292 Journal of Agricultural Research voi. xviii.no. s 



There is quite clear evidence of the exertion of mechanical pressure. 

 In cell VII another hypha was observed growing toward the potato 

 cell wall. At first the hypha was straight; then as growth pressed it 

 against the wall it bent upward, at the same time making a dent in the * 

 wall near the center of the wall face. This hypha did not penetrate the 

 wall, for when it had reached the position given in the figure, rapid 

 growth was begun by the growing tip just below it, and the pressure of 

 the upper hypha seemed to be insufficient to break the wall. 



Where the hypha approaches the cell wall at right angles it usually 

 passes through as shown in cell I; but when the tip strikes the wall 

 obliquely it does not usually penetrate but pushes along the wall and 

 may go entirely around the cell, forming a coil within it. In traveling 

 around the cell the tip may reach a corner, in which case the wall is 

 frequently penetrated and the hypha grows between the cells. This is 

 characteristic, and hyphae are frequently found following the middle 

 lamellae. When the growth of the young tip is stopped for a few moments 

 as by strong light, a cell wall is apparently formed over the tip; and on 

 the resumption of growth this wall is broken at its weakest point by the 

 pressure developed within the hypha. The formation of a strong hyphal 

 wall requires about two minutes under the conditions of these experi- 

 ments. The swelling of the tip shown in cell I took place in about 

 two minutes — that is, the cell wall of the hypha seemed to become strong 

 enough in that length of time to withstand the pressure within. There 

 must, then, be a rapid transformation of the fluid protoplasmic material 

 to form this wall. This transformation may be of the nature of a pre- 

 cipitation at the boundary between the hyphal sap and the potato cell 

 sap. It is possible that the precipitation of substances to form the strong 

 hyphal wall occurs only in contact with the cell sap of the potato, at least 

 occurs more rapidly in contact with the potato cell sap than in contact 

 with the cell wall. If this is true the hypha would form a tube of plastic 

 materials against the cell wall and the growth pressure of the fungus 

 filament would be applied directly to the cell wall of the potato. This 

 might be the explanation of the mechanics of cell-wall penetration by 

 the fungus. Evidence that the hyphae are sometimes cemented fast to 

 the cell wall was secured during observations of hyphae that strike the 

 cell wall obliquely, slide along it for a short distance, and then stop and 

 penetrate the wall. 



DISCUSSION OF RESULTS 



In a consideration of the results brought out in the foregoing pages it 

 is apparent that there is much evidence that the fungus makes its way 

 through the cell walls of the potato mechanically. It is, of course, 

 impossible to prove in work of this kind that some enzym is not secreted 

 at the tip of the hypha which softens or destroys the portion of the cell 

 wall with which it is directly in contact. If, then, no evidence of the 



