AECD-3446(App. )
7
A cross-section of the normal mature stem of Ipomoea tuba just below the
first true leaf shows a phellogen producing a thin-walled phellem several cells
in depth on the periphery.
Progressing toward the center of the stem from this
is located the collenchyma, four or five cells in depth; considerable assimilatory parenchyma containing lactiferous ducts and secretory cells; a uniseri-
ate layer which may be an endodermis, but cannot be strictly identified as such
since it lacks Casparian strips and is not a starch sheath; a pericycle; external
phloem groups; the cambium, several cells in thickness; a ring of secondary
xylem traversed by pith rays; radial rows of primary xylem; and a central pith
containing lactiferous ducts and strands of internal phloem (Figure 6).
Both
internal and external phloem contain well developed sieve tubes and companion
cellse
The cortex and central parenchyma contain abundant starch and there are
many cluster crystals.
Bands of fibers occur in the xylem of the older stem.
Younger portions of the stem are much the same as to tissues and organization,
except that there are no xylem fibers and no phellogen, the stem being covered
by a uniseriate epidermis.
A longitudinal section of the normal primordium shows a uniseriate tunica
covering the central, homogeneous-corpus.
Just back of the corpus region is
an area of cell elongation and pronounced procambial development.
Phloem is dif-
ficult to distinguish in the longitudinal section, but xylem elements are easily
recognized ome to two mmo back of the apexe
The origins of lactiferous ducts in
both the central and cortical regions may be distinguished at about the same
level and many of the parenchyma cells contain large cluster crystals (Figure 7).
The tumors consist largely of parenchymatous tissue with relatively smell
and, it would seem, inadequate amounts of xylem and phloem.
The parenchyma cells
are about the same size as those occurring in the central pith of the normal
ceéllse
The xylem and phloem cells, on the other hand, are extremely small and
there is apparently no cambial activity.
The phloem varies from almost com-