114 Lectures on Bacteria. [^ x. 



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 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 

 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 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 

 itself at once in the tissue into which it has penetrated, and 

 grows on in and with the growing plant, and produces the 

 disease. The germ-tubes of Cystopus may indeed make their 

 way for a short distance into all the other parts of the plant, but 

 are unable to establish themselves inside it and continue their 

 development. The plant is for the future safe from the attacks 

 of the parasite as soon as the cotyledons have fallen off. The 

 two or the twenty rusted plants in the bed are the ones in which 

 the Fungus attacked the cotyledons in good time; if it had 

 attacked the thousand others in equally good time, all would 

 have been rusted. They continued healthy, because they were 

 not infected in the stage in which they were open to infection, 

 that is, predisposed. 



