105 
MYCELIUM 
The young, vigorous mycelium growing within the agar is smooth, 
quite straight, nearly or quite hyaline, and of very uniform diameter. The 
general aspect near the edge of a colony is shown in Fig. 6,d. In older 
parts of the colony the submerged mycelium is somewhat thicker and is 
less rich in protoplasm. The mature to old submerged mycelium has in 
agar a barely perceptible tinge of straw-color. The cells are quite strongly 
constricted at the septa and the content is highly vacuolate. Ravn (91) 
states that in the old immersed mycelium both plasma and cell-walls may be 
blackish green, grayish brown, or entirely black as in Alternaria. No such 
coloration as this has been observed in my cultures. The same author (91) 
discussing colony zonation, attributes it to variation in the color of the my- 
celium, while in my cultures it appears to be almost entirely due to variation 
in quantity of conidia and of aerial mycelium. 
The quantity of aerial mycelium was highly dependent upon air- 
moisture and agar-conditions. When these favored there was a some- 
what conspicuous, even floccose, white aerial mycelium 2 to 3 mm. high, 
consisting of either smooth, even, single filaments or, when quite abun- 
dant, of several filaments twined into a rope (Fig. 5,c).’ When conditions 
were less favorable the aerial mycelium, though inconspicuous, was still 
present in considerable quantity. The difference between various species 
of Helminthosporium and their saltants as regards the abundance of 
aerial mycelium is well shown in Plates IX-XIII. H. No. 36, a very dis- 
tinct species from H. No. 1, was chiefly characterized by the great 
profusion of aerial mycelium (Pl. X). This character is the only one 
I have found by which to separate H. No. 1 and H. No. 3, and it serves 
only in the large cultures (Pl. IX-XIII), the distinction often failing in 
ordinary Fetri-dish culture. The character is apparently so much mod- 
ifed by varying humidity as to make its utility for diagnosis of very 
questionable value (see pp. 95,97). Ravn (91), however, distinguishes H. 
avenae, H. gramineum, and H. teres, growing on sterilized straw, by the 
different individual characters of the aerial mycelium and sclerotia; and 
H. gramineum and H. avenae on beerwort by the fact that H. gramineum 
forms a snow-white uniform mass of aerial hyphae, while H. avenae has 
sparse, white aerial mycelium which forms solid clumps, the black sub- 
stratum-mycelium being visible between them. 
The mycelium in wheat tissue is somewhat more irregular in contour 
than that grown in agar and is often much thicker. Occasionally upon the 
wheat surface it branched in a close fan-like fashion (Fig. 22, page134). The 
