Aue. 5 . i9*a Anatomy of the Potato Plant 229 
A close examination of the xylem (Pi. 42, A) shows that the larger 
outer cells are arranged somewhat in radial rows, in which the individual 
cells either abut on one another or are separated by small thin-walled 
parenchyma cells. The walls of these elements are scalariform, reticulate, 
or pitted (Pi. 32, C; 33, E). Occupying the tip of the wedge-shaped mass 
are smaller xylem elements scattered among parenchyma cells. These 
have secondary thickenings in the form of rings or spirals and are the 
elements which mature before elongation ceases, the so-called protoxylem 
(PI. 29, A, C; 42, C). Frequently these protoxylem elements appear 
crushed (Pi. 29, C), and show only part of the secondary thickening of 
the wall—that is, half a ring or part of a spiral band. The larger ele¬ 
ments external to these cells make up the metaxylem. Though the 
arrangement of the xylem cells is somewhat irregular, the smallest, 
oldest, elements are found adjacent to the pith, showing that the order 
of development is from within out or centrifugal, and that the arrange¬ 
ment of these cells is therefore endarch. 
At this stage both the xylem and the phloem are entirely primary— 
that is, of procambial origin. The phloem consists of small groups of 
sieve tubes with their companion cells and thin-walled conducting 
parenchyma (Pi. 31, A, B). Phloem fibers are first observed in slightly 
older tissue. When present, they occur singly or in small groups on the 
inner face of the endodermis, and in the outer region of the pith. Groups 
of primary phloem occur not only to the inside and outside of the primary 
xylem groups but are equally prominent in the interfascicular region where 
they may be seen at varying distances on both sides of the well-developed 
interfascicular cambium (PI. 29, A). The outer phloem groups are small 
and close to the cambium; the inner are larger and more distant. 
Since the structure and development of the phloem are the principal 
objects of this study, the cells of the xylem are considered only briefly. 
The vessels are porous, and of the type usually found in herbaceous 
angiosperms. The vertical extent of the individual cell is about two to 
three times its diameter. The end walls are usually somewhat oblique 
(PI. 33, E), but sometimes nearly transverse (PI. 32, B). As is usual in 
vessels, the walls are heavily pitted, the pits being arranged in trans¬ 
verse series (PI. 33, E). Typical tracheids and wood parenchyma cells 
are found scattered among the larger vessels. In places these parenchyma 
cells are arranged in radial rows, forming narrow bands one or two cells 
wide. These are the innermost cells of the first-formed medullary rays, 
those extending from the pith itself, rays which may be called “primary 
medullary rays.” In stained sections these cells are distinguished not 
only by their arrangement/ regular size, and shorter tangential diameter 
but also by their darker stain. The protoxylem elements have already 
been considered and will be described in detail in the study of the 
ontogeny. 
