Dec. 1, 1919 Parasitism of Pythium deharyanum on the Potato Tuber 283 



tional to the area of the cross section of the needles. The determinations 

 made with different needles are thus comparable. 



In the experiments, the results of which are shown in Tables V to VII, 

 inclusive, the potatoes were inoculated first in the cortex. This was 

 done by removing a piece of the potato skin about i mm. in thickness 

 and 5 to 10 mm. in diameter. A Van Tieghm cell was cemented to the 

 surface of the potato around this wound with vaseline, and a drop or two 

 of sterilized distilled water was placed within the ring and inoculated 

 with mycelium of the fungus. The top of the cell was then closed 

 with a cover glass and vaseline. The inoculated tubers were then 

 placed in an incubator maintained at 30° C. and examined daily. If 

 the tubers became infected, they were removed before they were more 

 than half rotted. They were sliced through the point of inoculation, 

 the distance the rot had progressed was measured, and the weight 

 necessary to puncture the tissue in the two different regions of the 

 sound portions of the tubers was determined. If the inoculation was 

 unsuccessful, a second inoculation was made in the cortex in the same 

 way as the first ; and if this inoculation did not result in an infection the 

 potato was inoculated beneath the cortex in the central part. If the 

 results from this third inoculation were negative, as they usually were 

 with McCormick potatoes, the tuber was considered to be immune, and 

 the weight necessary to puncture the tissue was determined as described 

 above. It is worthy of note that Bliss Triumph and Green Mountain 

 potatoes usually rotted as a result of the first inoculation. 



The tables in which the results of these experiments are given show 

 the diameter of the needle in microns and the pressure required in grams 

 per square centimeter to penetrate the tissue of the freshly cut potato 

 in the cortex and central portion, respectively. In every case the 

 numbers given as the pressure required to penetrate the tissue are 

 averages of 10 determinations. The same needle was always used for 

 both the cortex and the central portion. The number of inoculations made 

 on each tuber, the region in which they were made, and the result after 

 the length of time indicated are also given. In Table VII, under "results 

 of inoculation," the term "slight rot" appears. This was used to char- 

 acterize the results of inoculations when there was a browning and slight 

 softening of the tissue immediately around the point of inoculation which 

 seemed to indicate that infection had occurred. The rot had not pro- 

 gressed a measureable distance, however, and the tuber thus affected 

 was apparently practically immune. 



