Dec. 1, 1919 Parasitism of Pythium debaryanum on the Potato Tuber 295 



than to puncture the tissues of the two susceptible varieties, Bliss Triumph 

 and Green Mountain. Correlated with this resistance to puncture is a 

 resistance to infection by Pythium debaryanum. 



(4) The resistance to puncture in McCormick tubers is also correlated 

 with a higher crude-fiber content, which was considered to be due to 

 more secondary thickening in the cell walls. 



(5) The cut surface of the cortex of Bliss Triumph and Green Mountain 

 when dried for three hours was much more resistant to puncture than the 

 freshly cut surface. Here also there is a correlation between resistance 

 to infection by this fungus and resistance to mechanical puncture. 



(6) The osmotic pressure within the fungus filament, as determined by 

 plasmolysis in this work, was sufficient to develop the pressure necessary 

 to puncture the cell walls in the potato tubers in all cases in which 

 infection occurred, with one exception. It was not sufficient to develop 

 the pressure necessary to puncture the tissue of the potatoes in the cases 

 where no infection occurred, with three exceptions. 



(7) Mechanical pressure exerted by the fungus hyphae seems to be 

 the most important factor in cell-wall penetration by this fungus, and 

 resistance to infection is apparently due to resistance of the cell walls to 

 mechanical puncture. Microscopical observations of cell wall penetra- 

 tion by the fungus hyphae seem to corroborate this theory, 



LITERATURE CITED 

 (i) Appel, Otto. 



1915. DISEASE RESISTANCE IN PLANTS. In Science, n. s., v. 41, no. 1065, p. 



773-782. 



(2) Averna-Sacca, Rosario. 



I91O. l'aCIDITA DEI SUCCHI DELLE PIANTE IN RAPPORTO ALLA RESISTENZA 



CONTRO GLi ATTACHi DEI PARASSiTi. In Staz. Sper. Agr. Ital., V. 43, 



p. 185-209. 



(3) Bary, a. de. 



1886. UEBER EINIGE SCIvEROTINIEN UND SCLEROTIENKRANKHEITEN. In Bot. 



Ztg., Jahrg. 44, No. 22, p. 377-381, i fig.; No. 23, p. 393-404; No. 24, 

 p. 409-426; No. 25, p. 433-441; No. 26, p. 449-461; No. 27, p. 465-474. 



(4) BehrEns, Johannes. 



1898. BEiTRAGE zuR kenntniss der obstfaulnis. Iu Centbl. Bakt. [etc.], 

 Bd. 4, No. 17I18, p. 700-706. 

 (s) Blackman, V. H., and Welsford, E. J. 



1916. STUDIES in the physiology OF PARASITISM. II. INFECTION BY BOTRYTIS 



cinEREA. In Ann. Bot., v. 30, no. 119, p. 389-398, 2 fig., pi. 10. Lit- 

 erature cited, p. 397. 



(6) Brown, William. 



191 5. STUDIES IN THE physiology OF PARASITISM. I. THE ACTION OF BOTRYTIS 



CINEREA. In Ann. Bot., v. 29, no. 115, p. 313-348. 



(7) 



1916. STUDIES IN THE PHYSIOLOGY OF PARASITISM. III. ON THE RELATION 



BETWEEN "infection DROp" AND THE UNDERLYING HOST TISSUE. In 



Ann. Bot., v. 30, no. 119^ p. 399-406. 



