Pethybridge — The Verlicillium Disease of the Potato. 77 



were then subjected to careful microscopical and cultural examination, but 

 in no single instance could the fungus be found present in a sprout. In one 

 case it was found in the wood vessels of a tuber close to the point of origin 

 of the sprouts, but even here the sprouts themselves were free from it. 



The young shoots produced by affected tubers always appear healthy 

 (although they may be small) when first they come above ground ; and if they 

 are examined microscopically at this stage, mycelium may not be found in 

 their wood vessels. But if they are allowed to remain longer, sooner or later 

 symptoms of disease will begin to develop in their foliage, such as rolling of 

 the leaflets, or dying off of the lower leaves. When this occurs, or at any 

 rate very soon afterwards, mycelium will be found in the wood vessels of the 

 lower portions of the stalks at least, while often it has not yet reached the 

 upper parts. This mycelium has been traced down the stalk, back to the 

 point of insertion of the latter on the parent tuber, and finally into the wood 

 vessels of the tuber itself at this point, thus permitting of no reasonable 

 doubt but that it has grown from the wood vessels of the tuber into those of 

 the stalk. No evidence was found of the presence of the fungus in the 

 cortical tissues of these stalks, and consequently it could not have reached 

 the wood vessels from this source. Hence it is believed that the mycelium 

 reaches the young stalks, not by growth over the outside of the tuber, as 

 Eeinke and Berthold supposed, but by direct growth through the xylem 

 system common to tuber and stalk. 



There is, however, a distinct pause of longer or shorter duration before 

 the mycelium proceeds from the tuber to the stalk, during which temporarily 

 healthy stalks are developed from affected tubers. If this pause is sufficiently 

 prolonged, either because the amount of mycelium in the tuber was originally 

 very small, or its location was removed so far from the developing shoot that 

 the latter could not be reached in time, then it is possible to understand how 

 an affected tuber may give rise to a plant which remains healthy throughout 

 the season, and which produces healthy progeny. 



Statistics compiled for the seasons 1912-1915 show that when affected 

 tubers are planted 96 per cent, of them give rise to diseased plants bearing 

 further affected progeny, while the remaining 4 per cent, produce healthy 

 plants with healthy progeny. 



VI. The Fungus in Pure Cultures. 



Since when placed under suitable conditions of moisture and temperature, 

 the fungus present in infected tissue grows out into the surrounding air, 

 and produces 'a plentiful crop of conidiophores and conidia, it is a com- 



