432 



BULLETIN 387 



of infection, but the adhering particles of soil, which could not be washed 

 off without sacrificing the hairs, not only were detrimental to the micro- 

 tome knife, but also obstructed a clear view of the cell walls. Finally a 

 method was devised whereby infected roots could be procured free from 

 any other contamination. Diseased roots that contained spores but were 

 not far enough advanced to be invaded by bacteria were sterilized on the 

 surface with mercuric chloride and transferred to agar slants in test tubes. 

 After two weeks cooling in the ice chest they were finely minced in the agar, 



and incubated 

 until it was clear 

 that no bacteria 

 were present in 

 the tissue, from 

 which they might 

 have been liber- 

 ated by the cut- 

 ting. After enough 

 time had elapsed 

 to insure perfect 

 freedom from any 

 saprophytes, a few 

 drops of sterilized 

 muck-soil filtrate, 

 and a young cab- 

 bage seedling 

 which had been 

 grown from disin- 



FlG. IOI. THE AMCEBA OF PLASMODIOPHORA BRASSICAE IN A f ected Seed On a^ar 

 ROOT HAIR . . 



A, A root hair with an amceba showing two nuclei. B, A uninucleate ] b ' 



amceba in a root hair which shows an abnormal swelling in the immediate were added It 

 vicinity of the organism. C, A uninucleate amoeba in a tangential section 



of a root hair; the nucleolus has elongated, as it ordinarily does just before W aS neCCSSarV to 

 nuclear division. D, A host nucleus in a root hair, showing its size as com- 



pared with that of a uninucleate amoeba, 

 shrunken, distorted root hair. X 1600 



E, A uninucleate amoeba in a 



exercise care in 

 adding sufficient 



liquid to permit spore germination and not have an excess, which would 

 injure the root. A few drops would not evaporate until all the swarm- 

 spores had ample time to be set free and attack the root hair. The 

 process was somewhat long, and very often roots were chosen which were 

 too old and were already contaminated with bacteria. In spite of all the 

 difficulties, enough pure cultures were obtained to provide a large number 

 of sections which showed all sizes of amoebae. 



The first and most important thing shown by the stained sections was 

 that Plasmodiophora Brassicae enters the root hair as a uninucleate amceba, 



