Dec. 1, 1919 Parasitism of Pythium deharyanum on the Potato Tuber 289 



tipped glass needles, and the pressure required was considerably more than 

 that required for any portion of the cut surface of tubers tested. 



No direct evidence was obtained that the fungus could exert sufficient 

 pressure upon the cell walls of susceptible potato tubers to puncture 

 them. However, some indirect measurements were made of the pressure 

 the fungus might be capable of exerting under certain conditions. When 

 fungus filaments were plasmolyzed in cane sugar solution it was found 

 that it required a solution capable of exerting about 54 atmospheres, or 

 55,773.3 gm., per square centimeter to plasmolyze them. If, then, 

 the protoplasm of the fungus is not permeable to cane sugar, the fila- 

 ments are capable of withstanding nearly 55,773.3 gm. pressure per 

 square centimeter; or, stated in another way, the filaments are capable 

 of exerting that much pressure. This is considerably more pressure 

 than is required to puncture the tissue of the central parts of Bliss 

 Triumph and Green Mountain potatoes. It is sufficient pressure to 

 puncture the cell walls of the cortex of all tubers of these varieties 

 which rotted when inoculated in that region except one. This exception 

 is tuber 31 in Table V, a Green Mountain tuber which required 57,571.5 

 gm. per square centimeter, or 1,798.2 gm. more than the osmotic pressure 

 of the fungus filament as found in this study. It is also sufficient to 

 puncture the tissue of the two McCormick tubers which rotted when 

 inoculated in the central portion and the one which rotted when inocu- 

 lated in the cortex. 



The pressure would not be sufficient to puncture the cell walls of 

 McCormick tubers when they were resistant to infection, and it is lower 

 than that required for the cortex of two of the three Green INIountain 

 tubers that did not rot when inoculated in that region. The third Green 

 Mountain and the two Bliss Triumph tubers that did not rot when in- 

 oculated in the cortex required pressures considerably below the osmotic 

 pressure of the fungus filament to puncture the cell walls. Just why 

 these three potatoes did not rot is not apparent. It is, of course, possible 

 that the 10 determinations of the pressure required to puncture cells 

 of the cortex were made on less resistant cells than those upon which 

 the inoculations were made. Another possibility is that a weak culture 

 of the fungus was used. These 3 potatoes, however, were exceptions to 

 the rule. 



The experiments for the determination of the pressure required to 

 puncture the tissue of the potatoes on the fresh surface and when dried 

 at 30° C, as detailed in Table X, show that the pressures required for 

 the cortex of the dried tubers, which were resistant to infection, were 

 considerably higher than the osmotic pressure of the fungus filament. 

 This is in agreement with the evidence just brought out from data in 

 Tables V to VII. It would seem from this work that the mechanical 

 pressure of the fungus filament against the cell wall of the potato is an 

 important factor in the penetration of the potato tissue by the fungus. 



