312 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^ list of lichens in the Species Plantar um 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 state- 

 ments the stiident is referred to purely systematic works. 



There are two main types of spore production in the "pycnidia" which 

 have been shortly described by Steiner^ 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. 



