814 
Journal of Agricultural Research voi. xxvn, no. h 
to the cells of the pith, however, the cortex shows a greater degree of 
specialization. Some of the cells, especially in the region of the eyes, 
have greatly thickened walls, forming typical stone cells or sclereids. 
Not all varieties develop these stone cells. The peripheral cortical cells 
either abut abruptly on the periderm or form a transition zone three to 
eight cells wide. The cells in this transition region are smaller than the 
typical cortical cells, more elongated, and nearly rectangular. They are 
almost devoid of starch, but contain proteins, especially in the form of 
cubical crystals, and, in the case of colored varieties, also the pigment. 
The epidermis of the tuber primordium has only a temporary existence. 
Anticlinal walls early appear in certain of the epidermal cells; later peri- 
clinal walls are also added. Simultaneously with the division of the cells 
of the epidermis, division walls also appear in the subepidermal layer. 
Soon a regular cork cambium becomes distinct, and cell division in this 
layer continues until a layer of tissue from 6 to 15 cells wide is produced, 
which assumes the protectivedunction of the epidermis. The cork cam¬ 
bium, or phellogen, consists of a single layer of cells which divide tangen¬ 
tially and which constitute the inner row of daughter cells produced by 
the first division of the cells of the hypodermal layer. While most of 
the periderm arises from the phellogen derived from the hypodermis, the 
young tubers contain in addition a superficial periderm derived from the 
epidermis. 
In the young tuber primordium the first periderm cells naturally appear 
in close proximity to the stem end, but soon the periderm extends over 
the entire tuber, and by the time the latter has reached only the size of 
a pea the characteristic number of rows of periderm cells has developed. 
The cork cambium remains active throughout the season, forming new 
cells to replace those which are sloughed off at the surface as the tuber 
expands. Loss and gain are thereby balanced, and the tuber shows 
through the various stages of its existence the normal thickness of skin, 
unless seasonal changes constitute a factor which changes the normal 
development. 
The individual periderm cell is approximately brick shaped; its walls 
remain thin and later become suberized. The cells abut on each other, 
almost invariably without intercellular spaces. Small simple perfora¬ 
tions are rarely observed (PI. io, K). The mature periderm cell is de¬ 
void of content. The younger cells contain small granules of tannin 
and the remains of the nucleus. Occasionally starch grains are found, 
but the latter are small, round, and do not show the striation charac¬ 
teristic of the large grains. In colored varieties the periderm cells con¬ 
tain the pigment, which, as has been previously noted, is also found in 
the peripheral cortical cells. 
Simultaneously with the formation of the periderm the cells under¬ 
neath the stomates, which are found widely scattered over the surface 
of the tuber primordium, begin to divide actively, producing a mass of 
loose tissue of roundish cells. Under favorable conditions these cells 
break through the epidermis and proliferate, becoming visible even to 
the naked eye as small white dots. Since the periderm is quite imper¬ 
vious to gases, this new structure—the lenticel (PI. 8, E) —performs the 
function of aeration. This is facilitated by the shape of the cells and 
by the course of the intercellular spaces which run in radial direction. 
Under certain abnormal environmental conditions it may happen that 
the periderm departs from the normal development. Such a condition 
was observed in Bliss Triumph potatoes grown in Ithaca, N. Y. The 
