Feb. 16, 1924 
Flag Smut of Wheat 
483 
however, they differ from the sporidia produced by such genera as 
Tilletia and they present slight resemblances to an undifferentiated 
promycelium. 
MYCELIUM AND SPORE FORMATION 
A study of the epidermis stripped from inoculated colepotiles indicated 
that entrance was chiefly effected by a germ tube which arose from a 
single sporidium and not from conjugated sporidia. 
The mycelium is for the most part from 1.5 to 2 /x in width. In the 
early stages of its growth the mycelium is intracellular but it soon becomes 
intercellular. Its growth throughout the plant is also typically inter¬ 
cellular (fig. 1, c, and PI. 3, E and G). In the older tissues—e. g., 
in the parenchymatous cells at the nodes of infected plants—it was 
observed that a large proportion of the mycelium was intracellular. 
The intracellular portions of the mycelium may perhaps be considered as 
haustoria, for they were observed to be associated frequently with 
intercellular mycelium, but in many cases appeared to represent a 
characteristic form of dormant mycelium (fig. 1, d, and PI. 3, D, a). 
In the tissues of the young leaf the mycelium was very characteristically 
intercellular. In heavily infected leaves the hyphae sometimes wedge 
the host cells apart (fig. 1, c, and PI. 3, G). When growing rapidly, 
they are characteristically nonseptate, and may become branched; 
haustorialike structures sometimes are observed (fig. 2, c). It is a 
question, however, whether these structures always represent haustoria 
or whether they represent mycelium which has commenced to take up 
an intracellular position. The mycelium varies considerably at spore 
formation. Those portions utilized in the formation of spores are at 
first narrow and nonseptate, whereas those portions not utilized become 
vacuolate, somewhat irregular in outline, and are most frequently 
septate. 
The nuclear phenomena were not followed with certainty in all instances, 
because it was difficult to stain satisfactorily both nuclei and septa in the 
same mycelium. It appeared, however, that the nuclear content was not 
constant, one to four nuclei sometimes being observed in the cells. Hy- 
phal fusion frequently was observed, and there were indications that nu¬ 
clear migrations had occurred (fig. 1, c). The mycelium prior to spore 
formation, however, was fairly constantly binucleate. Fusion of hyphae 
was observed to occur at various stages prior to spore formation. 
The method of spore formation in this genus has been studied in some 
detail by Wolff (57) and others. Although it appeared that the spore 
balls sometimes originate from a single hypha, more frequently two 
distinct hyphae appeared to be involved in spore formation. Branches 
arise which become very much coiled on one another and the spores 
and the spore ball envelope of sterile cells arise simultaneously in this 
manner (fig. 1, f, and PI. 3, E and G). 
relation to nonsusceptible plant 
While studying the relation of the fungus to a susceptible plant, 
studies were made concurrently on the relation of the fungus to plants 
resistant to flag smut. The relations between certain parasites and 
nonsusceptible plants have been studied by a number of investigators. 
Various aspects of the question of resistance and immunity have been 
studied in this connection, especially by investigators of the cereal 
rusts. 
