Aug. 5, 1918 
Anatomy of the Potato Plant 
239 
Though the protoxylem elements are easily recognized in cross section 
by the thickness of their secondary walls, the protophloem cells have 
little to distinguish them from the procambium except their location. 
In vertical section, however, they do exhibit a few differences, though 
only under high power. The cells of the inner phloem which lie adjacent 
to the pith, though having the general proportions of the normal pro¬ 
cambium cell, are slightly more elongated, and the content of the cell^ is 
richer and more granular. In fixed material the protoplasm has with¬ 
drawn from the side walls and has formed a slime string, which in the 
region of the transverse wall of the cell widens, filling almost the entire 
lumen. A similar condition is found in the adjacent cell, suggesting a 
protoplasmic connection between these cells, but actual perforations in 
the wall are not noticeable as yet. These elements are the first sieve 
tubes of the phloem, and from their location, it is noticed that they 
have developed farthest away from the protoxylem elements, differen¬ 
tiating progressively outward. Their development is, then, centrifugal, 
like that of the protoxylem. A little later, sieve tubes in the outer 
phloem begin to appear. These are similar in structure in every way 
to those of the inner phloem, but develop centripetally (fig. 3, D). The 
sieve tubes of the inner phloem are, then, the first to differentiate, a fact 
which might have been expected, since here the first visible differentiation 
of phloem group initials takes place. The number of sieve tubes at 
such a stage is very small, for rarely are more than one or two parallel 
rows seen in a section. 
A cambium becomes distinct very early in the ontogeny of the stem 
even before the sieve tubes of the inner phloem are differentiated. The 
first-formed cambium initials divide rapidly forming two or three tiers 
of cells in the region of the projecting points of the procambium,.long 
before a tangential connection between all the cells of this layer is estab¬ 
lished. 
The protoxylem elements, maturing at this stage, are still small; they 
have secondary thickenings in the form of close rings and flat spirals. 
The secondary thickenings of the first-formed elements, owing to constant 
elongation of the cells, have become separated and form scattered rings 
(PI. 32, D). They are further distinguished from the later-formed ele¬ 
ments by their location and smaller size. 
In somewhat older stages several changes in progressive differentiation 
have taken place. Plate 41, B, shows the vascular cylinder at such a 
stage. The number of protoxylem elements has increased from about 8 
to 56, and the phloem groups have more than doubled in number. 
Though it is not always possible to distinguish the individual groups in 
their entirety, 76 groups were counted in the inner and 68 in the outer 
region (fig. 3, E). Still farther away from the distal end is found what 
is probably the maximum number of phloem groups, 116 in the inner 
