I 9 20.] 



Decay in Potato Clamps. 



79 



apart from a few exceptional cases, no effect beyond the 

 softening of the surface tissue to a depth of one or two milli- 

 metres. At .20 J ( ., in every case the whole of the tissue rotted 

 completely in three or four days. The colour of the rotted 

 tissue was at first quite white, but became greenish-yellow on 

 the second day and remained this colour for seven or eight 

 days, when the colour changed to pink. 



From the greenish-ye'low tinge of the tissue B. xanthochlorum 

 (Schuster) was suspected as the cause of the decay, but on 

 transferring repeatedly from the margin of the diseased area 

 to fresh potato slices this colour was observed to become less 

 prominent, and it eventually failed to appear at all. The 

 rotting effect continued actively in the absence of the yellow- 

 producing organism, which was evidently a saprophyte. The 

 character of the rotted tissue indicated the "Black-Leg" 

 bacillus (B. atrosepticus) as the causal organism. The rotted 

 tissue was perfectly white in the unexposed portions and 

 fieshy-pink at the surface, where it was exposed to light and 

 air. Platings which were made from these rotted potato 

 slices gave pure cultures of an organism which agreed in 

 morphological characters with Bacillus atrosepticus and gave 

 also all the reactions by which this organism is recognised. 

 Inoculations of potato stems and bean stems gave rise to the 

 symptoms of " Black- Leg," thus leaving no doubt as to the 

 identity of the organism. 



" Black-Leg " is well known as a cause of rotting in stored 

 potatoes. The disease had not been observed by the farmers 

 in the fields from which these crops had been taken, but this 

 signifies little, since it is well known that a late attack of 

 " Black- Leg " will not show itself to any very obvious extent 

 in the " tops," and the presence of the organism in the tubers 

 can only be recognised by cutting the tuber longitudinally 

 through the " heel." 



From the cultural experiments recorded above it is certain 

 that the " Black-Leg " bacillus was largely responsible for the 

 damage in the particular cases investigated, but the question 

 as to whether it was responsible for the commencement of the 

 trouble or whether one of the other agencies enumerated by 

 Cotton and Taylor was concerned must be left open. It is 

 also unknown whether the presence of B. atrosepticus was 

 general in the rotted potatoes throughout the country, but it is, 

 at least, significant that trouble from " Black-Leg " was worse 

 than usual during the past summer. 



