Nov. 39.1915 
Varietal Resistance of Plums to Brown-Rot 
383 
PHYSIOLOGICAL RELATION OP FUNGUS TO HOST 
That resistance is not entirely due to the partial inability of the fungus 
to gain entrance to the tissues of the resistance forms is shown by the 
difference in rapidity of rotting after infection has taken place. A 
study of the further penetration of the fungus in the resistant and sus¬ 
ceptible forms was therefore undertaken. 
Previous investigators do not agree as to the manner in which the fun¬ 
gus penetrates the host tissues, some holding that it penetrates the cell 
walls wherever it comes in contact with them and that it shows no par¬ 
ticular affinity for the middle lamella (Cooley, 1914), while others hold 
that the fungus follows the middle lamella and may or may not split it 
completely (Schellenberg, 1908; Bruschi, 1912). 
The method used in the present study of the relation between the host 
and the fungus cells was the same as that used in the determination of 
the method of infection—i. e., a study of prepared slides of infected plum 
and apple tissue. The stains already mentioned were used. The mate¬ 
rial consisted of small blocks of plum and apple tissue cut from the edge 
of the rotting spots and also blocks cut from plums which had been in¬ 
fected within 12 to 30 hours of the time of killing. For this study of the 
penetration of the fungus, over 220 slides were prepared from material 
collected from 17 varieties of plums and 4 varieties of apples. In 80 of 
these slides the fungus hyphae were clearly differentiated from the host 
tissue. 
PENETRATION 
In all cases the fungus shows a very strong affinity for the middle lamella 
(PI. XXXVIII, fig. 2, and XXXIX, fig. 1, 2, 5, and 6). No instances 
were found where the hyphae had actually pierced the cell walls and en¬ 
tered the cell cavity, so that it seems certain that the hyphae of S. cinerea 
are unable to penetrate the cell walls of the plum and apple fruits. No 
record has come to notice of other investigators having extracted from 
the brown-rot fungus a cellulose-splitting enzym which has the power of 
dissolving the plum cell walls. Furthermore, that such an enzym is not 
produced by the fungus in the host tissues is clearly demonstrated by 
the fact that in completely rotted plum tissue (PI. XXXIX, fig. 5) and in 
sclerotia which have been buried in the ground for over 18 months and 
have produced apothecia, the cell walls are still intact. 
From the appearance of the infected tissue it is evident that the fun¬ 
gous hyphae secrete a substance which splits out the middle lamella 
slightly in advance of its penetration through the tissue (PI. X 1 XXIX, fig. 
1, 2, 3, 5, and 6). Eventually the middle lamella is completely dissolved, 
leaving the cells in the rotted area entirely free from one another. In¬ 
stances comparable to those illustrated were found in nearly all of the 
slides examined. 
