PDF - CES (IISc)

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MORPHOLOGY
occur on the lower surface of the lichen, correspond to those of the upper
in general structure, but there is no intermixture of thalline gonidia. That
Nostoc cells can grow and retain the power to form chlorophyll in adverse
conditions was proved by Etard and Bouilhac 1 who made a culture of the
alga on artificial media in the dark, when there was formed a green pigment
of chlorophyll nature.
Endotrophic cephalodia occur in many groups of lichens' Hue 2 states
that he found them in twelve species of Aspicilia. As packets of blue-green
algae they are a constant feature in the thallus of Solorinae. The species of
that genus grow on mossy soil in damp places, and must come frequently
in contact with Nostoc colonies. In Solorina crocea an interrupted band of
blue-green algae lies below the normal gonidial zone and sometimes replaces
it a connecting structure between cephalodia and a true gonidial zone.
c. PSEUDOCEPHALODIA. Under this section have been classified those
cephalodia that are almost independent of the lichen thallus though to some
extent organically connected with it, as for instance that of Lecidea panaeola
which originate on the hypothallus of the lichen and maintain their position
between the crustaceous granules.
The cephalodia of Lecanora gelida, as described by Sernander 3
, might
also be included here. He watched their development in their native habitat,
an exposed rock-surface which was richly covered with the lichen in all
stages of growth. Two kinds of thallus, the one containing blue-green algae
(Chroococcus}, the other bright-green, were observed on the rock in close
proximity. At the point of contact, growth ceased, but the thallus with
bright-green algae, being the more vigorous, was able to spread round and
underneath the other and so gradually to transform it to a superficial flat
cephalodium. All such thalli encountered by the dominant lichen were
successively surrounded in the same way. The cephalodium, growing more
slowly, sent root-like hyphae into the tissue of the underlying lichen, and
the two organisms thus became organically connected. Sernander considers
that the two algae are antagonistic to each other, but that the hyphae can
combine with either.
The pseudocephalodia of Usnea species are abortive apothecia; they are
surrounded at the base by the gonidial zone and cortex of the thallus, and
they contain no foreign gonidia.
E. AUTOSYMBIOTIC CEPHALODIA
Bitter 4 has thus designated small scales, like miniature thalli, that develop
constantly on the upper cortex of Peltigera lepidophora, a small lichen not
uncommon in Finland, and first recorded by Wainio as a variety of Peltigera
1 Etard and Bouilhac 1898.
2 Hue I9 , o 3 Sernander 1907.
* Bitter 1904.