lichens, because they are not single organisms,

but the lichen symbiosis is unlikely to have evolved

until the Ascomycota developed, more than 300 mil-

lion years ago.

The range of forms in lichens

Lichens exhibit a variety of forms, ranging from

almost casual associations of fungi and photosyn-

thetic cells in some of the desert crusts (discussed

later) to highly differentiated structures with a clear

zonation of tissues. Germinating ascospores can produce

typical fungal colonies on normal culture media, but

a suitable photobiont is required to trigger morpho-

genetic changes leading to several different forms of the

lichen thallus(the “body” of the lichen) which are

commonly grouped into four categories (Figs 13.12–

13.15):

1 Foliose lichens, which have a flat, leaf-like structure.

2 Fruticose lichens, which have an erect or pendulous

structure.

3 Squamulose lichens, which produce small, scale-like

plates.

4 Crustose lichens, which produce flat crusts on rock,

soil or tree surfaces.

268 CHAPTER 13

Fig. 13.11The lichen Xanthoria parietina, which commonly grows on rocky shores exposed to salt spray. (a) The lichen

thallus. (b) Close-up of part of the thallus, showing the disk-shaped apothecia which release ascospores.

(a) (b)