Zea mays

fibres often develop in the outer procnmbium (1.4,
3.2) and replace the crushed, isolated files of
prorophloem, while the protoxylem is sometimes
represented by lacunae (1.11) after the 'hole}"
primary walls (2.66) of the rrachcary elements
become over-extended. In dicotyledons the
metaxylem vessels are frequently arranged in
radial files separated by parenchyma or sclerenchyma
(3.5)_ In monocotyledons the relatively few
vessels arc usually larger and parenchyma or
sclerenchyma often occurs benveen them (1.11).
Modifications ofthe primary stem
In submerged aquatic stems the epidermis usually
lacks both a cuticle and stomara. Chloroplasts
often occur in the epidermis and tHe normally
abundant throughout the aerenchymatous cortex
(6.16). The vascular tissue is concentrated in a
central cylinder with the xylem greatly reduced
and restricted to annular or spiral tracheids whilst
protoxylcm lacunae are common (6.17). Desert
perennials are commonly succulent and store
water in their pith and cortex. They have small or
vestigial leaves (3.33, 6.9) and the stem cortex is
the main site of photosynthesis. The epidermis is
often multiseriatc and is covered by a thick cuticle
but a transparent cork layet may develop in
£If!Jhorbia and some cacti. In plants growing in
salt marsh or other saline environments the leaves
are also often reduced and the stems are succulent.
Secondary growth
Most dicotyledons and all gymnosperms undergo
some degree of secondary thickening (1.30, 3.2,
3.27,3.44). The alllouO[ of thickening produced
depends upon whether the mature plant is herbaceous
(6.18, 6.19) or arborescent (1.2, 3.44). The
fascicular vascular cambium de ....elops from a
narrow strip of procambium between the xylem
and phloem which remain meristematic after the
primary vascular tissues have matured in a bundle.
At the onset of cambial acti\·iry the divisions are
normally localised within the individual vascular
bundles (1.28, 3.5, 6.18). They then spread
laterally through the adjacent interfascicular
parenchyma cells so that a continuous cylinder of
vascular cambium eventually results (1.30). The
vascular cambium normally commences activity
by the end of the first season's growth.
118
Anatomy of the woody stem
The vascular cambium consists of fusiform and
ray initials (6.20, 6.21). The cambium is often
storeyed, with the fusiform initials arranged in
approximately horizontal layers when viewed in
tangential longitudinal section (6.21). However, in
a non-srore)'ed cambium (6.21) the fusiform
initials tend to be longer and their end walls taper
more acutely than in the storeyed cambium. The
fusiform initials give rise to the axial components
of the woody stem: vessels, tracheids, fibres, sie ....e
rubes, companion cells and axial parenchyma
cells. These may be storeyed or otherwise (3.63)
according to the patrern of the cambium from
which they arc formed. The tangenrial (periclinal)
walis of the fusiform initials are wider than the
radial walls (6.20). The initials divide tangentially
(6.20), cuning off xylem centripetally and phloem
centrifugally (3.27, 6.22). During the gtowing
season the cambial initials are actively dividing.
They are highly vacuolate cells and the expanding
cell plate formed after mitosis is invested at its
periphery by a prominent phragmoplast (2.61,
6.20,6.22).
Each cambial initial produces radial rows of
derivatives and in an acti ....e cambium a fairly wide
cambial zone is apparent (6.1 S). In this zone
tangential divisions also occur within rhe potential
xylem and phloem elements (6.22). In gymnosperm
and some angiosperm xylem tbe radial
seriation is retained as the trachear}' e1emenrs
mature (3.17, 3.53, 6.23). In mosr angiosperms
this radial pattern is more or less severely
disturbed by the maturation of large-lumencd
vessels (3.67, 3.6S, 6.18). The r.:'ly inirials arc
approximately isodiamctric .:'Ind they divide
tangentially (6.20) to form the rays which run
radially across the secondary vascular system
(3.17, 3.44). To accommodate the increasing
circumference of the stem, as secondary thickening
progresses, the fusiform initials sometimes divide
radially (anticlinally) to form additiollal fusiform
initials. They also give rise to further initials which
form new rays to meet the increased requirements
for radial transport of water and nutrients in the
expanding srem.
The expansion of the stem brought about by
secondary growth is accompanied by various
changes. The primary phloem and xylem cease to
function in translocation and mlilspiration. The
pith often remains more or less inract over a
number of years but may breakdown ro form a
central cavity. The cortex may persist for some