Dec. 22, 1923 
Cytology of Wheat Stem Rust 
587 
At any rate, in material of Mindum fixed seven days or more after 
inoculation with this rust, haustoria are to be found which have not caused 
a complete collapse of the host cell. The nucleus and a portion of the 
cytoplasm move to the haustorium, but the remainder of the cell con¬ 
tents keeps its place, and the cell as a whole retains its original shape. 
There are no empty lobes of the cell with walls collapsed. 
A typical example is represented in Plate 5, B. At b is a group of 
hyphae with scant contents, and at a is a haustorium connected by a 
slender neck to its mother cell outside. The nucleus / near by is dead. 
Enveloping both the body and neck of the haustorium is a thick irregular 
sheath, greater in bulk than the haustorium it covers. Both the hausto¬ 
rium and its sheath are dead. When newly killed, it makes a most con¬ 
spicuous object in the cell, as it stains intensely. Eater on it loses power 
to stain and becomes glistening and transparent. The damage done by 
the fungus in this case has passed beyond the limits of the cell it entered, 
for there is plasmolysis in the next cell at c, a swollen wall at d, and a mass¬ 
ing of nucleus and cytoplasm in an adjoining cell at e. 
Dozens of haustoria of this same general appearance are to be met with 
in older infected material. The body of the haustorium usually remains 
small and dense, often spherical, as if unable to expand freely in its 
heavy sheath, and it soon dies. 
An attempt was made to study the nature and origin of this haustorial 
sheath. In Plate 5, C, is a haustorium, a , and condensed around it in 
more or less concentric layers are materials continuous at their periphery 
with the cytoplasm of the host cell. The nucleus b t in attendance as 
usual, is collapsed and dead. Another case was seen in which host cyto¬ 
plasm was banked around the neck and the basal half of the body of the 
haustorium. This haustorium had expanded almost normally and its 
contents opened into a reticulum. In Plate 6, A, at /, is another haus¬ 
torium, about which the host cytoplasm is concentrating. It is inter¬ 
esting, too, in this connection that in Plate 5, B, at e, a cell not attacked 
directly by the fungus, the nucleus, plastids, and cytoplasm have banked 
up on the side of the cell nearest to the cell attacked by the fungus. 
While the evidence is not conclusive, it seems probable that the sheath 
originates from host cytoplasm, although the possibility is not excluded 
that the haustorium itself contributes to it by secretions of some sort, 
or that host, or fungus, or both, secrete more or less of cell wall substances 
around the haustorium. 
For some reason the presence of the haustorium seems to induce host 
cytoplasm to move toward it and bank around it. Each seems to be 
toxic to the other; at least both die very soon. The host nucleus almost 
invariably is found alongside of the haustorium, and it, too, dies quickly. 
The end of this struggle appears to be the digestion of both the haus¬ 
torium and its covering and often the death of the cell containing them. 
In Plate 5, D, at a, fixed 11 days after inoculation, haustorium and sheath 
are transparent, having lost all affinity for stain, and only by reducing 
the light could the limits of the former haustorium within be discerned. 
The dead nucleus b was the only stainable object left in the cell. The 
mycelium near by at d also is dead and transparent. 
Haustoria in mesophyll tissues fare ill, but in epidermal cells a haus¬ 
torium may attain to full size before causing any perceptible disturbance. 
Plate 5, E, shows a full-sized healthy looking haustorium, a, in an epider¬ 
mal cell. It has retained its normal connection with the empty hausto- 
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