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LICHEN GONIDIA 37
seen, he tells us, in crustaceous lichens associated with " Pleurococcus" or
" Cystococcus" \ they were much less frequent in the larger foliose or fruticose
lichens. Dead cells of Trentepohlia were also difficult to find.
In a second paper Elenkin records one clear instance of a haustorium
entering an algal cell, and says he found some evidence of hyphal branches
penetrating otherwise uninjured gonidia, round holes being visible in their
outer wall, but he holds that it is the cell-wall of the alga that is mainly
dissolved by the ferment and then used as food by the hyphae.
No allowance has been made by Elenkin for the normal wasting common
to all organic beings: the lichen fungus is continually being renewed,
especially in the cortical structures, and the alga must also be subject to
claims, nevertheless, that his observations have proved that the
change. He 1
one symbiont is always preying on the other, either as a parasite or as a
saprophyte. He has likened the conception of symbiosis to that of a balance
between two organisms, "a moveable equilibrium of the symbionts." If, he
says, we could conceive a state where the conditions of life would be equally
favourable for both partners there would be true mutualism, but in practice
one only is favoured and gains the upper hand, using its advantage to prey
on the other. Unless the balance is redressed, the complete destruction of
the weaker is certain, and is followed in time by the death of the stronger.
The fungus being the dominant partner, the balance, he considers, is tipped
in its favour.
Elenkin's conclusions are not borne out by the long continued and healthy
life of the lichen. There is no record of either symbiont having succumbed
to the other, and the alga, when set free, is unchanged and able to resume its
normal development. Without the alga the fungus cannot form the ascigerous
fruit. Is that because as a parasite within the lichen it has degenerated past
recovery, or has it become so adapted to symbiosis that in saprophytic conditions
it fails to develop ?
Another Russian lichenologist, U. N. Danilov 2
, records results which
would seem to support the theory of parasitism. He found that from the
clasping hyphae minute haustoria were produced, which penetrate the algal
cell-wall, and branch when within the outer membrane, thus forming a
delicate network over the plasma; secondary haustoria arising from this
network protrude into the interior and rob the cell-contents. He observed
gonidia filled with well-developed hyphae and these, after having exhausted
one cell, travel onwards to others. Some gonidia under the influence of the
fungus had become deformed and were finally killed. As a proof of this
latter statement he adduces the presence in the thallus of some gonidia
containing shrivelled protoplasm, of others entirely empty. He considers, as
further evidence in favour of parasitism, the finding of empty membranes as
1 Elenkin I9o6 2 .
5G01.1
2 Danilov 1910.