376 
Journal of Agricultural Research 
Vol. V, No. 9 
place. Is the difference in susceptibility to infection due to differences 
in morphological characters of the epidermis? 
It has been definitely proved from time to time that the fungus has 
the ability to “bore” through the uninjured skin of plums and peaches. 
Therefore, penetration must take place either through the rather thick 
cuticle of the epidermal layer or through the stomata. 
Morphology op the Skin and Flesh op the Plum 
For a better understanding of the entrance and penetration of the 
fungus in the plum fruit, a knowledge of the morphology of the “skin” 
and underlying layers of cells is necessary. 
Stomata. —The epidermis of the plum consists of a single layer of 
cells covered by a rather thick layer of a cutinized substance (PI. 
XXXVIII, fig. 2). On the surface of this is secreted a waxy “bloom.” 
Stomata are present in the young fruit. In fruit about half grown 
changes take place in the stomata leading to the formation of lenticels. 
The lenticels are formed in at least three ways: 
(A) In some cases a few flat disk-shaped cells are formed parallel to 
the epidermis and lining the stomatal cavity. The walls of these 
cells appear to be of the same material as those of the deeper lying 
parenchyma cells (PI. XXXVII, fig. 1). The guard cells often open wide 
and dry out. In other cases changes take place in the composition of the 
walls of about two layers of cells lining the stomatal cavity. These cells, 
the walls of which were originally cellulose, give the characteristic yellow 
staining reaction of cork with the iron-alum-hematoxylin safranin 
stain (PI. XXXVII, fig. 3). 
(B) In some varieties meristematic tissue develops from the paren¬ 
chyma cells and produces tissue which partially (PI. XXXVII, fig. 2) 
or completely fills the stomatal cavity (PI. XXXVII, fig. 4). Occasionally 
a column of cells even grows out through the stomatal opening. These 
cells appear to be of the same nature as the hypodermal cells underlying 
the epidermis, in no case giving the staining reaction of cork. 
(C) The lenticels, which appear as large, corky specks on the surface 
of ripe plums, are made of a pad of corky cells lying parallel to the 
epidermis. They probably develop at the stomata, splitting the guard 
cells apart and growing out through the opening. The details of their 
formation, however, have not been carefully studied in this connection, 
as only very few were encountered in the material examined. 
Hypodermae Parenchyma. —Directly underlying the epidermis are 
layers of oblong cells slightly larger than and lying parallel to the epi¬ 
dermal layer. These make up what is commonly known as the “skin” 
of the plum. In some of the thick-skinned varieties there are often 
as many as seven or eight layers of these cells (PI. XXXVIII, fig. 5), while 
in the thin-skinned forms often not more than one or two layers are 
present (PI. XXXVII, fig. 1, 2, and 5). 
