Agriculture Reference
In-Depth Information
The leaf joins the stem at the
node
and has in its angle
(axil) with the stem an
axillary bud
, which may grow
out to produce a lateral shoot or in some instances
flowers. The distance between one node and the next
is termed the
internode
.
Stems of herbaceous plants
need to maintain a high water content to provide turgor
pressure to support the plant (see p. 120). Winter
stems in deciduous woody plants often show a
leaf
scar
where the leaf was attached and a
bud scale
scar
(girdle scar) where last year's
apical bud
was
positioned and this can be useful in determining which
part of the stem is current, one-year-old or two-year-old
wood when pruning (Figure 7.4).
Young stems may be green and carry out
photosynthesis (Figure 7.5). In common with other plant
organs, stems are enclosed by the
epidermis
which
contains
stomata
, pores which allows gas exchange
between the air and the living tissues inside the
stem (see p. 80). When a stem becomes woody, the
epidermis gives way to a waterproof and gas-tight
bark
layer. The stomata are then replaced by breathing pores
called
lenticels
. Lenticels may be useful in identification;
in
Prunus
species they form very distinctive horizontal
lines on the smooth bark (Figure 7.6).
The colour and texture of plant stems are decorative
features which can also be an aid to identification.
For example,
Cornus alba
'Sibirica' (a dogwood) has
bright red stems in winter (Figure 7.7), many
Salix
spp. (willows) have bright yellow or green stems even
when woody. Stem colour is best in young stems so
these shrubs are often pruned to the ground each year
in winter to produce new stems the following season
(coppicing).
Many trees have highly decorative bark, which bring
winter interest to the garden. These include many
Eucalyptus
spp.,
Betula utilis
var.
jaquemontii
and
other white stemmed birches,
Prunus serrula
var.
tibetica
(Tibetan cherry)
with shiny red bark,
Acer
griseum
with peeling cinnamon-coloured bark and the
snakebark maples
Acer davidii
,
A. pensylvanicum
and
A. capillipes
(Figures 5.12 and 7.8).
Figure 7.3
Germinating
Vicia faba
(broad bean) seed
showing the radicle developing into a tap root with
laterals
grasses but can also be found in dicotyledons such as
Senecio vulgaris
(groundsel).
Adventitious roots
do not derive from the radicle of
the plant embryo. They tend to grow in unusual places
such as on the stem or other organs. Most fibrous
root systems are made up of adventitious roots which
grow from the bottom of the stem, with the primary
root failing to develop or dying away.
7
A
taproot (primary root)
is a single large root
which will have many
lateral
(
secondary
) roots
growing out from it at intervals.
Primary
roots
originate from the radicle of the embryo. A
fibrous root system
consists of many roots
growing from the base of the stem with no
dominant root.
Adventitious
roots grow in
unusual places and do not originate from the
radicle of the embryo.
Buds
A bud is a condensed stem which is very short
and has small leaves attached, both enclosing and
protecting it (Figure 7.9). Buds are found at the apex
of the shoot and in the axils of leaves with foliage
buds containing numerous folded leaves and flower
buds containing the immature flower.
On the outside of the bud, the leaves are often thicker
and darker forming
bud scales
to resist drying and
damage from animals and disease. They may contain
chemical inhibitors which delay bud break until the
Stems
The stem's main functions are to:
X
physically support the leaves in the optimum
position for photosynthesis
X
physically support the fl owers in the optimum
position for pollination
X
transport water, minerals and food between roots,
leaves and fl owers.
