Dec. 1, 1919 Parasitism of Pythium deharyanum on the Potato Tuber 279 



Table II. — Average rate of growth of Pythium deharyanum in tissue of Bliss Triumph^ 

 Green Mountain, and McCormick potatoes 



It is noticeable in Table II that the rate of growth of the fungus in 

 Bliss Triumph and Green Mountain tubers is from three to four times as 

 rapid as in McCormick under the conditions of the experiment. In 

 some cases the fungus was apparently unable to affect the cylinders 

 from the McCormick and had not, at the end of the experiment, pene- 

 trated into . the first millimeter of tissue. Other cylinders from this 

 variety were much more susceptible, however, so that the average rate 

 of growth of the fungus, as shown in the table, is fairly high. 



In order to relate this rate of growth to the number of cells traversed, 

 measurements of the cells in the cortex and central portions of tubers 

 of these three varieties were made. About 1,500 measurements were 

 made with each variety. The averages for the cortex and central por- 

 tions of the three varieties are given below: 



Table III. — Average size of cells in the three varieties of potatoes from i ,500 measurements 



on each variety 



Bliss Triumph. 



Green Mountain. 



McCormick. 



Cortex 



Central portion. 



269M X 303M 

 318M 



294/x X 311M 

 318M 



269M X 303M 

 347M 



If it is considered, then, that the same rate of growth holds for the 

 cortex and interior of the tuber, the average length of time required for 

 the fungus to pass through an average cell in the interior would be 43, 

 50, and 204 minutes, respectively, for the three varieties, Bliss Triumph, 

 Green Mountain,^ and McCormick. The fact that the cells are so nearly 

 of the same size in the three varieties would eliminate the possibility that 

 the relatively slow rate of growth of the fungus in the McCormick tubers 

 was due to the small size of the cells and the consequently larger number 



' In a former paper (//) one of the writers gives the size of the cell in a Green Mountain tuber as 13S.7M, 

 which is erroneous. The tuber mentioned was of the Burbank variety. The size of the cells and rate of 

 growth givea in the present paper are correct for this variety. 



