Nov. 29, 1915 Varietal Resistance of Plums to Brown-Rot 2>11 



Lying below the hypodermal layers of cells and in sharp contrast 

 to them are the large, isodiametric cells which make up the mass of the 

 fruit tissue (PI. XXXVII, fig. 6). In the ripening process in those varie- 

 ties which become soft these cells split apart at the middle lamella 

 (PI. XXXVII, fig. 5). The solution of the middle lamella apparently 

 takes place more readily in these cells than in those of the hypodermal 

 layers. 



METHOD OF ENTRANCE OF THE FUNGUS 



Two methods were used in the determination of the details of the 

 entrance of the fungus. The first consisted of macroscopic observations 

 on ripe or nearly ripe fruit shortly after infection had taken place. 

 In the second method fruits of a number of varieties of plums at various 

 stages of development Avere brought into the laboratory and inoculated, 

 in some cases by a suspension of spores in water and in others by laying 

 the plums in contact with moist mummies well covered with spores. 

 After infection had taken place and small decayed spots had appeared, 

 blocks of the flesh, including these spots, were killed and embedded in 

 paraffin, according to the usual methods employed. These were later 

 sectioned, mounted, and stained. Sections 8 to ii)u thick were found 

 most satisfactory. Various stains were used, including Harper's short 

 modification of the triple stain, Heidenhain's iron-alum-hematoxylin, 

 and also a modification of this in which safranin was used. This last- 

 named stain proved very satisfactory. 



It was noticed continually, particularly in ripe or nearly ripe fruit, 

 that when infection took place through the uninjured skin, the spot always 

 had in its center a lenticel or "dot." These observations indicated that 

 infection takes place, not through the cuticle, but through the lenticel in 

 ripe or nearly ripe fruit. Further evidence was obtained on this point 

 when sections were made of the skin from material in which the lenticels 

 were either forming or completely formed and through which infection 

 had taken place. It was found that the hyphge entered between the 

 guard cells into the stomatal cavity (PI. XXXVIII, fig. 3, 4, and 5). In 

 those stomata lined with corky material infection of the fruit tissue does 

 not take place immediately, as the fungus apparently has not the power 

 to pierce directly through the corky cells. The hyphae continue to grow, 

 filling up the stomatal cavity, and eventually exert enough pressure to 

 split away the epidermis from the lenticel cells (PI. XXXVIII, fig. 5). 

 It is through this opening that infection takes place into the fruit tissue 

 (PI. XXXVIII, fig. I and 2). 



In the young plums, before corky material has been formed, the germ 

 tubes also enter through the stomata. After entering they come in contact 

 with normal fruit tissue, and direct infection takes place (PI. XXXVIII, 

 fig. 4). In all, 44 instances of infection through stomata or lenticels were 

 noted, and although the surface of both ripe and green plums was often 



