Aug. 5, 1918 
Anatomy of the Potato Plant 
241 
These are arranged in no constant or typical manner, though often the 
larger cells form radial rows (PI. 42, A). 
The inner phloem groups meanwhile increase in size and in number of 
elements. Sieve plates are readily seen in both longitudinal and cross 
section. The outer phloem groups also become larger and more distinct. 
In many places they are separated by large, ovoid cells. The interfasci¬ 
cular cambium is almost completed between the six first-matured groups 
of vascular tissue, and is in places two or three rows wide (PI. 29, D). 
An endodermis has so far not been distinguishable from the cortex, 
but now becomes fairly distinct. The cells making up this layer are 
small, usually more regular in shape, and contain starch even when the 
latter is not present in other tissues. (PI. 29, C; 31, A.) 
The phloem fibers are the last to appear. The first of these are differ¬ 
entiated simultaneously in the inner and outer phloem. At first they 
are seen singly; later they may increase in number forming groups. 
Their walls are usually heavily thickened, but do not become lignified 
until later. In the inner phloem the fibers usually appear in groups 
either scattered among the peripheral pith cells or forming the inner limit 
of the phloem groups (PI. 33, C). In the outer region the fibers usually 
appear in groups, either scattered among the peripheral pith cells or 
abutting on the inner phloem groups (PI. 33, A). In the outer region 
the fibers form a single broken layer next to the endodermis; they also 
occur occasionally in groups. 
Not all of the procambium cylinder differentiates into conductive 
elements and fibers. Some of the cells of the outer region enlarge without 
specialization, forming the parenchyma between the outer phloem 
groups, the pericyclic region of the stele. In the inner region a similar 
change takes place. The cells between and around the protoxylem 
elements and the innermost phloem groups', though differing from the 
pith cells in being smaller, remain parenchymatous and unspecialized. 
They form, together with the protoxylem, a band of tissue limiting the 
pith on the outside and the vascular cylinder on the inside, a band which 
may be called the “perimedullary zone,” or “Markkrone” (PI. 29, C; 
30, B). Sometimes typical pith cells separate groups of inner phloem 
from this region. These isolated phloem groups then appear as though 
they do not belong to the stele (PL 29, A), and, hence, have been called 
“pith bundles.” 
Above is in brief the early ontogeny of the tissues and elements of the 
sprout of the potato plant. In comparison it is of interest to trace the 
development of a growing tip of a mature stem, and to note differences 
in order of appearance. 
In such an older stem there is already very near the growing point a 
fairly well differentiated vascular ring. The inner phloem groups are 
distinct and numerous; the outer groups are still undifferentiated pro¬ 
cambium. The protoxylem elements are found singly and scattered; 
