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CLASSIFICATION
THE morphological characters, on which descriptions of the Ustilaginales are
based, are few and relatively simple. The size and ornamentation of the spores
(chlamydospores) and the structure of the sorus are of paramount importance
for differentiating species on morphological grounds, and from this standpoint
smuts are among the most, convenient fungi with which to work. The usefulness
of herbarium material, when carefully preserved, does not deteriorate, and so the
identity of successive collections is easUy checked and the exchange of material
between different workers greatly faciUtated. Final proof that a species belongs
to the Ustilaginales, and the determination of which of the two families (and
frequently also the genus) in which a specimen should be classified depends,
however, on the course of events at spore germination. The spores of herbarium
specimens, after a longer or shorter time, lose their abUity to germinate and the
spores of fresh material may germinate with difficulty, so that many species have
been proposed, and often correctly proposed, by analogy. One object of giving,
whenever possible, details of the behaviour at germination and the conditions
under which this event occurs after the usual specific descriptions in the
systematic treatment of the British smuts is to draw attention to the need for
further studies on this phenomenon.
What constitutes sufficient grounds for the differentiation of a species varies
from one group of organisms to another. In general, two tendencies may be
observed among students of fungi. Morphological or biological characters may
be emphasized. Among smuts, as in other groups of parasitic fungi, the second
attitude has been commonly adopted, and many species have been proposed
from a consideration of differences in parasitic abUity towards a series of plants.
Hence the identity of the host becomes of primary importance for the identification
of the parasite, although not infrequently biometrical studies reveal small
differences in spore-size of other characters between morphologically similar
'species' from different host plants.
Butler (1929) reviewed with pertinent illustrations the criteria for the definition
of species among fungi, and Ciferri (1932) expressed his views on the same
subject with special reference to the smuts. Both these authors agreed that, in
general, the most useful course is to defiiie-species by morphological characters
and to reserve physiological and biological characters for the definition of groupings
of subspecific rank, and this has been adopted as a guiding principle for
the present work. Our attitude to the taxonomy of the British smuts has been
conservative. We have made as few alterations as possible in both groupings
and names, and whenever there is any doubt or the evidence appears to be
inadequate no change has been made. Certain of the changes advocated call for
comment as they affect smuts of economic importance.
Cunningham (1924) and Rodenhiser (1926) each proposed the consohdation
of the loose smut of wheat {Ustilago tritici) and the loose smut of barley {U.
nvda) as a single species, and more recently this view has been supported by
Fischer (1943). These two smuts are morphologically aUke and only differ in
their pathogenicity. They may be compared with the specialized races of black
rust (Puccinia graminis) which attack wheat and oats, and they should, it is felt.
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