49 



generation of the cells in the interior of the lenticel, and the consequent, 

 extension of cork formation into the interior of the tuber. 



That this hypertrophied formation of lenticels is dependent upon an 

 unusual amount ot moisture we conclude from the following facts: 

 During a long continued season of wet weather the lenticels of alder 

 trees stand out in the form of thick white protuberances; by immers- 

 ing a piece of cherry stem in water a luxuriant growth of cork can 

 be artificially produced ; finally, in case of the potato, these cork warts 

 may be actually produced by keeping the potato for a long lime in moist 

 air. Xobbe 1 found, by a water culture of the potato, that the tubers 

 grown in water produced small warts while still very young. These warts 

 were caused by a local increase in growth in the cork formation, and 

 were not present in tubers produced in the air. 



The view that superfluous moisture at an unseasonable time is the 

 cause of the increased growth of cork that produces scab is indorsed by 

 Caspari 2 and Schacht 3 as well by Frank. 4 A later work by Stahl es- 

 tablishes the fact that if transpiration is stopped lenticels develop 

 beneath the stomata. 5 On the contrary, farmers assert that in the ma- 

 jority of cases the adding of lime and marl to the soil, and manuring 6 

 with dung and street-sweepings causes the disease. Kuhn agrees with 

 Wallroth 7 in the opinion that the formation of scab is caused by a fun- 

 gous growth (Erysibe subterranea, Wallr.). 



On the other hand, iron, when present in a low state of oxidation, is 

 considered injurious. In a field of potatoes at Kewmarket three rows 

 which had been marled with light colored earth mixed with iron oxide 

 were found free from scab, while the rows that had received marl that 

 was dark colored from the presence of ferrocyanate of peroxide of iron 

 were very scabby. 8 



The assertion has frequently been made by practical farmers that fresh 

 animal manure causes scab, especially in the thin-skinned varieties, 9 

 and that the disease makes its appearance after an application of soap- 

 suds, while after a strong application of potash, which had proved very 

 injurious at another time, the tubers were free from scab. 10 



1 Versuchsstationen, 1864, p. 58. 



2 Bot. Zeitung, 1857, p. 116. 



3 Bericht an,d. Kgl. Landes-Oecoinomie-Collegium iiber die Kartoffelpflanze uiid 

 deren Krankkeiteu, Berlin, p. 15. 



4 Die Krankheiten der Pfiauzen, 1880, p. 141. 



fi Biedermann's Centralbl., 1879, p. 714. 



6 Landwirtksch. Zeitschr. f. Westphalen und Lippe, 1864. 



7 LinnaBa, 1812, p. 332. Der Knollenbrand der Kartoffel, von Hofrath Dr. Wallroth. 

 He says: "The much discussed disease of the potato known in economic writings under 

 the name of potato scab, potato wart, scab disease, brown stain, and potato rot, I have 

 known for a long time as a kind of vegetable rust ( Uredo, Ustilago, and Cceoma of 

 authors, Erysibe Theophr., Adans, Murr., Wallr., not DC). 



8 Landwirth, 1875, p. 352. 



9 Landwirth, 1875, p. 319 and 352. 



i0 Janovski, Kartofflenbauversuche. Prager Landw. Wochenbl., cited in Central 

 bl. f. Agrik.-Chemie, 1876, I, p. 430. 

 11244— No. 8- 4 



