462 
Pole Evans . — The Cereal Rusts , 
The sub-stomatal vesicle is of a definite shape for each species. It 
may be non-septate s septate, or even multi-septate, and may give rise 
to one, two, or more infecting hyphae, according to the species. In some 
species a well-defined appressorium is present, in others it is not so 
apparent. 
The vegetative hyphae of the different species closely resemble each 
other in size and general behaviour, except in the case of P. glumarum , 
where they are much thicker than any of the other forms, and contain 
a much greater number of nuclei. 
The haustoria of some of the species are very distinctive, e. g. the 
hammer-headed form is one most commonly met with in P. Symphyti- 
Bromorum , whereas in P. glumarum , especially in the cells surrounding 
the vascular bundles, they are frequently very branched, while in the 
chlorophyll-containing cells the typical form of haustoria is that of a small 
club-shaped body. 
Whether such slight morphological differences as are exhibited by 
the mycelia of P. graminis on Wheat and P. Phlei-pratensis are sufficient 
to warrant them specific rank, may at first sight seem doubtful. Con- 
sequently, I will leave the discussion of these points, until I treat of the 
so-called ‘ physiological species ’ which will, I think, have an important 
bearing on the matter. 
