534 Scientific Proceedings, Rojjal Dublin Society. 



The external appearance of affected tubers varies to some extent, depending 

 chiefly upon the degree of development of the attack at the time of digging. 

 In fig. 1, Plate XLII, a tuber (Champion) is illustrated, in which about one- 

 fourth of its substance is affected with the rot, and another is illustrated in 

 fig. 5, Plate XLIII. The skin becomes somewhat discoloured, and not 

 infrequently the still healthy part of the tuber is marked off from the diseased 

 portion by a more or less definite blackish band of varying width, and" 

 rather irregular course around the tuber. 



The lenticels on the affected portions of the tuber usually stand out as 

 small black spots ; and particularly when the rot is well advanced, fungus 

 pustules of various kinds break out through the skin of the affected portions. 

 It may be stated at once that these pustules have nothing whatever to do 

 with the primary decay of the tuber ; and that the organisms constituting 

 them are of varied character. It is quite common to find on digging tubers 

 which have almost completely rotted in the ground that their surfaces are 

 covered to a large extent with the common green mould PenicilKum. 



When only slightly affected tubers, having no pustules on them at the 

 outset, are placed in a covered glass dish the disease extends rapidly, at room 

 temperature, until at the end of a few days the tubers become wholly rotten, 

 and their surface more or less dotted over with pustules. The same thing 

 occurs even if the tubers, after being well washed, are thoroughly disinfected 

 externally by soaking them in a dilute solution of formalin. It is abundantly 

 evident that following very closely upon the heels of the particular fungus 

 which causes the death of the tuber are a host of camp-followers consisting of 

 the most varied assortment of micro-organisms. 



On the other hand, when well-washed, perfectly healthy tubers are 

 inoculated at the heel end with a pure culture of the fungus, and particularly 

 if they are previously disinfected, the rot proceeds normally, but fungoid 

 growths do not appear on the surface for a period of two or three weeks. If 

 such tubers were kept entirely free from extraneous sources of infection for 

 so long a period, and if the exudation of a watery juice very favourable to 

 the development of the stray organisms which find an entrance could be 

 inhibited, doubtless the tubers could be kept free from these pustules for an 

 indefinite period. Since, however, repeated microscopic investigation showed 

 that the organisms forming these pustules were of various kinds, both animal 

 (eel worms) and vegetable (bacteria, various fungi, such as species of Fusarium, 

 Mucor, &c.), and had nothing to do with the phycomycetous fungus causing 

 the rot, no attempt was made to do this. 



As just remarked, there exudes very often through the lenticels a 

 liquid which, coming in contact with the particles of soil surrounding the 



