382 
Journal of Agricultural Research 
Vol. 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 OF STOMATA AND LENTICEES 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 lining the cavity 
(PI. XXXVII, fig. 1) 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 X Wi 5 and A X 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 (Pl. 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, 
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 through those in which 
only the corky tissue was present (PI. XXXVIII, fig. 1, 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. 
