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A few
lichens produce special spores in surface structures known as
pycnidia. These are buried in the thallus, appearing as tiny black
dots on the surface where the spores escape. The pycnidial spores
are formed by budding. The shapes and sizes of these spores usually
are characteristic of a genus. |
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Lichens
usually reproduce by distributing clumps containing algal cells and
fungal hyphae. The presence of each is necessary for this type of
reproduction. Such clumps include isidia, small growths with cortex
and medulla that can break off and propagate the lichen, and
soredia, powdery clusters, lacking cortex, that break through the
surface of the thallus. |
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This book
is concerned with recognition of some of the common and conspicuous
lichens in the field. Although the examples are taken from the
Rocky Mountain region, most of these lichens occur in mountain
areas throughout the world. For convenience, lichens can be divided
into four categories: crustose, squamulose, foliose, and
fruticose. |
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Crustose
lichens are closely anchored (appressed) to the substrate. They may
appear almost "painted" on the substrate, and because of their
close attachment they are difficult to collect unless part of the
substrate is taken as well. The thallus of a crustose lichen has no
lower cortex, and consists of an upper cortex, an algal layer, and
a medulla which functions to attach the lichen to the substrate.
Some crustose species are areolate, cracked into small islands or
areoles. See, for instance, Rhizocarpon geographicum (Plate
3). |
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Squamulose
lichens have a thallus made up of scalelike units or lobes
(squamules), for the most part closely attached, but free or
upturned at the edges. A good example is Psora decipiens
(Plate 12). A strikingly different squamulose lichen is the genus
Cladonia, which often appears on soil, moss, or rotting wood
such as old fence posts or fallen trees. Its thallus sends up
fruiting stalks, called podetia. In some species these may be
branched, or may look like tiny wine goblets or golf tees. In
others, the stalks may be straight posts, perhaps a quarter of an
inch tall. For examples, see Cladonia ecmocyna and C.
pyxidata (Plates 27 and 28). |
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Foliose
lichens are leaflike or have distinct, often large, lobes. They are
attached more loosely at the substrate. The thallus of most foliose
lichens has both an upper and lower cortex, and many have hairlike
structures called rhizines, which grow from the lower cortex and
serve to attach the lichen to the substrate. Some foliose species
are attached to a central umbilical point from which the lobes
radiate. For illustrations, see Parmelia, Umbilicaria, and
Dermatocarpon. |
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