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Since the life story of the fungus causing the Black 
Rust of wheat (Puccinia graminis) is typical of many 
rusts, it will be described in some detail. The disease 
usually appears in the summer on the leaves and stalks of 
the growing wheat in the form of yellowish streaks, 
at first shining through the epidermis. The orange 
patches on the leaves consist of the developing uredo- 
spores of the fungus causing the disease. As this pro- 
gresses, the epidermis is ruptured and the bright orange 
uredospores are liberated as a fine powder. These spores 
bring- about the rapid spread of the malady from one 
plant to another throughout the summer months, so that in 
a short space of time whole fields of wheat may be rusted. 
As the disease pustules increase in number and size the 
leaves lose colour and become paler day by day and a 
badly diseased field may thus give the appearance of 
premature ripening. 
With the advance of the season the streaks on the 
plants gradually change colour from orange almost to 
black, and although for a time some uredospores continue 
to arise in the pustules, mingled with them are now dark, 
brown spores of a different appearance. Finally, in the 
autumn, none but the dark spores are produced in the 
pustules. These spores are the teleutospores which serve 
for carrying the fungus over to the next season. Before 
considering in detail the form and behaviour of the uredo- 
spores and teleutospores, reference must be made to an 
opinion which prevailed at least a hundred and fifty years 
ago with regard to the wheat rust. 
It was strongly held by farmers that the presence of 
bushes of the common barberry near to wheat fields, bore 
some relation to outbreaks of rust, but no very definite 
reasons were given for this belief. The farmers of Massa- 
chusetts were so convinced of the connection between the 
barberry plant and the wheat rust, that a law was passed 
in 1755 compelling the destruction of all the barberry 
bushes. About a century ago Sir Joseph Banks suggested 
that a certain bright yellow fungus common on the bar- 
berry might be the same as that causing the rust of wheat. 
This fungus on the barberry, however, when examined 
microscopically was so unlike the fungus on wheat that 
for a long time the relationship was not understood. 
If a leaf of wheat is cut across through one of the 
yellow streaks and examined under the microscope, it is 
