Pole Evans . — The Cereal Rusts . 
455 
Morphologically the mycelium of P. dispersa more closely resembles 
that of P. Symphyti- Bromorum than that of P . glumarum, but it is at once 
distinguished from either of these two species by its characteristically 
shaped sub-stomatal vesicle. 
When the spore germinates a delicate germ-tube is formed, which 
may branch repeatedly until the tip reaches a stoma, where it swells up and 
forms the appressorium (as is seen in the oblique section of Fig. 25). 
A short tube grows out from the appressorium, which passes through 
the stoma, and then immediately swells out into the large substomal vesicle, 
which differs very strikingly in shape from that of P. Symphyti- Bromorum , 
which is more triangular and does not become transversely septate. 
In diameter the substomatal vesicle is from 11-14 \jl, and like that 
of P. glumarum , it lays itself with its long axis parallel to the stomatal slit 
(see Figs. 36 and 27), but it differs from P. glumarum in that it gives off at 
each end an infecting hypha, and it very soon becomes divided by a 
transverse septum, which never takes place in the substomatal vesicle 
of P. glumarum. 
The Hyphae. 
The hyphae in general appearance and structure are very like those of 
P. Symphyti- Bromorum, containing few nuclei, which are usually seen in 
pairs and arranged in single file in the hyphae. The hyphae measure 
from 3-5 n across, and in the young condition are full of protoplasm 
and non -septate, but as they grow older from the sixth day onwards 
they lose their protoplasmic contents and become septate, so that at the 
time of pustule formation the hyphae become quite empty and closely 
septate, forming a dense pseudo-parenchyma. 
Nuclei. 
The nuclei in the young and actively growing hyphae are of an 
oval shape consisting of a single deeply staining body surrounded by 
a clear space, the nuclear zone. Within this clear space a distinct 
centrosome can usually be made out towards the polar end (Fig. 33). 
No definite nuclear membrane can be made out in connexion with the 
nuclei in this condition. But in the resting condition the typical nucleus, 
as has already been described under P . glumarum, is seen. 
The Haustoria. 
The haustoria of P. dispersa are much more uniform in shape than 
those found with P. glumarum or P. Symphyti- Bromorum . 
The common forms met with are those shown in Fig. 34 ; branch forms 
are very rarely found. 
