290 



Journal of Agricultural Research 



Vol. XVIII, No. s 



One consideration detracts from the value of this indirect method for 

 the determination of the pressure the fungus filament is able to exert 

 against the cell walls. In the osmotic pressure determinations, an 

 attempt was made to determine the total pressures within the filaments 

 of the fungus, and these may or may not be the pressure the fungus is able 

 to exert against the cell wall of its host plant. The cell walls of the fungus 

 filament are apparently able under ordinary conditions to withstand 

 the pressure within the filament, except at the growing points. The 

 pressure exerted on the cell wall of the potato under the most ideal 

 conditions would be the pressure that the contents of the filaments 



Fig. I. — Drawing to illustrate growth of a Pythium hypha in potato tissue. Note the constriction of 

 the hypha where it penetrates the wall. 



were capable of exerting minus the pressure necessary to push out the 

 wall or rudimentary wall of the tip of the fungus filament. 



Further evidence on the method by which the fungus penetrated the 

 cells of the potato was furnished by direct observations of the hyphae 

 of the fungus within the tissue of the potato. In these experiments 

 sections of raw potato were prepared as nearly sterile as possible and 

 inoculated with the fungus. When kept overnight in hanging drop 

 cells at 30° C. a good growth of hyphae was usually obtained. Numerous 

 instances of cell-wall penetration were observed, and the method of 

 penetration was followed both by serial drawings and by motion photo- 

 micrographs (Pi. 36, 37). The part of the section selected for observa- 



