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You will
see, below the colored upper surface, even with the naked eye, a
thin line of bright green. This is the algal (now called the
photobiont) layer. Although a lichen is two different organisms
growing together, the form of a thallus is remarkably similar to
that of a green leaf. A leaf has a colorless upper layer
(epidermis). The lichen has a usually colored upper layer (cortex);
it consists of densely packed fungal hyphae. A leaf has a layer
below the epidermis, of vertically-elongated green cells closely
packed together (the palisade layer). The lichen has green algal or
bluegreen algal (cyanobacteria) cells. The fungus feeds on these
green cells, but they reproduce faster than they are consumed. The
leaf has a lower layer of pale cells forming a very loose network
(the spongy mesophyll). The lichen has a loose organization of
fungal hyphae (medulla). There may or may not be a lower epidermis
(in the leaf) or a lower cortex (in the lichen). Leaves have
stomates (doughnut-shaped pairs of cells that open and close,
allowing the exchange of oxygen and carbon dioxide). Lichens do not
require openings, but frequently have cracks or holes
(pseudocyphellae) in the cortex through which hyphae and algae may
poke. The lower cortex often produces hairlike rhizines or
suction-cup stalks (hapteres) that anchor the lichen to the
substrate. |
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The
arrangement of the various layers in the lichen and the leaf are
extremely similar, and is the most efficient for the process of
photosynthesis. The fact that the end result is obtained from
completely different materials is one of the wonders of plant life.
For the most part, it is thought that the spores are very
infrequently involved in the reproduction of the lichen, but are
leftovers from the days long ago when the fungus lived
independently. Try to imagine how this curious relationship between
fungus and alga began, and how the various lichens evolved over
millions of years. This is the great question for which no one yet
has the answer. |
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Apothecia
are very important in the recognition of many lichens. They are
usually small, rimmed cups or disks, appearing on the surface of
the thallus. The margins are sometimes, but not always, the color
of the thallus. The disks themselves are almost always a different
color. Perithecia also produce spores, but these structures,
flask-shaped, with a small opening at the top, are imbedded in the
thallus. Apothecia and perithecia produce spores (produced by
meiosis) which are diagnostic of species, but they probably play an
insignificant role in reproduction, because the lichen must
reproduce by fragmentation of a thallus containing both fungus and
alga. The strains of algae found in lichens evidently do not occur
free-living in nature. |
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