398 



smoother skin, a lower starch content and a considerably higher potassium 

 content. They are also smaller in size and have a smaller drj^ weight. Under 

 favorable conditions, healthy plants may often be grown from such tubers. 



Among the characteristics given, we have emphasized the length of the 

 existence of the parent tuber, which remains turgid and retains starch, 

 because Hiltner^ has recently described such a case belonging here and, in 

 fact, a partial subsequent enlargement of the parent tuber. Different people 

 •have made the same observations. In Hiltner's case it was also observed 

 that the plants produced from the turgid tuber developed no tubers below 

 the soil, attached to the stolens, but bore them directly on the lower inter- 

 nodes of the green stems. These stems, however, were only half as long 

 as in normal plants and bore leaves, rolled together, which reminded Hiltner 

 of leaf curl. He thinks that these processes are a result of the use of im- 

 mature tubers for seed. These tubers, after developing the stem, had uti- 

 lized in their own further growth the material obtained by the action of 

 the leaves. Naturally too little organic substance remains for the tubers of 

 the current year. 



If we accept Hiltner's theory as to the production of tubers which re- 

 main turgid, we can infer that leaf curl results from the use of unsuitable 

 seed. The tubers were not sufficiently matured in the previous year. This 

 must also make itself felt in the full development of the individual eyes. 

 While the majority of these had time to develop normally, some may have 

 remained immature and have retained this character when sprouting in the 

 following year. This will explain the fact that often only isolated shoots 

 are found which show leaf curl. The characteristic of immaturity is the 

 marked abundance of potassium and nitrogen compounds with a scanty 

 deposition of carbohydrates as reserve substances. We find such conditions 

 favored by the use of fresh manure with early varieties and drought stops 

 the growth of the tubers prematurely. 



If an over-supply of nitrogenous compounds, not normally utilized, 

 determines the appearance of leaf curl in the potato, the shrivelling disease 

 of the mulberry tree, and other diseases, to be mentioned under "Enzymatic 

 Diseases," then the symptoms of the blackening of the ducts and rapid 

 bacterial infection, already found, may be explained easily. 



This theory is further supported by a study made by Appel-, who, under 

 the name "Bacterial-ring disease," describes the phenomena which often 

 suggest leaf curl. He makes bacteria responsible for the ring disease 

 and "indeed, as in black-leg, not one species alone but a few closely re- 

 lated forms." "These bacteria are undoubtedly present normally in many 



soils " Influenced by these statements I should like to include 



bacterial ring disease under those diseases in which a constitutional weak- 

 ness in the plant and not a parasite determines the phenomenon and favors 



1 Hiltner, L.., Zur Frag-e des Abbaues cler Kartoffeln. Prakt. Bl. f. Pflanzcn- 

 bau und Pflanzenschutz 1905, Part 12. 



~ Appel, O., Die Bakterien-Rinskrankheit der Kartoffel. Flugblatt 36 d. Kais. 

 Biolog:. Anst. Dahlem. 1906. 



