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34
CONSTITUENTS OF THE LICHEN THALLUS
Lindau 1 has described the association between fungus and alga in
Pertusaria and other crustaceous forms as one of contact only (Fig. 12).
He found that the cell-membrane of the two adhering organisms was unbroken.
Occasionally the algal cell showed a slight indentation, but was
otherwise unchanged. The hyphal branch was somewhat swollen at the tip
where it 'touched the alga, and the wall was slightly thinner. The attachment
between the two cells was so close, however, that pressure on the cover-
glass failed to separate them.
Generally the hypha simply surrounds the gonidium with clasping
branches. Many algae also lie free in the gonidial zone, and Peirce 2 claims
that these are larger, more deeply coloured and in every way healthier
looking than those in the grasp of the fungus. He ignores, however, the case
of the soredial algae which though very closely invested by the fungus are
yet entirely healthy, since on their future increase depends in many cases
the reproduction of new individual lichens.
In a recent study of a crustaceous sandstone lichen, "
Caloplaca pyracea"
Claassen 3 has sought to prove a case of pure parasitism. The rock was at first
covered with the green cells of Cystococcus sp. Later there appeared greyish-
white patches on the green, representing the invasion of the lichen fungus.
These patches increased centrifugally, leaving in time a bare patch in the
centre of growth which was again colonized by the green alga. The lichen
fruited abundantly, but wherever it encroached the green cells were more
or less destroyed. The true explanation seems to be that the green cells
were absorbed into the lichen thallus, though enough of them persisted to
start new colonies on any bare piece of the stone. In the same way large
patches of Trentepohlia aurea have been observed to be gradually invaded
by the dark coloured hyphae of Coenogonium ebeneum. In time the whole of
the is alga absorbed and nothing is to be seen but the dark felted lichen.
The free alga as such disappears, but it is hardly correct to describe the
process as one of destruction.
This algal genus Trentepohlia (Chroolepus) forms the gonidia of the
Graphideae, Roccelleae, etc. It is a filamentous aerial alga which increases
by apical growth. In the Graphideae, many of which grow on trees beneath
the outer bark (hypophloeodal), the association between the two symbionts
may be of the simplest character, but was considered by Frank 4 to be of an
advanced type. According to his observations and to those of Lindau 5 , the
fungal hyphae penetrate first between the cells of the periderm. The alga,
frequently Trentepohlia nmbrina, tends to grow down into any cracks of the
surface. It goes more deeply in when preceded by the hyphae. In some
species both organisms maintain their independent growth, though each
shows increased vigour when it conies into contact with the other. In some
1 Lindau 1895'.
* Peirce I899