Nov. 29,1915 
Varietal Resistance of Plums to Brown-Rot 
387 
RESISTANT AND SUSCEPTIBLE VARIETIES 
The fungus hyphae of 5 . cinerea in both resistant and susceptible fruits 
show practically no constant differences. In both cases they are large 
and densely protoplasmic over their entire length. In a few instances 
hyphae in resistant forms appeared more knotted and irregular than in 
susceptible ones, but this could be explained in those cases by mechanical 
pressure of the small cells of the hypodermal layer, which in the resistant 
plums appear to be less easily collapsed than in the susceptible varieties. 
Considerable difference, however, could be noticed in the rapidity with 
which the hyphae developed in the two forms. The hyphae in the 
susceptible varieties usually completely filled the intercellular spaces as 
the rot spread, while in the resistant ones fewer hyphae were produced. 
A few instances were noticed in resistant varieties of cells lying com¬ 
pletely or nearly completely surrounded by hyphae from which the 
middle lamella had not been dissolved. This and the fact that in these 
forms the middle lamella seldom appeared to be dissolved out far ahead 
of the penetration of the fungus lead to the conclusion that this partial 
resistance is due to the inability of the toxic material secreted to dis¬ 
solve the middle lamella as rapidly in the resistant as in the more 
susceptible varieties, owing possibly to very slight differences in its 
composition. 
That there is an actual difference in the composition of the middle 
lamella material seems fairly certain. It is well recognized that varieties 
of plums, apples, and other fruits and vegetables vary greatly in the time 
required for cooking. Some remain firm after a long period of boiling, 
while others soften and become mushy after very short heating. An 
examination of boiled-apple tissue which had become soft revealed the 
interesting fact that the softening was due in part to a separation of the 
cells as a result of the middle lamella having been dissolved. The cell 
walls appeared not to be ruptured at all. In those varities which do not 
become soft on boiling it is assumed that the middle lamella material is 
less soluble and therefore is probably of a slightly different chemical com¬ 
position. It is recognized, of course, that the dissolving action of the 
fungus upon the pectic substances and solution by hot water are entirely 
different processes and, therefore, resistance to the fungus and firmness 
after cooking may or may not be correlated. 
In view of the fact that eventually in both resistant and susceptible 
forms the middle lamella is completely dissolved, the difference in sporu- 
lation (PI. XXXVIII, fig. 6, 7, 8, and 9), as described above, could hardly 
be explained by variations in middle lamella composition, but rather points 
to a small amount of some toxic substance being produced either by the 
host cells or fungus hyphae, which is not enough to completely stop the 
growth of the fungus, but merely to retard slightly its normal functioning. 
