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36
CONSTITUENTS OF THE LICHEN THALLUS
separating the cells which, from yellow, change to a green colour and
become rounded off (Fig. 13). The mature lichen, a white thallus dotted
with black fruits, contrasts strikingly with the yellow membranous alga.
Lichen formation usually begins near the edge of the leaf and the margin of
the thallus itself is marked by a green zone showing where the fungus has
recently come into contact with the alga.
More recently Hans Fitting 1 has described " Mycoidea parasitica" as it
occurs on evergreen leaves in Java. The alga, a species of Cephaleuros,
though at first an epiphyte, becomes partially parasitic at maturity. It penetrates
below the cuticle to the outer epidermal cells and may even reach
the tissue below. When it is joined by the lichen fungus, both constituents
grow together to form the lichen. Fitting adds that the leaf is evidently but
little injured. In this lichen the alga in the grip of the fungus loses its
independence and may be killed off: it is an instance of something like
intermittent parasitism.
J. RECENT VIEWS ON SYMBIOSIS AND PARASITISM
No hyphal penetration of the bright-green algal cell by means of
haustoria had been observed by the earlier workers, Bornet 2 Bonnier , 3 and
others, though they followed Schwendener 4 in regarding the relationship as
one of host and parasite. Lindau, also, after long study accepted parasitism
as the only adequate explanation of the associated growth, though he never
found the fungus actually preying on the alga.
In recent years interest in the subject has been revived by the researches
of Elenkin 5
a Russian , botanist who claims to have established a case for
parasitism or rather "endosaprophytism." He has demonstrated by means
of staining reagents the presence in the thallus of large numbers of dead
algal cells. A few empty membranes are to be found in the cortex and in
the gonidial zone, but the larger proportion occur below the gonidial zone
and partly in the medulla. He describes the lower layer as a "necral" or
"hyponecral" zone, and he considers that the hyphae draw their nourishment
chiefly from dead algal material. The fungus must therefore be regarded in
this case as a saprophyte rather than a parasite. The algae, he considers,
may have perished from want of sufficient light and air or they may have
been destroyed by an enzyme produced by the fungus. The latter he thinks
is the more probable, as dead cells are frequently present among the living
algae of the gonidial zone. To the action of the enzyme he also attributes
the angular deformed appearance of many gonidia and the paler colour and
gradual disintegration of their contents which are finally used up as endo-
saprophytic nourishment by the fungus. Dead algal cells were more easily
1
Fitting 1910.
2 Bornet 1873.
3 Bonnier i88 9 2 .
5 Elenkin 1903! and 1904!, 19042.
4 Schwendener 1867.