TEN YEARS' PROGRESS IN LICHENOLOGY. 283
The author describes various growth forms of the lichen
thallus as exhibited in Peltigera canina, Physcia parietina,
foliose forms, Usnea barbata, and Cladonia sylvatica, fruticose
forms, and claims that such raise doubts as to whether the
fact of the association of fungus and alga has anything to do
with the matter of form ; and adds further, "that such doubts
are confirmed when one realizes that not one of the form factors
is anything new after all, as they are in fact only a repetition of
the commonest common-place factors of the somatic organiza-
tion of algae as seen in modern sea-weeds." From this it is in-
ferred that the fugus symbiont, after association with the alga,
continues to develop a growth form similar to that exhibited
by its ancient ancestry which, as Dr. Church asserts, was evolved
in the sea. "It is to the sea that one must look for the analogue
of any specialised lichen-thallus."
A subsequent paper, "The Lichen as a Transmigrant," by
the same author (25), although not published within the period
now under review, has been included, owing to its intimate
association with the one entitled "Lichen Symbiosis."
The reader is reminded that in an ordinary Fucoid the photo-
synthetic chlorophyll cells are in close contact with the external
food solution, for they constitute a thin brown external film
surrounding the mechanical tissue of the central axis. "Stripped
of the outer layers, the whole plant reduces to a system of
hyphal strands as an interwoven mechanical tissue of descending
hyphae, to all intents and purposes the mycelium of a
fungus-axis." The more internal tissues having previously
lived at the expense of the surface layers and not feeling the
effect of the change of environment so readily would continue
to live for a time.
"A higher fungus of. the land is in short a skinned sea-weed
. . . of which, on the death and decay of the older meta-
bolic and autotrophic surface layers the exposed internal tissues
continue their existence at the expense of the soluble carbohyd-
rates of the standing and non-aerated medium."
The writer's view is that "the main series of higher Eumycetes
are derived from stripped sea-weeds, which emerge from the
water. . . . The Lichen represents the case of similar
simple or branched algal somata, remaining denuded of auto-
trophic tissue in standing pools, and, hence, soon smothered