Dec. 22,1923 
Cytology of Wheat Stem Rust 
585 
beyond the stoma to adjoining epidermal cells. In Plate 4, B, is shown 
an unusual case in which a mesophyll cell happens to be in direct contact 
with the guard cell at a, and it, too, shows disordered and partly dissolved 
contents. 
Plate 4, B (taken from material fixed seven days after inoculation), 
illustrates one or two other points. The guard cell walls are weakened, 
as is evidenced by the fact that one at b was badly broken and torn in 
sectioning. This is of common occurrence at this stage. Although the 
wall is weakened, it is not appreciably swollen. This is in contrast with 
the adjoining walls of the epidermal cells, as at c, which have swollen 
enormously, enabling one to see clearly the layers of which they are com¬ 
posed. The appressorium d shows signs of degeneration. It is shrunken 
and its contents are somewhat disorganized, the nuclei being scarcely 
distinguishable. 
There is nothing unusual about the manner of entry of the fungus. . It 
pushes a thin wedge-like projection through the stomatal aperture and 
the protoplasm flows through, usually forming the substomatal vesicle 
just inside the stoma. From the vesicle, the infecting hypha grows out 
over the end of the guard cell and skims the inner surface of the epidermis 
until the tip strikes a mesophyll cell. In exceptional cases the infecting 
hypha develops directly without the formation of the substomatal vesicle, 
Sometimes two infecting hyphse are found growing in opposite directions* 
or a single hypha may branch into several. 
Development of the Fungus After Entrance 
A general idea of the early relations between fungus and host may be 
gained from Plate 3, D, a low-power drawing of part of a longitudinal 
section through the leaf four days after inoculation. In contact with 
the outer side of the guard cell is the empty appressorium d , and on its 
inner surface is the substomatal vesicle of the fungus, also empty. Two. 
infecting hyphae have formed. The first, at e , has already attacked the 
host cell /, with the result that both it and its host are dead. The 
second, at g, is just beginning to form a haustorium in the mesophyll cell, 
and a branch hypha is growing off toward deeper-lying tissues. Cells at 
h and i f at some distance from the dead cell /, are plasmolyzed. 
Material was available for a detailed study of this first attempt of the 
fungus to establish relations with its host. As soon as the tip of the 
infecting hypha meets the mesophyll cell, changes set in preparatory to 
the formation of a haustorium. Its pair of nuclei divide, one daughter 
pair of nuclei moves out into the tip of the hypha and a septum forms 
back of them, separating a short terminal cell, the haustorium mother 
cell. The nuclei of this cell decrease rapidly in size. 
The haustorium mother cell usually is wedged into the angle between a 
mesophyll cell and an epidermal cell (pi. 4, C), and the haustorium may 
be formed in either of the two host cells. The history of the fungus may 
vary somewhat, according to the course taken at this point, for the 
haustorium usually is killed promptly if it forms in the mesophyll cell, 
but it may live and function for several days if formed in the epidermal 
cell. 
In the majority of cases the first haustorium forms in the mesophyll 
cell. An early stage in this procedure is represented in Plate 4, C, which 
was drawn (X 1460) from material taken two days after inoculation. 
The infecting hypha ran obliquely to the plane of the section, the proximal 
