Introduction to Fungi, Third Edition

GENERAL ASPECTS OF LICHEN BIOLOGY
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to higher plants. Lichens are classical pioneer
organisms, e.g. on bare rocks or infertile soils.
Lichens can cause the weathering of rocks by
secreting oxalic acid which reacts chemically
with the rock surface; the rate of degradation
may be 0.5 3.0 mm century 1 (Hale, 1983). Dirina
massiliensis f. sorediata has been shown to cause
much more rapid weathering of limestone surfaces,
including those of historical monuments,
at a rate of up to 2 mm in 12 years (Seaward &
Edwards, 1997). Extensive lichen communities
also exist on the bark and foliage of trees (corticolous
lichens). Additionally, freshwater and
marine species have been described. Lichens
occur in all climatic zones from the Arctic and
Antarctica, where they provide the dominant
vegetation (Seppelt, 1995), to the tropics.
Some lichen thalli live for over 1000 years
and can be used for determining the age of rock
surfaces because of their slow growth rate. This
discipline is known as lichenometry (Hale, 1983;
Innes, 1988). It has been applied, for example, to
date the standing stones on Easter Island or the
time point of exposure of rock surfaces caused by
avalanches or earthquakes. Crustose lichens are
commonly used for lichenometry because they
have the slowest growth rate. An example is the
‘map lichen’, Rhizocarpon geographicum (Plate 8b;
O’Neal & Schoenenberger, 2003).
A wide range of lichens has been examined
by different research groups. Therefore, in the
present chapter we will give an introduction to
the general features of lichen biology, followed
by brief profiles of common examples taken from
the Lecanorales, which is by far the largest order
of lichenized fungi. Good general textbooks on
lichens are those by Hale (1983), Ahmadjian
(1993) and Nash (1996a). Richardson (1975) has
written a stimulating account of the importance
of lichens to mankind and in natural ecosystems.
16.2 General aspects of lichen
biology
16.2.1 Morphology of the lichen thallus
Lichen thalli come in three basic shapes
crustose (crust-like), fruticose (shaped like a
miniature shrub) or foliose (leaf-like). It should
be noted that these are purely descriptive terms
which have no taxonomic meaning. Intermediate
forms also exist. Good summaries are those by
Büdel and Scheidegger (1996) and Honegger
(2001).
By far the most common type is the crustose
thallus which forms a thin spreading crust
firmly attached to the substratum by its entire
lower surface (Plate 8a,b). In the morphologically
simplest crustose lichens, fungal hyphae are
loosely associated with photobiont cells but do
not form a protective upper cortex. Such lichen
thalli appear powdery and are referred to as
leprose. They often have a highly hydrophobic
surface. Other crustose lichens produce a thicker
thallus often held together by mucilage, as in the
gelatinous lichens. In more highly differentiated
crustose thalli, the photobiont cells are positioned
in a defined layer located underneath an
upper cortex formed exclusively by the mycobiont.
The photobiont cells are thus protected
from adverse environmental factors. Air spaces
in the photobiont layer and the medulla underneath
permit gas exchange (see Fig. 16.1). The
differentiation of the lichen thallus into horizontal
layers is called stratification. Insquamulose
lichens (Plate 8e), the crustose thallus forms
small scales (squamules) which become partially
raised from the substratum, giving the surface a
scurfy appearance.
The stratification is developed further in the
second thallus type, the foliose lichens (Plate
8c,d) by the development of a lower cortex.
Attachment to the substratum is often by bundles
of hyphae termed rhizinae. As a result, the
thallus appears leaf-like or lobed and can be
detached from the substratum without being
damaged.
The third thallus type is called fruticose. Here
the thallus has a shrub-like or branched appearance
and is raised from the substratum (Plate 8f)
or hangs down from it (Plate 8g). In some cases
a fruticose thallus may develop from a basal
crustose or foliose thallus (Plate 8e). Stratification
in fruticose thalli is often tubular/
concentric rather than horizontal, and it resembles
the more complex crustose types in possessing
an outer cortex overlying a photobiont layer