382 Journal of Agricultural Research voi.v.No.9 



the spores on this freshly cut surface of the plum in a drop of water. The 

 plums were kept in a moist chamber. The same relative differences in 

 rapidity of rotting were noted in these cases as when the infection took 

 place through the uninjured skin, indicating that mere thickness of skin is 

 not the deciding factor in resistance, as the cells underlying the skin show 

 the same relative resisting powers. 



However, it will be seen by referring to Table IV that the varieties 

 which are the most susceptible are the thin-skinned, tender-fleshed ones, 

 while the more resistant varieties are thick-skinned and of a firmer, 

 tougher texture. An examination of prepared slides of the skin of the 

 different varieties confirms these observations, in that all of the very sus- 

 ceptible varieties have a thin skin (PI. XXXVII, fig. 4), consisting of one 

 or two layers of cells besides the epidermis; while the resistant varieties 

 all have a very thick skin (PI. XXXVIII, fig. 4), consisting of from five to 

 eight layers of cells. The varieties appearing to be intermediate in 

 resistance have skins varying in thickness, but in all cases examined they 

 are thicker than the susceptible forms. It would seem, then, that there 

 is a rather close correlation between skin thickness and resistance to the 

 brown-rot fungus. 



RELATION OP STOMATA AND LENTICELS TO RESISTANCE 



In studying the method of infection, a comparison of the stomata and 

 lenticels of the different varieties revealed some interesting and important 

 facts relating to resistance. The lenticels described above, in which no 

 change other than the production of a few flat cells fining the cavity 

 (PI. XXXVII, fig. i) took place, were found only in the thin-skinned 

 varieties, as Gold and some of the sand-cherry hybrids. Those in which 

 the lining cells became corky (PI. XXXVII, fig. 3) were found in the 

 thicker skinned varieties. 



In two of the most resistant varieties, B XW15 and Ax W9, the forma- 

 tion of lenticels, due to filling of the stomatal cavity with parenchyma 

 cells, was very common (PI. XXXVII, fig. 4). This condition was not 

 entirely confined to these varieties, as instances were found in many others 

 of the thick-skinned varieties and also in such a thin-skinned variety as 

 Gold (PI. XXXVII, fig. 2 and 4), where, however, only a few cells were 

 formed that did not in any case completely fill the cavity (PI. XXXVII, 

 fig. 2). 



That the complete plugging of the stomata is a factor in resistance is 

 shown by the fact that many instances were noticed in which these stom- 

 ata were completely covered by germinating spores, with no resulting infec- 

 tion. It did take place, however, through stomata the cavities of which 

 were only partially filled with these cells and also thrpugh those in which 

 only the corky tissue was present (PI. XXXVIII, fig. i, 2, and 5). This 

 may explain why it was possible to obtain so few infections in A X W9 and 

 B X W15, even when their surfaces were covered with germinating spores. 



