4 
Journal of Agricultural Research 
Vol. XXVII, No. i 
The chemical changes which are responsible for color development in 
the apple have not, so far as the writers are aware, been determined. 
The cells in which the pigment is located are primarily subepidermal. 
There is no pigment in the cuticle, or wax covering, and no color in the 
epidermis, or outer cell layer. Below the epidermis, however, chloro¬ 
phyll pigment, and later the red pigment, is very abundant. These 
pigments are largely limited to the rather flat, thick-walled cells lying 
in the immediate 2 to 10 layers beneath the epidermis, though in certain 
varieties some pigment, both green and red, occurs well down into the 
flesh of the apple. 
As the red pigment develops it apparently replaces the chlorophyll, 
being developed in the same cells in which chlorophyll was formerly 
present. If red color does not develop, the disappearance of the chloro¬ 
phyll leaves the yellow color characteristic of ripe and mellow fruit. 
The amount of red which develops is apparently directly associated with 
the exposure of the fruit to the light. The side of the fruit exposed to 
the direct rays of the sun is always more highly colored than the side 
away from the sun. Those regions of the country which have the greatest 
amount of sunshine during the late growing season produce the most 
highly colored apples. Treatments such as cultivation, or fertilizing with 
nitrogenous materials, and which result in a more dense foliage on the trees, 
will invariably result in more poorly colored fruit. On th& other hand, 
sod culture, summer pruning, etc., which result in a more direct exposure 
of the fruit to the light, produce a more highly colored fruit. Shading 
will almost entirely prevent the development of red color. Whitehouse 
(27) has estimated color development of Fameuse and Tompkins King, 
and concludes that the greatest color development occurs during the last 
few days of the season. This would seem particularly to hold true after 
some of the leaves had fallen, thus exposing the fruit more directly to the 
sun. Since color development is largely dependent upon direct sunlight, 
picking the fruit and removing it from the sun will stop the development 
of the red pigment. 
The change of the green or ground color to yellow, which occurs in 
most blushed varieties, is entirely independent of light. This change 
occurs regardless of whether or not the fruit is separated from the tree, 
and is one of the best indexes to the actual condition of maturity of the 
fruit. Corbett ( 8 ) has discussed this color change as a means of determin¬ 
ing the time of picking the fruit. Although there is some variation in 
different varieties, practically all blushed apples should not be picked 
before they show a distinct yellowish tinge to the green of the unblushed 
side. This test, being largely independent of light exposure of the fruit, 
is very valuable as a picking aid. 
CHANGES IN THE SKIN OP APPLES DURING RIPENING 
The changes in the skin of the apple during its development on the 
tree have been studied in detail by Zschokke ( 28 ). He found the skins of 
young apples 4 to 5 weeks old to contain 2 to 10 stomata per square milli¬ 
meter. No further stomata were formed, and as the fruit increased in 
size the stomata became farther and farther dispersed. During the latter 
part of the growing season cork formation usually occurs below the stom¬ 
ata openings. The openings are later ruptured by the growth of the fruit. 
The corky tissue with the ruptured stomata constitute the “dots” by 
which many varieties are characterized. In some varieties, however, 
