428 Brooks: The Fruit Spot of apples 



The structure and development of the apple 



Epidermal structure. — The epidermis of the apple consists of 

 a single layer of cells, the outer walls of which are strongly thick- 

 ened. These outer walls consist largely of cutin, which as the 

 apple matures is impregnated and covered with a resinous and 

 waxy substance. The apple is thus furnished with a covering that at 

 most points is practically impervious to water and fungi. Many uni- 

 cellular hairs are found on the surface of the young fruit. The some- 

 what conical bases of these are inserted between the other epidermal 

 cells. These hairs disappear when the apple is a few weeks old, 

 but before this time the cuticle of the apple has considerable thick- 

 ness. Zschokke (6) found that in dropping from the apple the 

 hairs might break even with the outer edge of the cuticle, leaving 

 their conical bases behind, but that more frequently they loosened 

 themselves from the surrounding epidermal cells, leaving a deep 

 scar, which, in the later development of the fruit, might become a 

 definite break in the epidermis. The writer's observations agree 

 with those of Zschokke. As late as the first of August these hair 

 pits were quite common in the epidermis. 



In the young fruit the stomata furnish such openings in the 

 epidermis as are necessary for the passage of gases and liquids. 

 They are very numerous early in the year and while the majority 

 of them disappear later they are still quite common on the mature 

 fruit. From three to four weeks after the fall of the blossoms 

 numerous cork-like flecks begin to appear on the apple surface. 

 The majority of these are lenticels (plate 32, figure 4). When 

 the epidermis of the young apple is broken, either through a nat- 

 ural or foreign agency, the opening is rapidly covered with cor 

 cells. According to Zschokke (6) the lenticels develop as a result 

 of breaks in the epidermis caused by the rapid enlargement of the 

 apple, the stomata being the points that yield most readily to the 

 strain. In a typical lenticel the cells are arranged in definite 

 layers. In the apple the structure of these corky specks varies a 

 the way from that of a typical lenticel to a few thick-walled c<* 

 cells promiscuously arranged beneath a minute break in the cutic e - 

 Their early structure leads the writer to the opinion that while t e 

 majority of them develop at stomata others probably originate from 

 the pockets left by the falling hairs. 



