40 



BITTER PIT INVESTIGATION. 



Thus the peeled pear had lost over four times as much as that of the whole pear, and the 

 pared apple had lost over five times that of the unpared. 



The skin consists of several layers of cells, an outer single colourless layer known as the 

 epidermis, and several inner layers of coloured cells containing chlorophyll, and known as hypodermis. 

 The outer walls of the epidermal cells in the mature fruit are strongly thickened, and there is a 

 continuous layer of a transparent substance of varying thickness overlying all, which is known as 

 the cuticle (Figs. 106 and 107). In some varieties there is a surface covering of wax, which is finely 

 granular and dust-like, easily rubbed off, the so-called " bloom." Inside the skin there are the cells 

 of the " flesh," or pulp cells, which at first are somewhat similar to the cells of the hypodermal 

 layer, and then become large and bladder-like. 



This is the structure found in the mature fruit, but if the young fruit is examined, the cells of 

 the epidermis are seen to be much smaller, the outer thickening is not yet strongly developed, and there 

 is a dense layer of hairs which is subsequently dropped before ripening (except in the case of the 

 quince). There are also openings in the skin to admit air, and to allow the escape of gases, generally 

 of a different nature to those in the fully-formed fruit. 



As the young fruit grows, there will necessarily be an enlargement of the surface, and the 

 epidermis must keep pace with it. How this is accomplished will now be shown. There is a 

 tangential growth of the epidermal cells, so that there is sufficient stretching to keep up the continuity 

 of the skin. At the same time the shape of the cells undergoes a change, and they become plate-like, 

 while they soon divide into two or more daughter cells, forming the so-called " window-cells " 

 (Fig. 105). But, if growth is very rapid, and the daughter-cells cannot stretch fast enough, each 

 daughter-cell may in turn divide like the parent, and there may be not only primary and secondary, 

 but tertiary and even quaternary subdivision. In this way arises the mosaic seen on a surface 

 view, and, as division and subdivision go on, the cell-walls become relatively thinner and thinner, 

 so that the amount of subdivision taking place can be determined thereby. It can readily be 

 understood that when after a dry spell, for instance, there comes a rush of sap, so that the growth 

 of the flesh is so rapid that the skin cannot keep up with it, then something must give way, and 

 small rents are produced, as well as a disruption of the stomata, to be replaced by minute openings, 

 which, in their function and mode of origin, correspond to the lenticels. 



The cuticle varies in thickness in different varieties, and generally it is thinner in early than 

 in late varieties ; but no general rule can be laid down. The width of the cuticle is sometimes 

 given as characteristic of certain varieties, but, since fruits taken from the same tree on the shady 

 and sunny side respectively show greater differences than that between different sorts, it is not a 

 reliable test. The bloom or wax on the outside will prevent the adhesion of water, and thus tend 

 to render the fruit less liable to the attacks of fungi, 



STOMATA AND LENTICELS. 



^ If the skin of a young apple is examined microscopically, it is found to be invariably studded 

 with numerous breathing pores or stomata (Fig. 101), and even in full-grown apples they sometimes 

 persist. They are more numerous in smooth-skinned than in rough-skinned varieties, and their 

 distribution is not always quite symmetrical in the ripe fruit. In apples they are more crowded 

 towards the crown end, and in pears towards the stalk end, the greater relative growth of the lower 

 portion of the apple and the swollen upper portion of the pear having caused a greater amount of 

 stretching, and consequently the stomata are further apart. 



When, later in the season, the thickened outer membrane of the epidermis is subjected to 

 considerable surface tension on account of the rapid growth, rupture occurs, and it is generally in 

 the direction of the stomata. In this way the stomata are replaced by star-shaped points studding 

 the surface, characteristic of several varieties. They are often recognised by the naked eye as white 



