Nov. 29. 1915 Varietal Resistance of Plums to Brown-Rot 387 



RESISTANT AND SUSCEPTIBLE VARIETIES 



The fungus hyphae of S. cinerca 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 

 hyphge 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 hyphse 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 boiUng, 

 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 hyphse, which is not enough to completely stop the 

 growth of the fungus, but merely to retard slightly its normal functioning. 



