292 



exceedingly fine, 2-4 fi in diameter, hyaline or pale brown. The 

 cell-walls appear to be somewhat thickened and cuticularised and 

 to be lined with a brown substance, due probably to the decomposi- 

 tion of the cell-contents. The fungus penetrates below the 

 periderm and invades the parenchyma, often to a depth of 12-15 

 lis. The starch grains in the invaded cells entirely disappear. 



ceils. 



fungu 



cularisation of the cell walls, but attempts are also sometimes 



made 



below the affected part (see Fig. 1). In some cases successive 



Fig. 1 



layers of cork are formed. Early in April, when the 

 fungus appears to be most vigorous, the hyphae in the 

 diseased cells are very numerous and for the most part hyaline; 

 but later in the season, if similar areas are examined, very few 

 hyphae are found, and these are mostly brown, probably as the 

 result of age. Judging from the smallness of the area which 

 results from each infection, and from the fact that after a certain 



period of time the hyphae, instead of penetrating to fresh cells, 



* 



gradually die, the fungus can be regarded as only a very weak 

 parasite on the potato tuber. 



Puke Cultures of the Fungus. 



No difficulty has been experienced in isolating the fungus in 

 pure culture. The tubers were washed in running water and 

 then placed for ten minutes in '002 per cent, mercuric chloride 

 solution and afterwards washed in sterile water. In order to 

 lessen still further any chance of contamination the tubers were 

 sometimes dipped in spirit and allowed to dry. The top of a scab 

 was then sliced off with a clean sharp knife and the brown tissue 

 below lifted with a sterile needle and placed on suitable media. ' 



In this way a large number of inoculations have been made from 

 different tubers on various media, and in every *case a similar 



