830 
Journal of Agricultural Research voi. xxvn, no. n 
Daber, which is resistant to bacterial wilt, new cork cells appeared 6 
hours after injury, while in the thin-skinned and susceptible variety 
Apollo, new division walls in the periderm formed only after 36-48 hours 
had elapsed. Lutman (8) emphasizes the fact that it is almost impossi¬ 
ble to eliminate the personal equation in determining the thickness of 
the periderm, since on the same tuber thin and thick areas are commonly 
found. He noticed that application of fertilizers does not affect the 
thickness of the skin and that varieties resistant to scab have a thick 
periderm associated with a type of lenticel which is close textured and 
partly buried under the skin. 
In the present study of the structure of the periderm similar variability 
which could not be correlated with the variety or the external conditions of 
growth was encountered. This must be borne in mind in connection 
with the following descriptions of the periderm, as it appeared to be most 
typical in the varieties studied. 
structure of the periderm in potato varieties grouped 
ACCORDING TO STUART ( ig) 
Cobbler. —Periderm normally thick with somewhat rough or scaly 
surface. Layer measures 130-156 microns and is composed of 8-9 rows 
of cells. Variation in thickness is not infrequent, as can be seen in Plate 
10, A, B, and Figure 7. The cells vary greatly in size and arrangement. 
The radial rows are interlocked and the cells narrow, which gives the 
periderm a compact texture. The average size of most cells is 78 by 13.5 
microns. The transition zone is rather wide and gradual (4-5 rows). 
The cells are elongated, rectangular, or pointed. 
Triumph. —Periderm of medium thickness, covered by a thin, rough 
crust. Layer measures 130 microns and is composed of 6-7 rows of cells. 
Occasionally as many as 10 rows can be counted, the total width of the 
layer reaching 156 microns or even more. The periderm cells show a 
fairly regular arrangement in radial rows; the width of the rows, however, 
varies considerably. The average cell dimensions are 82 by 13-15.6 
microns. The transition zone is wide and the change in cell type is 
gradual. All transition cells are usually narrow and elongated, but near 
the cortex they become oblong or even cubical. (PI. 10 and fig. 8.) 
Michigan. —Periderm of medium thickness; surface usually smooth, 
but in Early Eureka rough and scaly. Thickness of periderm layer 130 
microns; average number of rows about 9. The cells are rather uniform 
and show a distinct arrangement in unbroken radial rows. The average 
length of the cells is 85 microns. In Early Eureka the cells are wider, 
measuring on the average 104 microns. The transition to the cortex is 
gradual. (PI. 10, D, and fig. 7.) 
Rose. —Periderm of medium thickness and smooth. Layer measures 
135 microns and is composed of 6-8 rows of cells. The cells are arranged 
in even radial rows. The width of the rows varies. There are commonly 
two types of cells—wide ones, measuring 82 by 13-15.6 microns, and very 
narrow ones. There is only a narrow transition zone, but transition is 
nevertheless gradual because the transition cells greatly resemble the 
periderm cells. (PI. 16, E, F, and fig. 7.) 
Early Ohio. —Periderm of medium thickness and covered with a thin 
crust. Layer measures 117-130 microns and is composed of 6-8 rows. 
The cells are arranged in even radial rows of elongated, mostly narrow 
cells. The two types of cells measure 78 by 15.6 and 38 by 14.8-15.6 
microns, respectively. There is a broad transition zone; transition itself 
