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Since the life story of the fungus causing- the Black 

 Kust ot wheat (Pucctnia graminis) is typical of mativ 

 rusts, it will be described in some detail. The diseasV 

 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, arid although for a time some uredospores continue 

 to arise in the pustules, mingled with them arc 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 telcutospores 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 reo-ard 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 



