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FUNGAL SYMBIOSIS 269

Fig. 13.12Two foliose lichens. (a) Lobaria pulmonaria(lungwort), which grows on tree trunks in unpolluted parts of
Britain. The lobes are bright green, about 1–2 cm diameter, and can have brown fungal fruiting bodies (apothecia) on
the ridges. (b) Peltigera canina(the “dog lichen”) is a common lichen that produces flat, gray lobes about 2–3 cm dia-
meter on mossy banks. (Note the conspicuous root-like projections – rhizinae– on the lower surface.)

(a) (b)

Fig. 13.13Two fruticose lichens. (a) Cladonia rangiferina(“reindeer moss”) which produces a gray-green, brittle,

multiple-branched thallus. There are several Cladoniaspp. similar to this in upland heathland habitats. They provide a

major source of winter food for reindeer in Scandinavia. (b) Long, pendulous, bright green strands of the lichen Usnea

sp. attached to the branches of trees.

(a) (b)

Structural organization of lichens

Many of the larger lichens have a well-defined struc-

ture, with a clear zonation, as shown in a cross section

of Xanthoria parietina(Fig. 13.16) and in the “tissues”

of Peltigera(Fig. 13.17). Typically, there is an upper

cortexof tightly packed fungal cells, which progres-

sively merges into a medullaof more loosely arranged

hyphae. The cells of the photobiont often occur at the

junction of the cortex and medulla. In many lichens,

including Xanthoria, there is often a lower cortex

which sometimes has anchoring projections termed

rhizinae. The significant feature of this zonation is that

the photosynthetic cells are protected from desiccation

and exposure to intense sunlight by the tightly packed

cells of the upper cortex. But the algal cells also need

gas exchange for photosynthesis, so they are associated

with a loose network of fungal hyphae which are

probably coated with hydrophobins, providing air

spaces for gas exchange (Fig. 13.16).