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LICHEN ALGAE 63
algal cells had been also noted by Krabbe 1 in the young podetia
of some
species of Cladonia, the change in form being due to the continued pressure
in one direction of the parallel hyphae.
Isolated algal cells have been observed within the cortex of various
lichens. They are carried thither by the hyphae from the gonidial zone in
the process of cortical formation, but they soon die off as in that position
they are deprived of a sufficiency of air and of moisture. Forssell 2 found
Xanthocapsa cells embedded in the hymenium of Omphalaria Heppii. They
were similar to those of the thallus, but they were not associated with hyphae
and had undergone less change than the thalline algae.
C. CONSTANCY OF ALGAL CONSTITUENTS
Lichen hyphae of one family or genus, as a rule, combine with the same
species of alga, and the continuity of genera and species is maintained.
There are, however, related lichens that differ chiefly or only in the characters
of the gonidia. Among such closely allied genera or sections of genera may
be cited Sticta with bright-green algae and the section Stictina with blue-
gr-een; Peltidea similarly related to Peltigera and Nephroma to Nephromium.
In the genus S0/orina,some of the species possess bright-green, others blue-
green algae, while in one, 5. crocea*, there is an upper layer of small bright-
green gonidia that project in irregular pyramids into the upper cortex ;
while below these there stretches a more or less interrupted band of blue-
green Nostoc cells. The two layers are usually separated by
strands of
hyphae, but occasionally they come into close contact, and the hyphal
filaments pass from one zone to the other. In this genus cephalodia con-
taining blue-green Nostoc are characteristic of all the "bright-green" species.
Harmand 4 has recorded the presence of two different types of gonidia in
Lecanora atra f. subgrumosa\ one of them, the normal Protococcus alga of the
species, the other, pale-blue-green cells of Nostoc affinity.
Forssell 5 states that in Lecanora (Psoroma) hypnorum, the normal bright-
green gonidia of some of the squamules may be replaced by Nostoc. In that
case they are regarded as cephalodia, though in structure they exactly
resemble the squamules of Pannaria pezizoides, and Forssell considers that
there is sufficient evidence of the identity of the hyphal constituent in these
two lichens, the alga alone being different.
It may be that in Archilichens with a marked capacity to form a second
symbiotic union with blue-green algae, a tendency to revert to a primitive
condition is evident a condition which has persisted wholly in Peltigera
with its Nostoc zone, but is manifested only by cephalodia formation in the
1 Krabbe 1891.
2 3
Forssell 1885. Hue 1910.
5 Forssell 1886.
* Harmand 1913, p. 1050.