Mar. is, 1924 
Studies on the Potato Tuber 
833 
distinct arrangement in radial rows. The large cells measure 104 by 
14.8 microns, the narrow type 52 by 14.8 microns. Transition is gradual, 
but the transition cells resemble those of the cortex rather than the peri¬ 
derm (PI. 10, K, L, and fig. 7). 
Rural. —Periderm thin or of medium thickness, usually more exten¬ 
sive in the Russet type. The surface is smooth or, in Russet Rural, 
covered with a thick, scaly crust. Layer measures 130 microns, num¬ 
ber of rows about seven. Though sometimes uniform, the periderm is 
more conspicuous by irregularity of the arrangement of the rows and of 
the individual cells. The most common type of cell measures 91 by 
18.2 microns, but cells as wide as 156 microns are not uncommon. 
The transition to the cortex is narrow and abrupt. The transition cells 
are rectangular, but sometimes elongated and narrow, thus more closely 
resembling those of the periderm (PI. 10, M, N, and fig. 8). 
Pearl. —Periderm thick and covered by a rough thick crust. Layer 
measures 156 microns and is composed of 8-10 rows. Arrangement of 
rows somewhat irregular with much variation in the width of the rows. 
Most cells measure 91 by 13 microns on the average. Transition zone 
is of medium width. The cells are elongated and irregular. Transition 
appears to be more abrupt than gradual (PI. 10, O, and fig. 7). 
McCormick. —Periderm thick and covered by rough crust. Layer 
measures 156 microns and is nine rows wide. The cells show a regular 
arrangement in radial rows of fairly even width. In general the peri¬ 
derm cells are narrow, measuring only 52 by 18.2 microns. Transition 
is very abrupt; often only a single transition row is present. In the 
Peachblow several rows of somewhat elongated cells are commonly 
found (PI. 10, P, Q, and fig. 8). 
Up-To-DaTE. —Periderm of varying width, smooth or covered by thin 
crust. Layer measures from 104 to 160 microns and is composed of 
7-14 rows of cells. The cells show a fairly even arrangement, but the 
width of the rows varies. On the whole the periderm layer has a very 
compact appearance. The most common type of cells measures 78 by 
15.6 microns, on the average. The transition zone is wide; transition 
itself is gradual. The cells of the transition zone are usually much 
elongated (PI. 10, R, and fig. 8). 
SUMMARY 
(1) Developmental studies on the potato tuber lead us to conclude 
that : 
(a) The periderm is formed jointly from the epidermis and the hypo- 
dermis. Continuity of the periderm is assured by the development of 
a phellogen arising in the hypodermis. 
(b) The cortex of the tuber forms a very narrow band of tissue between 
the periderm and the outer circle of phloem groups. The cells contain 
the pigment in case of colored varieties; protein crystals, tannins, and 
a small amount of starch. 
(c) The pith forms the narrow central core of the tuber, but is con¬ 
tinuous with the eyes by means of lateral branches. The cells of the 
pith are poor in starch and have a higher water content than the rest 
of the tuber issue. 
(d) The vascular tissue: The vascular ring as it appears to the naked 
eye constitutes a narrow band of tissue which contains the xylem and 
the secondary phloem. The broad bands of storage parenchyma in 
