288 



Journal of Agricultural Research voi. xviii. No. s 



Table X. — Pressure in grams per square centimeter required to puncture tissue in slices 

 oj potatoes freshly cut and after drying for j hours at J0° C. 



GREEN MOUNTAIN 



BL,ISS TRIUMPH 



Average. 



42'52S.3 

 47,715 

 41,018. I 

 38,448-3 

 35.577 



41.057.3 



76,176.8 

 79.315-9 

 64,456 

 57.132-6 

 60,690. I 



67.554-2 



26, 496. 9 



29, 994. 8 

 27,624.4 

 22,601. 9 

 33,065. 6 

 26,368.8 



27,931- I 



48, 133- 6 

 46, 250. I 

 78,068 

 49,644.2 

 57,715 



55.962. I 



28,031. o 



From the results shown in Table X it is evident that the resistance of 

 the wounded surface of the potato to puncture is appreciably increased 

 in every instance by exposure to the air for 3 hours. In the five Green 

 Mountain potatoes the average increase in pressure required to puncture 

 the cortical tissue was 79 per cent, and the average increase for the 

 central tissue was 95 per cent. With the same number of Bliss Triumph 

 tubers the results were 64 per cent and 100 per cent for the cortex and 

 central portions, respectively. 



There is then correlated with the resistance to infection shown by 

 wounds after 3 hours' drying a very marked resistance to mechanical 

 puncture. If the fungus penetrates the tissue mechanically, it is quite 

 possible this increase in resistance, due to drying, would be sufficient 

 to prevent its entrance. It is noticeable that the pressure required to 

 puncture the dried cortex, the region in which the inoculations were 

 made in these experiments, most closely approaches the averages for 

 the inner portion of McCormick tubers which did not become infected. 



Another point which is of interest in this connection is the fact that 

 no cases of natural infection through the potato skin have been observed, 

 and repeated attempts in the laboratory to inoculate tubers on the 

 surface have yielded negative results. Correlated with this resistance to 

 infection is a very marked resistance to mechanical puncture. It was 

 exceedingly difficult to puncture the skin of the potato with the round- 



