Mar. 15, 1924 
Studies on the Potato Tuber 
811 
them are readily recognized. The groups of vascular tissue form bicol¬ 
lateral bundles. The xylem, occupying the middle of the bundle, stands 
out by the larger size of its elements and the secondary thickenings of 
the cell walls. About equidistant from the xylem internally and exter¬ 
nally are small groups of thin-walled cells—the phloem. The outer 
phloem groups form a more or less continuous band, limited externally 
by the endodermis and separated from one another by parenchymatous 
cells of irregular size—the pericy- 
cle. The inner phloem groups are 
separated from the xylem by thin- 
walled cells which in their entirety 
make up the Markkrone. Inter¬ 
jacent to the outer phloem and 
the xylem there is a single layer 
of procambium still in meriste- 
matic condition. It becomes the 
cambium of the bundles and later 
gives rise to secondary tissue. 
The entire xylem of the bundles 
is limited to a few annular and 
spiral elements surrounded by un¬ 
specialized parenchyma. (Fig. 1.) 
The first formed protoxylem ele¬ 
ments occur surprisingly close to 
the apex (PL 3) in a region in 
which there is much change and 
enlargement in the radial and tan¬ 
gential direction; these cells must 
therefore be able to accommodate 
themselves to increase in size and 
change in shape. This is made 
possible by their structure. The 
secondary wall is not laid down 
uniformly over the entire surface 
of the primary wall, but instead 
the secondary thickenings consist 
of rings or spirals. Elongation of 
the surrounding tissue causes the 
rings to be pulled farther apart, 
and as the stretching goes on the 
cells of the protoxylem may be¬ 
come completely flattened until 
the lumen is almost closed. The 
later formed protoxylem elements 
have thickenings in the form of 
close spirals and scalariform bands. 
These elements are shorter than those formed earlier and are evidently 
developed from more mature initials which had completed their longi¬ 
tudinal growth. The end walls of the protoxylem elements are sloping, 
occasionally strictly transverse. The elements communicate with each 
other through a single large pore. (PI. 2, E, D.) 
The phloem groups consist of sieve tubes, their companion cells, and 
thin-walled parenchyma. The sieve tube is a long cylindrical cell with 
Fig. i.—C ross section of stolon tip. X 375. (Draw¬ 
ing is exact copy of photomicrograph.) 
