Pole Evans . — The Cereal Rusts. 
445 
In South Africa, especially the Transvaal, Uredo gramims prevents the 
successful raising of summer cereal crops. 
Uredo graminis , the Black Rust, is so called from the dark colour 
produced on the straw by the teleutospore patches. In the field the Uredo 
stage is recognized by its rusty orange to brown ochre coloured pustules, 
which are from 12-15 mm. long or even longer. Under the microscope the 
spores are broadly elliptic, dirty yellow, spinulose, 17-40 x 14-22 /x. 
The Histology of Uredo graminis. 
Although Uredo graminis has probably received more attention at the 
hands of mycologists all the world over than any other parasitic Fungus, 
I think it will be clear that the complete sequence of infection as represented 
in my figures has never heretofore been observed or represented. Apart 
from de Bary (5), Marshall Ward (32), and more recently Bolley (7), have 
figured the germination of and infection by means of uredospores of Uredo 
graminis. 
De Bary has figured a slight swelling of the germ-tube over the 
stoma, but nothing more. Bolley has depicted the germ-tube passing 
straight through the stoma to the mesophyll-cells below. This I find never 
occurs. 
Further, Bolley (7), p. 630, says that the germ-tube from these 
uredospores ‘ may bore its way through the skin of a wheat plant and thus 
start another point of infection ’ ; in the case of uredospores of 5 Pttccinia 
rubigo-veral he illustrates some of the filaments passing directly through 
the leaf. I have now examined some hundreds of thousands of uredospore 
infections on all the cereal crops and various other Gramineae, and have 
never met with a single infection which was not brought about by way of 
a stoma. 
The germination of the uredospore on the epidermis is usually effected 
within the first twenty-four hours, and by the third day infection is well 
established. 
When the spore germinates, two germ-tubes frequently appear at first, 
but one quickly gets ahead of the other and arrests its growth. The tip of 
the germ-tube, as soon as it reaches a stoma, instead of entering straight 
away, swells up and forms the appressorium (Figs. 1 and 2). From the 
appressorium a narrow branch passes through the stomatal slit, always 
situated somewhat nearer to one of the corners. As soon as the neck has 
got through the aperture, it enlarges to form the sub-stomatal vesicle, 
which applies itself closely to the inner face of the stoma (Figs. 2 and 3). 
Into this vesicle the whole contents of the spore are poured, and the entry 
of the fungus is completed. The germ-tube and appressorium now quickly 
wither and are soon lost sight of. 
A point to be noted in connexion with the substomatal vesicle, both in 
