Zea mays

CHAPTER 6
The stem
Primary growth
The aerial stem bears the green photosynthetic
leaves and the reproductive organs (1.16,4.13,
6.1, 6.2), while underground stems are frequently
perennating and food storage organs (6.3). Most
unthickened stems are cylindrical (6.2), but ridged
and rectangular forms (6.4, 6.5) 3rc common.
Stems are sometimes flattened, leaf-like structures
(phylloclades, 6.6, 6.7) with their leaves reduced
to scales. The slender xcromorphic stem of
Casuarina (6.8) bears only scale leaves, so that
phorosynrhesis occurs in the stem cortex which
lies adjacent to the longitudinal, hair-lined
grooves.
[n succulents the stems are swollen and also
photosynthetic and in many species of cacti and
spurges (Euphorbia) the leaves 3rc rcpresenred by
spines (3.33, 6.9). Starch is commonly stored in
the parenchymatous ground tissue of the stem
(2.31) and is particularly abundant in the swollen
stems of succulents and the underground stems of
corms, tubers and rhizomes (6.3). On the
condensed shoots of rosette species (4.13) the
lea\'es are crowded and the internodes short, bur
at flowering the internodes commonly become
much more widely spaced as is dramatically
shown in Agave (6.2).
Anatomy of the mature
pnmary stem
The vascular system in the young internode
usually consists of separate vascular bundles (6.4,
6.5, 6.10, 6.11) that typically form a peripheral
cylinder in dicotyledons (1.28) but are scattered in
monocotyledons (1.27). The cortex lies external to
rhe vascular tissue and is bounded by an epidermis
which often bears stomata and trichomes (5.25,
6.5, 6.8). The ground tissue in which the vascular
tissue is embedded is basically parenchymatous
and the cortex is often photosynthetic (6.6, 6.8,
6.11). in dicotyledons a parenchymatous pith is
usually presenr, but vascular bundles occasionally
may be present centrally (6.10). In the majority of
monocotyledons the bundles occur throughout the
ground tissue (1.27), but sometimes a pith is
present (6.4). Sclerenchyma fibres are often
present in the ground tissue (6.4, 6.8, 6.10) and
the parenchyma may become lignified. Collen·
chyma frequently occurs JUSt beneath the epider·
mis, especially at the angles of the stem (6.5). In
some stems a prominent starch sheath occurs in
the innermOSt cortical layer and in underground
stems this may develop thickening to form an
endodermis (6.12).
Vascular bundles in the stem are commonly
collateral, with the phloem lying nearest to the
epidermis and the xylem situated internally and on
the same axis (1.11, 6.6, 6.10). Bicollateral
bundles may also occur in which the phloem lies
both external and internal to the xylem (6.11). in
many monocotyledons the bundles are amphivasal
with a cenrral strand of phloem surrounded by
xylem (6.12). Amphicribral bundles, in which the
xylem is surrounded by phloem (6. t3), occur in
ferns and a few angiosperms, while in others the
bundles may lack xylem. In the great majority of
dicotylcdonous stems, a cambial layer is located
betwcen the xylem and phloem (6.6, 6.11) but in
monocotyledons this is absent (1.11, 6.12).
The vascular anatomy at the node is more
complex than in the internode due to the branches
that pass outwatds from the axial vascular
bundles to the leaves and axillary branches (4.17,
4.18,4.23, 6.14). Apart from branching at the
nodes, the axial bundles normally interconnect
with adjacent vascular bundles at various levels
along the internodes. In monocotyledons, axial
bundles often run obliquely for some distance in
the internode and have frequent interconnections
(6.15) with numerous veins (leaf traces) passing
outwards to each leaf. In dicotyledons there are
usually fewer leaf traces. In species with few
interconnections between axial bundles, damage
to one part of the axial system may severely
disrupt the supply of water and nutrients to parts
of the plant lying above or below the injury site.
In most stems the protoxylem and protophloem
elementS (2.3) 3re damaged during elongation and
expansion growth (1.11), so that in the oldet
primary stem (3.5, 6.10, 6.11) only the metaxylem
and mctaphloem are normally functional. Pericylic
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