Dec. 1, 1919 Parasitism of Pythium deharyanum on the Potato Tuber 291 



tion was usually two or three cells thick, since if the section is much 

 thinner the hyphae are liable to grow over the surface. 



The hypha shown in figure i was watched continuously for three 

 hours. During this time it grew 1,976 ju at a room temperature of 

 about 70° F. The time required to penetrate the wall was about 5 

 minutes. The distance traversed and the time required for each cell 

 were as follows: 



The hypha in cell I, 

 The hypha in cell II, 

 The hypha in cell III, 

 The hypha in cell IV, 

 The hypha in cell V, 

 The hypha in cell VI, 



traversed 200.3 niicrons in 25 minutes, 

 traversed 493.9 microns in 35 minutes, 

 traversed 186.9 microns in 20 minutes, 

 traversed 387.2 microns in 20 minutes, 

 traversed 333.7 microns in 20 minutes, 

 traversed 373.8 microns in 30 minutes. 



Fig. 2. — Drawing to illustrate method of cell-wall penetration in cells I, IV, and V. For explanation 



see text. 



The drawings of figure 2, made at the time of observation to show 

 the relative positions assumed, give the characteristic methods of cell- 

 wall penetration as observed in this study. In passing through the 

 cell wall between cells I and II the hypha approached the cell wall 

 nearly at right angles; it formed a swelling at the end, bent slightly, 

 and penetrated the wall by a small tube. After passing through the cell 

 wall into the next cell the hypha expanded to its usual diameter. Con- 

 siderably more bending of the hypha is shown in cells IV and V. It 

 is noticeable that the wall in cell V bends outward under the pressure 

 of the hypha and that the hypha straightens after the wall is penetrated. 



