376 Journal of Agricultural Research voi. v, N0.9 



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 of the Skin and Flesh of 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 thjs 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 epidennis 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. i). 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 ca\nty (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 epidennis, 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, spUtting 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. 



Hypodermai^ 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). 



