Apr. i, 1921 
Leafroll , Net-Necrosis , and Spindling-Sprout 
73 
frost-necrosis of the net (PI. 4, B), ring, and blotch types. Neither 
of the last two types of injury has been seen in connection with the 
leafroll type of net-necrosis, which has developed in connection with 
leafroll tuber transmission under normal conditions of growing, harvest¬ 
ing, and storing, while at the same time thousands of other tubers, 
healthy in regard to leafroll, were in the same conditions but remained 
free from net-necrosis. Moreover, a number of lots of tubers have been 
selected from stocks grown at Aroostook Farm and Highmoor Farm for 
the study of net-necrosis or leafroll and have been kept in well-heated 
buildings, so that it was known that there was no chance for chilling after 
harvesting. These are described in Table XII. Notes upon the first 
seven sublots were taken by either Dr. W. J. Morse or Mr. G. B. Ramsey. 
The sources of many are described in Table VI. 
It is seen from Table XII that net-necrosis may develop without any 
exposure to chilling temperatures. Furthermore, a certain test indi¬ 
cates that there is no relation between higher temperatures of storage 
and the prevalence of net-necrosis. This test consisted of dividing the 
tubers from lot 1919-125 of Table XII into eight parts so that the range 
and average of the tuber weight were as nearly alike as possible for the 
various sublots. Four sublots were kept in constant temperature 
chambers at 30°, 25 0 , 15 0 , and 8° C., respectively. The other four were 
kept where the temperature fluctuated daily, the daily means averaging 
25 0 , 21 0 , 15 0 , and io°. This was done for about two months, from 
November 1 to January 7. At the end of this period examination of 
the tubers disclosed no differences between the various sublots in regard 
to net-necrosis that were correlated with differences in temperature. 
PHYSIOLOGICAL CHANGES 
It may be noted that three lots considered in Table XII were examined 
at the time of harvesting and at bimonthly intervals afterwards. The 
percentage of net-necrosis increased markedly up to about the middle 
of February. Cook (6, p. 26) reports that this discoloration may be 
absent at the time of digging but may develop in storage. Apparently 
changes that occur during the dormant period of the tubers may be 
involved with the development of the discoloration. 
During preliminary examination of net-necrosis tubers in comparison 
with healthy tubers, it was found that blackening appeared more quickly 
in cut or crushed tissues of the former tubers than in those of the latter 
and appeared more quickly than is possible for bacterial growth. The 
blackening substance was present in uninjured tissues only in the phloem 
of those with net-necrosis. It affected protoplasm and protein granules 
but not starch grains. It did not pass through the walls of vessels in 
the xylem of healthy tuber segments which were allowed to absorb some 
of the discolored juice. It developed most rapidly in crushed tissues 
exposed to the air and was characterized by being associated with acid 
