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216 PHYSIOLOGY
published an account .of various oil-cells in a large series of calcicolous
lichens (Fig. 117).
The occurrence of oil- (or fat-) cells is not dependent on the presence of any
particular alga as the gonidium of
Fig. 117. Lecidea immersa Ach. A, sphaeroid
fat-cells from about 8 mm. below the surface
x 550. B, oil-hyphae in process of :
emptying
a, sphaercid cells containing oil ; b, cells with
oil-globules x 600 (after Fiinfstiick).
the lichen. Funfstuck 1 has described
the immersed thallus of Opegrapha
saxicola as one of those richest in
fat-cells. The gonidia belong to the
a filamentous alga Trentepohlia um-
brina and form a comparatively
thin layer about 160/4 thick near
the upper surface; isolated algal
branches may grow down to 350/4
into the rock, while the fungal ele-
ments descend to 1 1-5 mm., and
though the very lowest hyphae were
without oil as were those imme-
diately beneath the gonidia the
interlying filaments, he found, were
crowded with oil-cells. Sphaeroid
terminal cells were not present.
Fiinfstiick 1 has re-examined the
thallus of Petractis exanthematica,
an almost wholly immersed lichen
with a gelatinous gonidium, a species
of Scytonema. The thallus is homoio-
merous : the alga forms no special
zone, it intermingles with the hyphae
dowr n to the very base of the
thallus; the hyphae are extremely
slender and at the base they measure
only about I/A in width. Oil-cells
are abundant in the form of inter-
calary cells about 3-5/4 in thickness. Nearer the surface sphaeroid cells
are formed on short lateral outgrowths ; they measure 14-16/4 in diameter
and occur in groups of 15 to 20. The superficial part of the thallus is a
mere film ; the hyphae composing it are slightly stouter and more thickly
interwoven.
Bachmann 2 3 and Lang have further described the anatomy of endolithic
thalli especially with reference to oil-cells, and have supplemented the
researches of previous workers. New methods of cutting the rock in thin
1 Fiinfstiick 1899.
2 Bachmann 1904' .
3 Lang 1906.