AMERICAN 
JOURNAL OF BOTANY 
Vol. VI January, 191 9 
BIOLOGIC SPECIALIZATION IN THE GENUS S 
Walter Spurgeon Beach 
Introduction 
The genus Septoria presents an excellent field for the study of biologic 
specialization on account of the very large number and the uncertain status 
of the described species, of which there are more than 1200. The mor- 
phological characters for the limitation of these are few, consisting chiefly 
of spore shape, length, thickness, and septation, and in some instances a 
slight tinting of the spore; pycnidium shape, size, and color; and ostiole 
size and character. The biological characters for the limitation of species 
are the size, shape, margin, zonation, and color of the disease spots, and 
their location and distribution upon the host. The specific descriptions in 
many cases are meager and really useless. The presence of a Septoria upon 
an unrecorded host is often made the basis of a new species. 
By tabulating (10), according to spore length, all the ''species" of 
Septoria given in Saccardo's ''Sylloge Fungorum," it has been found that 
nearly 700 fall withiix the limits of 20 to 50 11. Twenty-six species on grasses 
are between 20 and 40 ix. Yet in one case a Saccardian description gives a 
spore length ranging from 19 to 62 11. There have been great inaccuracies 
in measuring spores and pycnidia, with tendencies to report the measure- 
ments in round numbers. Elliott (7) has shown the degree of personal 
error with our modern technique. Among eleven observers measuring 
many spores of Alternaria, he found that there was a variation of over 
41 per cent from the highest measurement returned for maximum length, 
and that such a variation would make it impossible to distinguish the species 
of Alternaria on the basis of spore measurement. Septation is largely 
dependent upon the stage of maturity of the spores, is often difiicult to see, 
and was ignored by early mycologists. Environmental conditions cause 
fluctuations in the size of spores and pycnidia. The value of disease char- 
acters is doubtful, for they commonly change with the age, the part, and 
the species of the host, as well as with weather conditions. The confusion 
that Cavara (4) met regarding these points in his attempt to distinguish 
Septoria tritici and S. graminum on wheat, led him to state that both forms 
probably belong to the same morphological species. 
