Nov. 29. 191S Varietal Resistance of Plums to Brown-Rot 383 



PHYSIOLOGICAIy RELATION OF 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, 191 2). 



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. i, 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 m^onths 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 

 sHghtly in advance of its penetration through the tissue (PI. XXXIX, fig. 

 I. 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 aU of the 

 slides examined. 



