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THE THALLUS 287
Peltigera and Nephroma are also closely related in the character of the
fructification. It is a flat non-marginate disc borne on the edge of the
thallus: in Peltigera on the upper surface, in Nephroma on the under surface.
The remaining genus Solorina contains normally a layer of bright-green
algae, but, along with these, there are always present more or fewer Nosloc
cells, either in a thin layer as in S. crocea or as cephalodia in others, while,
in three species the algae are altogether blue-green.
The members of the Peltigeraceae have a thick upper cortex of plectenchyma
and in some cases strengthening veins, and long rhizinae on the
lower side. Some of the species attain a large size, and, in some, soredia
are formed, an evidence of advance, this being a peculiarly lichenoid form
of reproduction.
The Stictaceae form a parallel but more highly organized family, which
also includes closely related bright-green and blue-green series. They are
all dorsiventral, but they are mostly attached by a single hold-fast and the
lobes in some species suggest the fruticose type in their long narrow form.
A wide cortex of plectenchyma protects both the upper and the lower
surface and a felt of hairs replaces the rhizinae of other foliose lichens. In
the genus Sticta (including the section Sticlind) special aeration organs,
in Lobaria these are replaced by
cyphellae or pseudocyphellae, are provided ;
naked areas which serve the same purpose.
regarded the Stictaceae as the most highly developed of all
Nylander 1
lichens, and they easily take a high place among dorsiventral forms, but it
is generally conceded that the fruticose type is the more highly organized.
In any case they are the highest reach of the phylum or phyla that started
with Pyrenopsidaceae and Collemaceae ;
the lowly gelatinous thalli changing
to more elaborate structures with the abandonment of the gelatinous algal
sheath, as in the Pannariaceae, and with the replacement of blue-green by
considers the Stictaceae as evolved from the
bright-green gonidia. Reinke 2
,
Pannariaceae more directly from the genus Massalongia. Their relationship
is certainly with Pannariaceae and Peltigeraceae rather than with Par-
meliaceae ;
these latter, as we shall see, belong to a wholly different series.
D. EVOLUTION OF ARCHILICHENS
The study of Archilichens as of Phycolichens is complicated by the
many different kinds of fungi and algae that have entered into combination ;
but the two principal types of algae are the single-celled Protococcus group
and the filamentous Trenlepohtia : as before only the broad lines of thalline
development will be traced.
The elementary forms in the different series are of the simplest type a
somewhat fortuitous association of alga and fungus, which in time bears the
1
Seep. 126.
2 Reinke 1895.