45 2 
Pole Evans. The Cereal Rusts. 
the stomatal slit, so that^ in a transverse section of the stoma it is seen as 
a circular body blocking up the stoma (Fig. 1 6), and in longitudinal section 
as a long oval body fitting against the guard cells. 
With the formation of the sub-stomatal vesicle we may regard the 
F'ungus as having effected an entry into 
The next step in the development is the formation of the true 
infecting hypha. 
This springs from one end of the sub-stomatal vesicle, and either 
strikes straight across the sub-stomatal intercellular space or runs between 
the mesophyll-cells and the epidermis. 
The infecting hypha at once forms a haustorium in the first cell with 
which it establishes contact, and then we may say that infection has taken 
place. Frequently, in the same section, spore, germ-tube, appressorium, 
sub-stomatal vesicle, and the first infecting hypha with its haustorium, may 
be seen. 
Before describing the hyphae and their general behaviour, it should be 
pointed out at once that they always possess a definite membrane which is 
clearly visible from their first formation to the production of spores. 
Even in cases where the hypha fits itself into large and irregular inter- 
cellular spaces, the outer membrane is well developed. From the third 
day onwards to the seventh day the hyphae attain to the maximum 
development, and two distinct kinds of branches are seen. 
1. The short branches, which are frequently septate and ramify in the 
intercellular spaces between the chlorophyll-containing cells. 
2 . The long branches or runners. These are very vacuolated, very 
rarely septate, and apparently do not form haustoria. 
These run up and down the leaf, and are the hyphae which cause the 
rapid spread of the Uredo pustules (Fig. 22). 
One of these hyphae running from the margin of a Uredo pustule will 
in twenty-four hours attain a length of 12 mm., it then branches repeatedly 
at its apex, forming a nest of hyphae, which in the next twenty-four hours 
form the beginnings of a new Uredo pustule. 
Septation of the Hyphae. 
There are three distinct formations of septa : — 
1. Those formed in connexion with the haustoria. Directly a hausto- 
rium is formed from the tip of a hypha, a transverse septum makes its 
appearance and cuts off the tip. These septa may be formed as early as 
the third day. 
2. In the same way septa are formed in the short branches of the 
hyphae, which become so abundant in the intercellular spaces from the sixth 
day onwards. 
