PDF - CES (IISc)

38
CONSTITUENTS OF THE LICHEN THALLUS
well as of colourless gonidia filled with the hyphal network. This description
hardly tallies with the usual healthy appearance of the gonidial zone in the
normal thallus, and it has been suggested that where the fungus filled the
algal cell, it was as a saprophyte preying on dead material.
The gradual perishing of algal cells in time by natural decay and their
subsequent absorption by the fungus is undisputed. It is open to question
whether the varying results recorded by these workers have any further
significance.
These observations of Elenkin and Danilov have been proved to be
erroneous by Paulson and Somerville Hastings 1 . They examined the thalli
of several lichens (Xanthoria parietina, Cladonia sp., etc.) collected in early
spring when vegetative growth in these plants was found to be at its highest
activity. They found an abundant increase of gonidia within the thallus,
which they regarded as sporulation of the algae, and the most careful methods
of staining failed to reveal any case of penetration of the gonidia by the
hyphae.
Nienburg 2 has published some recent observations on the association of
the symbionts. In the wide cortex of a Pertusaria he found not only the
densely compact hyphae, but also isolated gonidia. In front of these latter
there was a small hollow cavity and, behind, parallel hyphae rich in contents.
These gonidia had originated from the normal gonidial zone. They were
moved upward by special hyphae called by Nienburg "push-hyphae." After
their transportation, the algae at once divide and the products of division
pass to a resting stage and become the centre of a new thalline growth. A
somewhat similar process was noted towards the apex of Evernia furfur acea.
Radial hyphae pushed up the cortex, leaving a hollow space over the gonidial
zone. Into the space isolated algae were thrust by "push-hyphae." In this
lichen he also observed the penetration of the algal cell by haustoria of the
fungus. He considers that the alga reaps advantage but also suffers harm,
and he proposes the term helotism to express the relationship.
An instructive case of the true parasitism of a fungus on an alga has been
described by Zukal 3 in the case of Endomyces scytonemata which he calls
a "half-lichen." The mature fungus formed small swellings on the filaments
of the Scytonema and, when examined, the hyphae were seen to have attacked
the alga, penetrating the outer gelatinous sheath and then using up the
contents of the green cells. It is only after the alga has been destroyed and
absorbed, that asci are formed by the fungus. Zukal contrasts the development
of this fungus with the symbiotic growth of the two constituents in
EpJiebe where both grow together for an indefinite time.
Mere associated growth however even between a fungus and an alga
does not constitute a lichen. An instance of such growth is described by
1 Paulson and Hastings 1920.
2
Nienburg 1917.
3 Zukaj l89I>