Agriculture Reference
In-Depth Information
Epidermis
Sclerenchyma
Sclerenchyma
sheath around
vascular bundle
Epidermis
Support
tissue
Some collenchyma
tissue
Phloem
Vascular bundles
scattered across
cross-section
of stem
Cortex
(parenchyma)
Xylem
(c)
(a)
Epidermis
Epidermis
Some collenchyma tissues
Sclerenchyma fibres
Phloem
Xylem
Sclerenchyma fibres
Phloem
Cambium
Xylem
Vascular
bundle
Vascular
bundle
Sclerenchyma
fibres
Cortex
(parenchyma)
Phloem
Cortex
Vascular
bundle
Pith cavity
Cambium
Xylem
6
(d)
(b)
Figure 6.4
Stems: (a, b) Transverse sections of a typical dicotyledonous stem in
Helianthus annuus
(sunfl ower);
(c) a typical monocotyledonous stem in
Zea mays
(maize); (d) a comparison between the stem tissue of
monocotyledons (to the left) and dicotyledons (to the right)
with large interconnected air spaces between the cells
(
aerenchyma
) to store air for aerobic respiration (see
Figure 9.5).
Inside the cortex in dicotyledonous stems is a ring of
vascular bundles
, so named because they contain
two vascular tissues that are responsible for transport.
The first,
xylem
, contains long, wide, open-ended
cells with very thick lignified walls, able to withstand
the high pressures of water (with dissolved minerals)
which they carry. The second vascular tissue,
phloem
,
consists again of long, tube-like cells (
sieve tubes
) and
is responsible for the transport of food manufactured in
the leaves to the roots, stems or flowers (see p. 112).
The sieve tubes, in contrast to xylem, have cellulose cell
walls and are living, unlignified cells. They are unusual in
not containing a nucleus. The end-walls are only partially
broken down to leave sieve-like structures (
sieve-
plates
) at intervals along the sieve tubes. Alongside
every sieve tube cell there is a small companion cell,
which regulates the flow of liquids through the sieve
tube. The phloem is found to the outside of the xylem
in most species. Also contained within the vascular
bundles of dicotyledons is the
vascular
cambium
.
This is a
lateral meristem
which contains actively
dividing cells producing more xylem and phloem tissue
to increase the girth of the stem in woody plants as it
grows
(secondary thickening
). The vascular bundles
of the stem in dicotyledons are arranged in a ring which
gives strength and support to the stem in much the
same way as steel rods do in reinforced concrete.
Pith
refers to the central zone of the stem, which
is mainly made up of parenchyma cells. It may
sometimes break down to give a hollow stem.
Collenchyma
and
sclerenchyma cells
, forming
collenchyma and sclerenchyma tissues, are often
found to the inside of the epidermis and around the
vascular bundles and are responsible for
support
in the young plant. Both tissues have cells with
specially thickened walls. When a cell is first formed,
it has a wall composed mainly of
cellulose
fibres. In
collenchyma cells the amount of cellulose is increased,
providing extra strength, but otherwise the cells
remain relatively unspecialized. Characteristically, there
are no air spaces between the cells. In sclerenchyma
