PHYSIOLOGICAL STUDY OF THE PARASITISM OF 

 PYTHIUM DEBARYANUM HESSE ON THE POTATO 

 TUBER 



By LoN A. Hawkins and Rodney B. Harvey 



Plant Physiologists, Office of Drug-Plant, Poisonous-Plant, Physiological, and Fermen- 

 tation Investigations, Bureau of Plant Industry, United States Department of Agri- 

 culture 



INTRODUCTION 



The physiology of parasitism and the relations existing between the 

 host and parasite have been the subject of numerous investigations, 

 many of which have taken up the method by which the fungus obtains 

 entrance into the plant or passes from cell to cell within the tissues of 

 its host. 



There are, of course, several possible ways by which a parasitic plant 

 may obtain entrance into the cells of its host plant. It may enter 

 through an opening already made; if it makes the opening itself, it may 

 push its way through mechanically, or it may soften or digest the cell 

 walls. It is possible, also, that the fungus hyphae might so stimulate 

 the cells of the host plant that enzyms secreted- by the host itself would 

 break down its own cell walls and allow the fungus to enter. A com- 

 bination of these methods is, of course, possible — for example, a fungus 

 might penetrate the cell wall by a small opening and then enlarge this 

 opening either mechanically or by a solution of a portion of the cell wall. 

 Some of the investigations on this subject will be considered here. 



In 1886 De Bary (j)^ showed that Sderotinia liberliania secretG:d a toxic 

 substance which killed the cells ahead of the growth of the fungus. He 

 concluded that the breaking down of the cell walls was due to an 

 enzym secreted by the fungus. 



Ward (27) concludes, in his study of Botrytis on lily, that the tip of 

 the fungus hypha "excretes relatively large quantities of ferment sub- 

 stance and dissolves its way into the cell wall." Nordhausen (21) con- 

 siders that Botrytis cinerea dissolves its way through the cell walls of 

 its host plant. Biisgen (9) considers that this fungus does not make its 

 way through cell walls or even cuticle by mechanical means alone. 

 Miyoshi (20) showed that Botrytis cinerea could force its way through 

 membranes of collodion, paper, and other substances, and details some 

 experiments in which Penicillium pushed its way through gold leaf. 

 Peirce {23) has shown that the haustoria of Cuscuta will puncture tinfoil 

 0.2 mm. in thickness. 



' Reference is made by number (italic) to "Literature cited," p. 295-297. 



Journal of Agricultural Research, Vol. XVIII, No. 5 



Washington, D. C. Dec. i, 1919 



sy Key No. G-i8i 



(275) 



