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A large number of lichens produce apothecia to

disperse their sexual spores (ascospores), as shown in

Fig. 13.20. When these spores germinate they must

contact the cells of a suitable photosynthetic partner

to establish a new lichen thallus. This process of

“reassembly” can be demonstrated in experimental

conditions, and evidence suggests that it also occurs in

nature. But its frequency may vary between different

types of lichen. For example, the separate dispersal

(and subsequent reassociation) of fungal and algal

(Trebouxia) cells might be the major means of dispersal

for the common lichen Xanthoria parietina(Fig. 13.20)

because this species does not produce soredia or other

specialized vegetative propagules.

Lichen ecology and significance

Lichens are classic “pioneer” colonizers in a wide range

of environments. They grow on the bark of temperate

trees or as epiphytes on the leaves of tropical rain

forest trees. Some lichens occupy the most inhospitable

environments on earth, growing on cooled lava flows

and bare rock surfaces. Other types grow abundantly

on tundra soils, providing a winter food source for

reindeer and caribou in arctic and subarctic regions.

Yet other lichens grow on or in the perennial leaves

of economically important tropical plants such as

coffee, cacao, and rubber. In all these respects, lichens

are significant components of ecosystems. But perhaps

their most significant role lies in their contribution to

soil formation, and in many cases this is brought about

by inconspicuous lichens that are so rudimentary that

they represent little more than a juxtaposition of

fungal and photosynthetic cells. For example, in the

soil cruststhat cover many semiarid regions of the

world the microbial population consists of mats of

cyanobacteria interspersed with fungal hyphae and

a few small lichens just visible to the naked eye

(Figs 13.21–13.23).

Summary of the lichen symbiosis

The great diversity of lichens and the many envir-

onments in which they occur attest to the fact that

lichen symbioses are highly successful. Lichens are

slow-growing organisms, so they do not necessarily

contribute greatly to biomass production. But their

unique symbiosis enables them to grow in a range

of environments that no other organism can tolerate

and, importantly, in conditions that none of the con-

stituent partners could tolerate alone.

Geosiphon pyriforme

Geosiphon pyriformeis a remarkable organism that was

first reported in 1996 and is known from only very few

sites in Germany. It is a soil fungus, closely related to

the arbuscular mycorrhizal fungi, but it incorporates

a cyanobacterium (Nostoc punctiforme) into its cells as

FUNGAL SYMBIOSIS 273

Fig. 13.20Part of a thallus of Xanthoria parietina, sectioned through an apothecium. (a) Many asci are seen just beneath
the upper surface of the apothecium. (b) Part of a crushed apothecium, composed of packing hyphae (paraphyses)
and asci containing ascospores.

(a) (b)