66 Transactions British Mycological Society. 
Eastern States; the neighbourhood of Paris and Rouen and 
probably also in the south of France. In Germany it was only 
known with certainty at Leipzig. 
Since 1889 it has spread in the United States as far west as 
Wisconsin and Iowa; being observed for the first time in the 
latter state in 1912 (3). 
The occurrence of the disease in this country has been de- 
scribed by Cotton(z) who also mentions the fact of its appear- 
ance in Denmark. 
GENERAL APPEARANCE OF THE DISEASE. 
The first indication of the disease is the appearance of one 
or more elongated, dark patches showing through the trans- 
lucent outer tissues of the cotyledon when the plant is in second 
leaf. These opaque patches, indicating the formation of 
chlamydospores, usually occur just below the “knee” of the 
cotyledon but occasionally also in the upper part of the leaf. 
At first the sporogenous mass is embedded in the tissues, but, 
as the spores mature the mesophyll and epidermis of the 
cotyledon are ruptured and the spores are exposed as a black 
fibrous mass in the longitudinal fissure so formed. At the 
Northumberland centres, where the crop is left unthinned, 
the death of the plant usually takes place at this stage, but in 
other localities where onions are grown as an ordinary crop 
after thinning, successively younger leaves become infected until 
the plant has developed a bulb of fair size. In those latter cases 
the crop will usually “grow out” of the disease. Both onions 
and leeks are susceptible. 
_ The Chlamydospores. The sporogenous mass is composed of 
immense numbers of chlamydospores which are usually con- 
sidered to be homologous with the teleutospores of the Ure- 
dineae. Each spore consists of one, or very rarely two, central 
fertile cells surrounded by as many as twenty sterile vesicles 
or pseudospores; the whole spore ball varying from 15-20 w in 
diameter. The fertile cell is from 11—14-5 » in diameter and its 
shape, at first globular, usually becomes irregularly polyhedral 
owing to the pressure exerted by the, developing sporogenous 
mass. It is uninucleated and contains dense granular cytoplasm. 
The wall is brown in colour, thick (1) and non-laminated. 
No division into exospore and endospore has been observed nor 
can germ pores be seen even with the highest magnification 
(x 2250). The sterile peripheral cells are closely adpressed to 
the central spore-wall, they are devoid of protoplasm and 
possess relatively thin walls. The spore ball is extremely re- 
sistant to the action of concentrated mineral acids. The vesicles 
becoming inflated as the acid penetrates the walls indicates the 
