Page:The Harveian oration 1888.djvu/34

26 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 conditions for their further development in the bed of cress, and are the instruments by which the white rust-disease is eminently 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 difference 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 cressplant 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 effect, 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 off. 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