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menon the name of the white rust in cress. This is a case of 

 disease, and so striking that every one notices it at once with 

 the naked eye. Now we find in a bed of cress at about 

 flowering time a certain number of rusty plants, two for example 

 or twenty. They are in the middle of the other hundred or 

 thousand plants, and these are healthy and free from the fungus 

 and continue so till the period of vegetation is at an end. This 

 is the case, though the Cystopus forms countless spores in the 

 white rust-pustules, and the spores are dispersed as dust and 

 are at once capable of germination, finding the necessary con- 

 ditions for their further development in the bed of cress, and 

 are the instruments by which the white rust-disease is emi- 

 nently infectious. Nevertheless those hundred or thousand plants 

 are not infected. All that has been hitherto said is strictly 

 correct, and if we limit our view to this, we shall see in the 

 phenomena which have been described a conspicuous case of 

 individual diflFerence in predisposition : a case too perhaps, if 

 we judge hastily, of sickly predisposition in the plants attacked, 

 for they do become sick and the others do not. And yet 

 this is not the true account of the matter. Every healthy cress- 

 plant is equally liable to the attacks of the Cystopus and to 

 the rust-disease which it causes, only the liability is confined 

 to a certain stage of the development, and ceases once for all 

 when that stage is past. The germinating cress-plant in efiect, 

 first unfolds two small three-lobed leaves, the seed-leaves or 

 cotyledons. When it has grown a little further and formed 

 more foliage -leaves, the cotyledons wither and drop ofi*. It 

 appears then, that the germ-tubes of the fungus of white rust 

 find their way into all the cotyledons and are able to develope there, 

 and if this development has once begun, the fungus establishes 



