72 Scientific Proceedings, Royal Dublin Society. 



those free from it, and proved that the cutting away of the heel-ends of the 

 tubers was not sufficient to eliminate the disease with certainty. 



It was clear that in any further experiments on these lines it would 

 be necessary to test individually each tuber before using it to ascertain 

 whether it contained the fungus or not, and the method of testing was 

 as follows : — 



The tubers were first thoroughly cleaned by careful scrubbing in plenty 

 of running water, and then dried in a clean linen towel. They were purposely 

 not treated with any disinfectant, lest some of it should be absorbed into the 

 wound at the heel-end of the tuber caused by its severance from the rhizome, 

 which might prevent the development of the fungus if present. 



A small portion of the heel-end of the tuber, about a millimetre or so thick, 

 was then cut in such a way that it remained hanging from the tuber by a 

 small bit of skin, the two cut surfaces being of course exposed. Tubers 

 prepared in this way were put into clean covered glass dishes the bottoms of 

 which were lined with moist filter paper. After standing for two or three 

 days at room temperature microscopic examination of the tubers was made. 



Verticillium when present in the wood vessels of a tuber does not produce 

 aerial growth and conidiophores at the cut surfaces with the luxuriance 

 generally observed when pieces of affected stalks, rhizomes, or roots are 

 suitably incubated. Nevertheless it does grow out in varying degrees, if 

 present, and by microscopical examination one can decide whether a tuber 

 possesses it or is free from it, although cases do arise occasionally where 

 definite decision is not possible. 



Examination was always carried out at three places — (1) the natural 

 wound at the heel-end ; (2) the cut ends of the vascular tissue of the small 

 portion of the tuber nearly severed ; (3) the cut ends of the vascular tissue of 

 the remaining part of the tuber. Only when the fungus was present or absent 

 at all of these three places simultaneously was the tuber regarded as infected 

 or free from infection respectively for the purpose in view. 



It may be stated that the mycelium can be traced definitely growing out 

 of the browned xylem portions of the vascular ring of the tuber, and, indeed, 

 the exit of an individual hypha from the cavity of a particular wood vessel 

 has in many cases been traced, so that there can be no doubt as to the source 

 from which the fungus comes. Occasionally the growth of other fungi makes 

 certain determination practically impossible, and this is especially the case 

 when Hypoclmus Solani is present on the surface of the tuber under examina- 

 tion, for it rapidly produces a luxuriant growth at the expense of the cells 

 killed by the process of cutting. Even in such cases, however, with some 

 experience it is sometimes possible to discriminate between the growth of 



