1 2 Proceedings of the Royal Irish Academy. 



to the development of a brown colour in the walls of the vessels themselves, 

 especially in those nearest the pith. Bacteria are to be seen in the vessels, 

 but not in very great numbers. When a stalk is cut across, a drop of 

 blackish liquid occasionally exudes from the cut ends of the vascular bundles, 

 which liquid may also contain bacteria, but, as a rule, not in great quantity. 

 The cavities of the wood vessels are also nearly always choked with a 

 development of thyloses. 



Summarized shortly, then, the chief characters by which the disease may 

 be recognized are : {a) the stunted appearance of the plants, together with the 

 yellowing of the foliage and the folding of the leaflets ; (6) the toughness of the 

 stalk on cutting, and the three prominent vascular bundles seen on the cross- 

 section; (c) the decay of the portion of the stalk below ground. Fig. 4, 

 Plate II, shows the general appearance of a stalk affected as described. 



As time goes on the affected stalks die away and shrivel up, and the 

 positions which such diseased plants occupied become obliterated owing to 

 the growth of the neighboui'mg plants. This is rather an important point, 

 because on looking casually over a field of potatoes — say, in August — the crop 

 may appear to be an entirely healthy one ; whereas, had it been examined 

 during June or at the beginning of July, quite a considerable proportion of 

 plants affected with Black Stalk-rot might well have been observed. 



Although by far the greater proportion of attacked plants are seen early 

 in the season, yet in some cases the disease does not develop and show itself 

 until considerably later, when the stalks have already made considerable 

 growth ; so that, as the season progresses, there is a more or less continuous 

 appearance of plants showing the disease. The number of these, however, is 

 generally small, and tends to grow less as the season becomes more advanced. 

 The symptoms here are very similar to those already given. There is, however, 

 perhaps somewhat more wilting and drooping of the foliage, and the stalks 

 themselves often show black lines or marks on their surfaces. Where leaves 

 break off or where branches break away, as frequently happens, the interior of 

 the stalks is seen to be black and rotten. Fig. 1, Plate I., shows such a 

 diseased plant in a drill amongst its healthy neighbours. Another somewhat 

 important difference between the earlier and later attacks is that whereas in 

 the former there has been but little chance for the formation of new tubers, 

 in the latter a considerable number of new tubers may have been formed ; and 

 some of them at least are always found to be attacked with the disease, entry 

 having occurred invariably at the heel end through the rhizome. The 

 difference between the healthy and attacked portions of such tubers can 

 usually be quite clearly discerned from an external examination of them. The 

 diseased portion is usually co\'ered with a more or less discoloured skin, which 



