Agriculture Reference
In-Depth Information
In
Hydrangea anomala
subsp
. petiolaris
and in
Hedera helix
(ivy),
adventitious climbing roots
develop along the stem attaching it to vertical
surfaces such as tree trunks and walls to increase
light for photosynthesis and raise the flowers up for
better pollination opportunities (see Figure 5.7). Ivy
is not parasitic and obtains all its water and nutrients
from its own roots in the ground, its adventitious
climbing roots are there just for support. Contrary
to popular belief it does not kill trees or damage
their bark and, where it grows into the crown, this
is most likely where the tree is already in decline
or is diseased. Similarly ivy rarely damages walls if
the brickwork is sound. Ivy is a valuable plant for
wildlife in the garden (see p. 47). On buildings, ivy
can insulate in the winter and cool the building in the
summer. Unlike ivy,
Viscum album
(mistletoe) is a
hemiparasite (see p. 47) which obtains some of its
nutrition from a host tree, commonly apple, poplar
or lime, to supplement its own photosynthesis. It
produces
haustorial roots
which penetrate the
vascular system of the host and tap into water
and sugars. Some plant roots associate with other
organisms such as bacteria (e.g in
nitrogen-fixing
roots
and fungi in
mycorrhizal roots
) to improve
their uptake of water and nutrients.
Epiphytes are plants which are physically attached
to aerial parts of other plants for support, enabling
them to reach more light for photosynthesis, such
as some ferns, bromeliads and orchids. Epiphytic
orchids have specialized aerial roots called
velamen
roots
.
These have a multilayered epidermis with an
outer velamen layer composed of dead cells giving
the root a silvery appearance. The velamen may
absorb water from the air and reduce water loss
from the root tissue and it seems that it is able to
take up nutrients dissolved in moisture in the air.
Unusually in roots, the underlying tissue is green and
photosynthesizes (Figure 7.14f).
Root tubers
develop near the base of the plant, often
from adventitious roots, as in
Dahlia
or
Ophrys
spp.
(bee orchids). Root tubers such as the
Dahlia
tuber
(Figure 7.14g) can be distinguished from stem tubers
in having lateral roots and no nodes. These, together
with
swollen taproots
(e.g.
Daucus carota
(carrot)
(Figure 6.1) and
Taraxacum officinale
(dandelion)) are
all perennating organs.
Figure 7.12
Stipules of
Rosa
spp. along the petiole
7
(a)
Stem adaptations
(b)
Many stems are adapted for climbing. In
Phaseolus
coccineus
(runner bean),
Lonicera
spp. (honeysuckle)
and
Wisteria
spp.,
twining stems
wind around other
Figure 7.13
Leaf arrangements: (a) alternate in hazel
(
Corylus avellana
); (b) opposite in
Acer
spp.
