Dec. aa, 1923 
Cytology of Wheat Stem Rust 
58 i 
host cell and its contents have expanded into an open reticulum. The 
sheath of host cytoplasm inclosing the haustorium is not demonstrable 
in this case, but in material which has shrunken slightly, a thin clear 
space is to be found between the limiting osmotic membrane of the 
haustorium and an outer delicate sheath inclosing it. The cytoplasmic 
sheath covering young haustoria is continuous with the peripheral 
cytoplasm of the host cell and it seems probable that this outer covering 
of mature haustoria is derived from it. 
Here, as in Baart, the sixth day marks a change in the activities of 
the fungus. Hyphae form long stolons which strike out through the 
intercellular spaces towards fresh areas of the leaf. In Plate 3, A, is 
represented a group of these stolons crossing a substomatal chamber 
along the inner surface of the epidermis. They are long, straight hyphae, 
with contents densest at the tips. They are composed of greatly elon¬ 
gated cells which branch but sparsely and make few or no haustoria. 
A runner may strike a host cell squarely, as in Plate 3, A, at b , stop, 
thicken as if about to make a haustorium, and then bend around the ob¬ 
struction and grow on. These stolons are obviously not self-supporting 
but derive their food from the central mycelium. 
Other hyphae push out towards the surface of the leaf and branch 
freely just below the epidermis, initiating uredinia. In Plate 3, B, is 
shown a portion of a young uredinium, a, which has not yet broken 
through the epidermis b. It is drawn from material taken seven days 
after inoculation. The spores and their stalks are binucleate and filled 
with dense cytoplasm. The uredinium is under considerable pressure 
from the resistance of the epidermis that it is lifting and the young 
spores are somewhat bent and flattened by it. Soon after this the epi¬ 
dermis is ruptured and the stalks of the spores, as well as the basal cells 
below them, elongate. The first spores are freed, their stalks wither, 
and other spores push up between them. 
While the spores and the matted hyphae just below them are richly 
supplied with food, the intercellular mycelium of the central area is nearly 
empty and the haustoria are pale and transparent. The host tissues 
are living and not seriously impoverished. 
Details from older material of the central mycelium are drawn in 
Plate 2, F and G. In F is a portion of a hypha which has contributed 
much of its substance to the peripheral growth of the fungus. The 
rich granular cytoplasm of younger hyphae (as seen in PI. 2, B, D, and 
E) is reduced here to a few delicate transparent strands. The nuclei 
persist longer, and this affords an exceptional opportunity to study the 
structure of vegetative rust nuclei (F, a and b) , for the large nucleolus 
and chromatin network of the nucleus stand out very clearly in the 
transparent medium. Later, the nuclei also disappear from these hy¬ 
phae, leaving only the orange-stained hypha walls. 
In Plate 2, G (taken from material 11 days old), the mycelium sur¬ 
rounding the host cell is practically void of stainable material. Three 
large haustoria extend into the cell. They, too, have lost much of their 
contents, and in the relatively transparent remainder of each one is to 
be seen a denser rounded body (a, 6, and c), having the size and appear¬ 
ance of a nucleus. The evacuation of the haustorium may be carried 
so far as to leave it nearly clear and outlined only by the delicate limit¬ 
ing membrane. 
In Plate 3, C, is drawn a small portion of a section through a 15-day 
infection. The host cells are exhausted and their contents partially ag- 
