May a6, 1923 
Graminicolous Species of Helminthosporium 
703 
represented usually by a compact velvety layer of a black olivaceous 
color, composed entirely of short crowded sporophores. If less agar- 
agar is used, leaving a slight excess of free water, a fluffy, dark greenish 
brown aerial mycelium, interspersed often with matted masses of white 
mycelium, is produced. Often the vegetative growth is largely present 
as a firm coriaceous bluish-black crust. In older cultures masses of 
compacted white hyphae of secondary origin may be thrust through the 
layer of sporophores giving rise to an appearance suggesting contamina¬ 
tion by another fungus. On com-meal agar or tap-water agar, con¬ 
taining only small amounts of organic food material, the growth below 
the surface of the substratum is relatively sparse, being limited to 
moderately remote hyphae branching and anastomosing freely but with¬ 
out the production of lobulate segments. (PI. 19.) Scattering spo¬ 
rophores arise at intervals as branches of these hyphae, and in the 
course of 4 to 6 weeks develop into long structures, intricately contorted, 
often bearing one or several branches, and scores of successively pro¬ 
liferated spores. (PI. 19, A, B, C.) Helminthosporium sativum is thus 
readily distinguished by its macroscopic cultural characteristics from 
congeneric species with subhyaline spores even when occurring on the 
same hosts, as H, teres and H, gramineum on barley and H, tritici-repentis 
on quack grass. Cultural characters, however, are of little assistance in 
distinguishing it from a considerable number of forms like H. monoceraSy 
that exhibit great similarity in development and in general habit of 
growth. 
The responsiveness of Helminthosporium sativum to obvious differences 
in environmental conditions, illustrated, for example, in the results 
obtained by Dosdall and Christensen (50) in their study of variations 
in length of spores, makes it less easy to define this, as well as many 
other species, with as great precision as might be desired. The difficulty 
is further accentuated by evidence of lack of uniformity under apparently 
similar conditions of growth. In a recent extensive paper, Stevens {142) 
has called attention to morphological differences existing between strains 
of Helminthosporium isolated from wheat affected with footrot. These 
strains this author assigned to the '' H, sativum group, which consists of 
a large number of elementary species.’* The mode of origin of pre¬ 
sumably new strains as aberrant sectors in plate cultures was designated 
as '‘saltation,** and found in some races to be of frequent occurrence. 
What the significance of such phenomena may be, and to what extent 
they correspond to realities in nature, constitute questions open to con¬ 
jecture. In any case, the fungus under consideration can hardly be 
regarded as especially extraordinary. Variability of the sort represented 
here, is probably more nearly the rule among species of fungi than the 
exception. The writer is inclined to believe that the amount of mor¬ 
phological versatility observed in a species is often contingent in a 
larger measure on the possibilities its distinctive structures possess for 
expressing readily demonstrable differences than on the degree in which 
it may be lacking in genetic unity. Thus, for example, the relatively 
short conidia of H, triseptatum with a maximum of three cross walls, 
could never exhibit the wide fluctuations in length and septation possible 
in spores of species like H. oryzae or H, rostratum. This obvious limita¬ 
tion in the possibility of variation, may very largely account for the 
uniformity of the small-spored species of Helminthosporium as compared 
with the larger-spored forms, Of which H. sativum is an example. 
