Dec. 1, 1919 Parasitism of Pythium deharyanum on the Potato Tuber 293 



existence of such an enzym was brought out in this study, the fact that 

 the fungus requires such a short time (about five minutes) to pass com- 

 pletely through a cell wall seems to indicate that the main factor, at 

 least in the breaking through the cell wall, is mechanical pressure, for in 

 enzym action it would be necessary to have a diffusion of the enzym 

 from the tip of the hypha into the cell wall and at least a softening, if not 

 a dissolving, of a portion of the cell wall at that point. Whether a sub- 

 stance, such as an enzym, with a relatively high molecular weight and 

 consequently low rate of diffusion could diffuse into and through the cell 

 wall with sufficient rapidity to soften or dissolve this tissue in the 

 time required for the fungus to pass through the cell wall is doubtful. 

 Another point in support of the hypothesis that the opening in the cell 

 wall is made mechanically is that there is apparently no considerable 

 increase in the size of the opening after the tip of the hypha passes through. 

 If the fungus secretes an enzym which acts on the cell wall, it would 

 seem probable that this enzym action would continue after the tip of the 

 hypha passed through and the opening would be larger than the hypha. 

 Hasselbring (jj) has figured the breaking down of the host tissue around 

 the fungus hypha. This phenomenon is common where a fungus breaks 

 down the cell walls of its host enzymically. With Pythium debaryanum 

 on potato, however, the opening in the cell wall is never larger than the 

 mean diameter of the hypha in the lumen of the cells and is usually 

 considerably smaller. 



Another point which supports the hypothesis of the mechanical punc- 

 ture of the cell walls of the host by the fungus is the fact that apparently 

 only the middle lamella of the potato cell wall is affected. The fungus 

 seems not to break down the secondary thickening of the cell walls, and 

 when a piece of well-rotted potato is teased out on a slide the cells full 

 of starch grains float free. 



If, then, as seems probable, the fungus makes its way through the cell 

 walls by mechanically puncturing them, a potato with cell walls strong 

 enough to withstand the pressure exerted by the fungus would be im- 

 mune to the disease. If we consider the osmotic pressure within the 

 fungus filament— as determined in this study — as the pressure the fungus 

 is able to exert against the wall of its host plant, then the resistance of 

 potatoes in all cases in which they did not become infected would be 

 explained, with the three exceptions mentioned earlier. This would 

 also account for the infection of all potatoes which became infected in 

 either the central portion of the tuber or the cortex, with the exception 

 of one Green Mountain tuber. Another point which should be noted 

 in this connection is that correlated with this resistance is a higher crude- 

 fiber content in the McCormick. This is probably due to more secondary 

 thickening in the cell walls. It is quite possible that the White INIcCor- 

 mick potato or some hybrid of this variety would be resistant to the 

 fungus when grown in the San Joaquin Valley and would thus solve 



