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3 i2
SYSTEMATIC
by fruit characters. The algae occupy a subsidiary position, but they also
are of importance in shaping the form and structure of the thallus. The
different phyla are often determined by the presence of some particular alga ;
it is in the delimitation of families that the algal influence is of most effect.
Zahlbruckner's system gives due weight to the inheritance from both
fungus and alga with, however, the fungus as the chief factor in development,
and as his work is certain to be generally followed by modern lichenologists,
it is the one of most immediate interest. His scheme has been accepted in
the following more detailed account of families and genera, and for the
benefit of home workers those that have not so far been recorded from the
British Isles have been marked with an asterisk.
It cannot be affirmed that nomenclature is as yet firmly established in
lichenology. Both on historical grounds and on those of convenience, the
subject is one of extreme importance, and interest in it is one of the main
avenues by which we secure continuity with the past, and by which we are
able to realize not only the difficulty, but the romance of pioneer work.
Besides, there can be no exchange of opinion between students nor assured
knowledge of plants, until the names given to them are beyond dispute.
According to the ruling of the Brussels Botanical Congress in 1910,
Linnaeus's 1 list of lichens in the Species Plantarum has been selected as the
basis of nomenclature, but since his day many new families, genera and
species have been described and often insufficiently delimited. It is not
easy to decide between priority, which appeals to the historical sense, and
recent use which is the plea of convenience. Here also it seems there can
be no rigid decision; the one aim should be to arrive at a conclusion
satisfactory to all, and accepted by all.
In the following necessarily brief account of families and genera, the
"spermogonia" or "pycnidia" have in most cases been left out of account,
as in many instances they vary within the family and occasionally even
within the genus. Their taxonomic value is not without importance, but,
in the general systematic arrangement, they are only subsidiary characters.
An account of them has already been given, and for more detailed statements
the student is referred to purely systematic works.
There are two main types of spore production in the "pycnidia" which
have been shortly described by Steiner 2 as "exobasidial" and "endobasidial."
In the former the sporophores are simple or branched filaments, at the
apices of which a short process grows out and buds off a pycnidiospore;
in the latter the spores are budded directly from cells lining the walls or*
filling the cavity of the pycnidium. The exobasidial type is more simply
rendered in the following pages by "acrogenous," the endobasidial by
"pleurogenous" spore production. In many cases the "spermogonia" or
1 Linnaeus 1753.
2 Steiner 1901.