8 l2 
Journal of Agricultural Research voi. xxvii, n 0 . « 
end walls strictly transverse. In diameter it varies between 2 and 9 
microns, in length between no and 250 microns. Wherever branching 
of the bundles and anastomosis occur, the sieve-tube segments are 
short and somewhat broader. The sieve tube has a thin protoplasmic 
lining and albuminous content in which small starch grains are occasion¬ 
ally embedded. After fixation and staining, the content appears as a 
deep-staining shrunken mass—the slime plug. Its nature is not under¬ 
stood. The companion cell, a sister element of the sieve tube, has an 
even smaller diameter. It is formed from the sieve-tube mother cell by 
longitudinal division followed by transverse divisions. Only the com¬ 
panion cell retains its nucleus; that of the sieve tube disappears at a 
later stage. The companion cells are heavily pitted with the sieve 
tubes and with the phloem parenchyma. The cells of the latter differ 
but little from the larger sieve tubes in a cross-sectional cut. In longi¬ 
tudinal section the cells taper at both ends. They do not always remain 
undivided, but two to three cross walls are formed. The pits are very 
numerous and arranged in clusters, simulating sieve plates. But this 
type of pitting is very common in all parenchyma cells and by no means 
limited to the parenchyma of the phloem groups. In cross section the 
phloem parenchyma cells are polygonal with a diameter varying between 
5 and 30 microns; in vertical direction they may attain a length up to 
140 microns. The cells have a thin layer of protoplasm and a nucleus. 
They frequently contain a small quantity of starch and occasionally 
crystal sand. In primary phloem groups the proportion of sieve tubes 
and parenchyma cells shows much variation. Ordinarily the paren¬ 
chyma cells are about twice as numerous as the sieve tubes. In the 
smaller groups, however, especially in the outer phloem, the representa¬ 
tion of these two kinds of elements is often about equal. Compared with 
the stem the relative number of sieve tubes in a group is larger. 
In close proximity to the primary phloem groups the phloem fibers 
differentiate. They are found either solitary, in the form of a tangential 
band, or in small clusters. The pits of these cells are much reduced, 
their slit-like openings either commonly parallel to the vertical axis or 
slightly inclined. They early develop a heavy secondary wall, where¬ 
upon their content disappears. The secondary wall, however, does not 
become lignified. 
In the young stolon a cambium is noticeable only in the fascicular 
region. The cells are of the general shape and size of a tracheid. The 
end walls are pointed; the terminal walls follow an oblique tangential 
course. In radial section the sloping character of the wall is not evident. 
Certain cells in the outer phloem region enlarge without specialization, 
making up in their entirety the pericycle of the stele. These cells are 
polyhedral, sometimes tangentially stretched. The young cells contain 
a large, spherical nucleus and parietal protoplasm. Before long, starch 
grains are deposited, and in the further growth of the tuber these cells 
enlarge and multiply rapidly, forcing the phloem groups farther and 
farther apart. In the mature tuber, as will be seen later, the tissue out¬ 
side the vascular ring is mostly pericyclic tissue in which numerous 
phloem groups are scattered. The procambium cells between inner 
phloem and protoxylem develop in a similar manner. Their rapid mul¬ 
tiplication causes a deflection of the inner phloem groups centripetally. 
As the tuber develops, the differences between cells of the Markkrone 
