Agriculture Reference
In-Depth Information
cells, the thickness of the wall is increased by the
addition of a substance called
lignin
, which is tough
and waterproof and causes the living contents of the
cell to disappear. These cells, which are long and
tapering and interlock for additional strength, consist
only of cell walls with a central cavity.
does differ: monocotyledon stems have their vascular
bundles scattered throughout the stem and do not
have a clearly defined cortex and pith whereas in
dicotyledons they are arranged in a ring between
the cortex and the pith tissue (Figure 6.4c and d).
Monocotyledonous roots often have stele with
multiple arms (Figure 6.7) rather than the relatively
few found in dicotyledons.
A major distinction is the absence of a vascular
cambium in monocotyledons which therefore do
not undergo secondary thickening and show limited
increase in stem diameter, generally not becoming
woody. In monocotyledons such as palms and
bamboos, a 'woody' stem does develop but this
comes about through a different mechanism to that
in dicotyledons. Mostly, the stem relies on extensive
sclerenchyma tissue for support that, in the maize
stem shown in Figure 6.4(c), is found as a sheath
around each of the scattered vascular bundles.
Dicotyledonous root tissues
The internal structure of a non-woody dicotyledonous
root is shown in cross section in Figures 6.5 and 6.6b.
The
epidermis
is comparable with the epidermis
of the stem; it is a single layer of cells which has a
protective as well as an absorptive function. Unlike
the stem it lacks a cuticle since reducing water loss
is unnecessary in the root. Inside the epidermis is the
parenchymatous
cortex
. The main function of this
tissue is respiration to produce energy for growth of
the root and for the absorption of mineral nutrients.
The cortex can also be used for the storage of
starch
where the root is an overwintering organ.
The cortex is often quite extensive and water must
move across it to reach the transporting vascular tissue
that is in the centre of the root. This central region,
called the
stele
, is separated from the cortex by a single
layer of cells, the
endodermis
, which has the function
of controlling the passage of water and nutrients
into the stele (see p. 122). Water passes through the
endodermis to the
xylem
tissue, which transports the
water and dissolved minerals up to the stem and leaves.
The arrangement of the xylem tissue varies between
species, but often appears in transverse section as a
star with several 'arms'. Since support is unnecessary
in roots surrounded by soil, this arrangement can
maximize water uptake. The root also has a
vascular
cambium
and in dicotyledons undergoes secondary
thickening to increase its girth. As in the stem,
phloem
tissue is present for transporting sugars from the leaves
to provide energy for the living cells of the root.
A distinct area in the root inside the endodermis,
the
pericycle
, has cells which are able to divide and
produce lateral roots, which push through to the main
root surface from deep within the structure. As roots
age they become thickened with waxy substances
and the uptake of water becomes restricted.
The structures of a dicotyledonous stem and root are
compared in Figure 6.6.
Growth and differentiation of the
stem and root
Plant increase in size (
growth
) (see p. 112) and cell
and tissue specialization (
differentiation
) take place
in a well-ordered sequence in particular areas such as
the tips of roots and shoots.
Differentiation
is the change that takes place in
a cell, tissue or organ enabling it to perform a
specific function.
Growth of stems, for example, is initiated in the
apical
or terminal
bud
at the end of the stem (the
apex) (Figure 6.8a). Deep inside the apical bud lies a
tiny mass of small cells, each cell with a conspicuous
nucleus but no cell vacuole. These make up the
apical
meristem
where cells divide frequently to produce
the tissues which will give rise to the epidermis, the
vascular bundles and the parenchyma, collenchyma
and sclerenchyma tissues of the cortex and pith.
In addition to its role in tissue formation, the apical
meristem gives rise to small leaves (bud scales) which
collectively protect the meristem. These scales and
the meristem together form the
bud
. Mutation or
damage to the sensitive meristem region by aphids,
fungi, bacteria or herbicides can result in distorted
growth such as fasciation where shoots and other
plant parts become flattened or fused together (see p.
272). Buds located lower down the stem in the angle
of the leaf are called axillary buds, each with their own
apical meristem which often give rise to side branches
(see p. 85). Leaf tissues similarly develop from the
Monocotyledonous stem and root
tissues
These have the same functions as those of a
dicotyledon, therefore the cell types and tissues are
similar. However, the arrangement of the tissues
