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The medley of lichen species to the left was
growing on a pear twig. Lichens are beautiful organisms composed of
both fungal and algal tissue, forming a body (thallus - singular, thalli -
plural) that appears to be a single organism. They are mutualists --
each partner gains something from the association. Lichens generally
do not harm their tree hosts. Although their thalli do poke some fungal
threads (hyphae) between the cells of the outer bark, only in rare instances
has it been found that the lichen takes up nutrients directly from the host
by this means. Lichens are not plants (like the bright green
moss seen growing with them here), but the algal partner does make food by
photosynthesis, and, there are several structural similarities between the
inside of a lichen and the inside of a plant leaf. They are rarely
the green color of grasses or moss, but usually some off green color such
as the mineral gray green of the Parmelia and Physcias here
or the yellow green of the Ramalina. Xanthoria is one
of the few orange lichens. |
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Physcia tenella, Physcia aipolia, Parmelia
sulcata and Xanthoria polycarpa are all foliose lichens, defined
as being flattened and leaf like, with a definite upper and lower surface.
Ramalina farinacea is a fruticose lichen, defined as being tufted
or drooping, with round or flat lobes but without a definitely different
lower surface.
Physcia aipolia and Xanthoria
polycarpa both exhibit the sexual fruiting structures (disk-shaped apothecia)
of their fungal partners. Parmelia sulcata and Ramalina farinacea
both have asexual reproductive bodies called soredia (see below). These
are powdery white structures, just visible as white markings on the lobes
of these species. Physcia tenella shows its lower root-like rhizines
at the upper left: these are anchors across the lower surface of many
lichens.
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A. A diagrammatic cross section
of a lichen shows the two partners of a foliose lichen. The basic substance
of the lichen is composed of fungal hyphae, with the upper and lower surface
(upper and lower cortex) built of cell-like compartments of closely adhering
hyphae and an internal, open network of hyphae (medulla). The algal
cells of most foliose lichens lie in a layer below the upper surface.
B. Soredia are
asexual reproductive structures that are composed of loose collections of
fungal hyphae and algal cells. They brush or blow off the thallus,
land on a new host and, in time, grow into the recognizable lichen.
Individual soredia are almost microscopic: they appear in mass as floury
eruptions from the thallus surface or margins. A handlens will resolve
the individual structures.
C. Isidia also
function in asexual reproduction, but differ from soredia in being a bit
larger and in having an outer covering (cortex). They also break off
the parent thallus and can start growth into a new lichen somewhere else.
(µm = micrometer; 1 µm = 1/1000 millimeter
= 1/1,000,000 meter)
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The upper righthand corner of clear and brownish
cells (A) are formed by the fungal partner and form the upper surface of the
lichen. The remaining structures below this surface are composed of
a network of fungal threads (hyphae) and photosynthetic cells. In most
lichens photosynthetic green algae are arranged in a layer (algal layer)
just under the outer lichen surface as seen here by the bright green cells
(B). In some lichens there are additional partners. This thin
section was made from the lichen, Lobaria oregana, which has three
partners: a fungus, a green alga and a bluegreen bacterium. The
olive green cells in the lower left half of the image (C) are this bluegreen
bacterium, a species of Nostoc. |
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This transmission electron microscope image,
again of the lichen Lobaria oregana, shows cross-sections through the
lower part of the green algal layer. Several fungal hyphae (A) and
two green algae (B) can be seen with the chloroplasts (organelles where photosynthesis
takes place) in the algal cells showing up as dark structures. |