14 Proceedings of the Royal Irish Academy. 



the leaves on the stalks ; and hence it happens that small aerial tubers are 

 formed in these positions. Such aerial tubers are also formed in other cases, 

 where, owing to the agency of grubs or other means, sometimes mechanical, 

 the base of the stalk becomes injured. Hence it is that although not infre- 

 quently such aerial tubers are found on stalks affected with Black Stalk-rot, 

 yet their presence there is by no means an infallible sign of the presence of 

 this particular disease. 



III. Isolation of the Oeganism causing the Disease. 



During the summer of 1909, attempts were made by one of us, and with 

 some success, to isolate the organism which was presumably responsible for 

 the disease. That the decayed, pulpy, dead tissues from affected stalks 

 contained some organism Irighly toxic to living potato-tissue, was easily and 

 repeatedly demonstrated by placing some of it — usually a not inconsiderable 

 amount, weighing perhaps a gramme — on the cut surface of h\dug tubers kept 

 for a day or two iii a covered moist dish, when a characteristic rot was nearly 

 always set up, control experiments conducted at the same time giving 

 negative results. From such raw material an organism was isolated on two 

 or three occasions by means of potato-gelatine plate-cultures, which, when 

 placed on slices of hving tubers, and inoculated into cut-pieces of stalks, 

 produced a characteristic rot in them. Inoculation with a pure culture into 

 the healthy hving stalk of a growing plant was also carried out ui one 

 instance, taking the usual aseptic precautions. Unfortunately, owing to a severe 

 attack of the ordinary bUght [Phyto'phthora), the inoculated plant, as 

 well as the remainder of the crop at the Investigation Station, became 

 seriously damaged, and the experiment had to be brought to a premature 

 conclusion at the end of sixteen days. During this time, however, a small 

 cavity had developed in the region of the stab, the sides of which were lined 

 with a blackish pulpy material swarming with motile bacteria. For a distance 

 of two or three inches up the stem from the wound, the three principal 

 vascular bvmdles had developed the brown stain characteristic of the disease, 

 and thyloses had also developed in the cavities of the vessels. 



The organism isolated was one which Liquefied gelatine rapidly, being rod- 

 shaped and exceedingly motile. Certain difficulties were encountered in 

 obtaining with certainty positive results right through a series of inoculations 

 on potato shoes, even when the same materials were used. It was found 

 that, on the whole, more certain results were obtained when the inoculating 

 material consisted of a fairly large quantity of the raw pulpy material from 

 an affected stem than when merely a platinum loopful of a pure culture was 



