45 § 
Pole Evans, — The Cereal Rusts. 
says the mycelium shows nothing remarkable and gives the measurement 
of the hyphae at 3 \x. 
The general development of P. simplex agrees in every way with 
the species already described, and the general structure of its mycelium 
very closely resembles some of these species, but it can easily be dis- 
tinguished from them by its characteristic and definitely shaped sub- 
stomatal vesicle. 
The spore on germination forms a delicate germ tube, the tip of 
which swells up to form the appressorium and then follows the sub- 
stomatal vesicle, which is at first a long sausage-shaped body, which 
always lays itself with its long axis parallel to the stomatal slit. At first 
(in shape) it is very much like P. glumanwi , but it soon begins to differ 
from it in the following important points. 
1. Infecting hyphae simultaneously spring from both ends of the 
vesicle. 
These hyphae only measure from 3-3*5/*. across, and contain few 
nuclei, which are usually seen in pairs, whereas in P. glumarum the sub- 
stomatal vesicle only gives rise to a single infecting hypha from one end. 
From this hypha, which measures 10-12// across, the other hyphae arise, 
and these are always crammed with nuclei. 
2. Soon after two or more pairs of infecting hyphae have sprung 
from the vesicle, it becomes divided transversely into two equal halves. 
Close to this partition wall, and on each side of it, a hypha then usually 
springs, and the vesicle may then become further sub-divided by transverse 
walls. (Figs. 35-37.) 
Thus the sub-stomatal vesicle in Uredo simplex becomes multi-septate. 
The Hyphae. 
The hyphae closely resemble those of P. Symphyti-Bromorum , as 
described by Marshall Ward ( 33 ), both with regard to size, vacuoles, and 
septation. 
As the hyphae begin to get older, they lose their protoplasmic 
contents, become very vacuolated, with a diminution in size of their 
contained nuclei, which are seen in the last stages of degeneration as 
small, round, dull, staining dots. Directly the Uredo pustule-formation 
begins, the mycelium that gave rise to it, including also the sub-stomatal 
vesicle, is seen to be quite empty and septate, except in the immediate 
neighbourhood of the pustule, where the hyphae are full of protoplasm and 
contain large and sharply staining nuclei. 
In P. simplex , the long runner-like hyphae so characteristic of 
P. Symphyti-Bromorum and P. glumarum are not anything like so 
numerous or well developed, but single ones are found here and there 
running through the tissue of leaf as a simple thread for some distance 
