PDF - CES (IISc)

REPRODUCTION
The thallus of this family of lichens is granular or furfuraceous ; it never
goes beyond the Lepra stage of development 1 . In some species it is scanty,
in others it is abundant and spreads over large areas of the trunks of old
trees. It is only when growth is especially luxuriant that oidia are formed.
Neubner was able to recognize the oidial condition by the more opaque
appearance of the granules, and under the microscope he observed the
hyphae surrounding the gonidia gradually fall away and break up into
minute cylindrical cells somewhat like spermatia in size and form. There
was no question of abnormal or unhealthy conditions, as the oidia were
formed in a freely fruiting thallus.
The gonidia associated with the oidial hyphae also showed unusual
vitality and active division took place as they were set free by the breaking
up of the encircling hyphae. The germination of the oidia provides an
abundance of hyphal filaments for the rapidly increasing algal cells, and
there follows a widespread development of the lichen thallus.
Oidial formation has not been observed in any other family of lichens.
B. REPRODUCTION BY CONIDIA
a. INSTANCES OF CONIDIAL FORMATION. It is remarkable that the
type of asexual reproduction so abundantly represented in fungi by the large
Conidia developed
om thallus of Arnoldia minutula
Born. x 950 (after
Bornet).
and varied group of the Hyphomycetes is practically
absent in lichens. An exception is to be
found in a minute gelatinous lichen that grows on
soil. It was discovered by Bornet 2 and called by
him Arnoldia (Physmd) minutula. From the thallus
rise up simple or sparingly branched colourless
conidiophores which bear at the tips globose brown
conidia(Fig. 108). Bornet 3 obtained these conidia
by keeping very thin sections of the thallus in a
drop of water 2 .
Yet another instance of conidial growth is given
by Steiner 4 . He
had observed that the apothecia
on plants of Caloplaca aurantia var. callopisma
Stein, differed from those of normal appearance
in the warted unevenness of the disc and also in
being more swollen and convex, the thalline margin
being almost obliterated. He found, on microscopical
examination, that the hymenium was
occupied by paraphyses and by occasional asci,
the latter seldom containing spores, and being
See 2
p. 143.
Bornet 1873.
3
Bornet's observations have not been repeated, and it is possible that he may have been dealing
with a parasitic hyphomycetous fungus.
4 Steiner 1901.