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LICHEN ALGAE 61
only less evident but much more faintly coloured. In Synalissa, a minute
shrubby lichen which has the same algal constituent, the tissue of the thallus
is more highly evolved, and in it the red colour can barely be seen and
then only towards the outside; at the centre it disappears entirely. The
long chaplets of Nostoc cells persist almost unchanged in the thallus of the
Collemaceae, but in heteromerous genera such as Pannaria and Peltigera
they are broken up, or they are coiled together and packed into restricted
areas or zones. The altered alga has been frequently described as Polycoccus
punctiformis. A similar modification occurs in many cephalodia, so that the
true affinity of the alga, in most instances, can only be ascertained after free
cultivation.
Bornet 1 has described in Coccocarpia molybdaea the change that the alga
Scytonema undergoes as the thallus develops : in very young fronds the
filaments of Scytonema are unchanged and are merely enclosed between
layers of hyphae. At a later stage, with increase of the thallus in thickness,
the algal filaments are broken up, their covering sheath disappears, and the
cells become rounded and isolated. Petractis (Gyalecta) exanthematica has
also a Scytonema as gonidium, and equally exact observations have been
made by Funfstiick 2 on the way it is transformed by symbiosis: with the
the thallus is immersed in the
exception of a very thin superficial' layer,
rock and is permeated by the alga to its lowest limits, 3 to 4 mm. below the
surface, Petractis being a homoiomerous lichen. The Scytonema trichomes
embedded in the rock become narrower, and the sheath, which in the
epilithic part of the thallus is 4/4 wide, disappears almost entirely. The
green colour of the cells fades and septation is less frequent and less regular.
The filaments in that condition are very like oil-hyphae and can only be
distinguished as algal by staining reagents such as alkanna. They never
seem to be in contact with the fungal elements : there
of parasitism nor even of consortism.
is no visible appearance
b. CHLOROPHYCEAE. As a rule the green-celled gonidium such as
Protococcus is not changed in form though the colour may be less vivid, but
in certain lichens there do occur modifications in its appearance. In Micarea
(Biatorina) prasina, Hedlund 3 noted that the gonidium was a minute alga
possessing a gelatinous sheath similar to that of a Gloeocapsa. He isolated
the alga, made artificial cultures and found that, in the altered conditions,
it gradually increased in size, threw off the gelatinous sheath and developed
into normal Protococcus cells, measuring 7 to IO/LI in diameter. The gelatinous
sheath was thus proved to be merely a biological variation, probably of
value to the lichen owing to its capacity to imbibe and retain moisture.
Zukal 4 also made cultures of this alga, but wrongly concluded it was a
Gloeocystis,
1 Bornet 1873.
2 Fiinfstiick 1899.
3 Hedlund 1892.
4 Zukal 1895, p. 19.