Of the Oak and the Ivy
Seven of the eight photos above show the same ancient oak (Quercus robur ?) viewed from different
angles and at different times (both before and after it was pollarded). Old ivy (
Hedera helix) covers this
tree and the smaller oak shown. The tree has suffered fire damage and its great trunk is hollow. I doubt
that the fire hollowed it out, though it might have done by burning away the dead wood, but rather I
suspect it had been hollow for some time before tinder that collected inside it caught alight, perhaps by
a lightning stroke. The heartwood of a tree is non-conducting and often described as 'dead' wood and
once a wound in the tree (such as from a fallen or felled branch) creates an opening to this heart
wood, fungi enter and break it down. The material rots down inside the trunk and the released nutrients
are reabsorbed by the tree's roots. Indeed, some trees, yews for example, grow roots down the inside
of the trunk and into this source of nutrients, in order to recycle the goodness into the tree. Thus, the
hollowing of an old trunk is a useful strategy for prolonging the tree's life by recycling nutrients. I would
estimate that this tree is about 450 years old.

This tree was pollarded, perhaps because it was feared that the hollow burnt trunk would collapse. This
can happen in old pollards, such as this, since when cut back these trees put out many shoots, and if
pollarding stops for a long time, then these numerous shoots grow too big and this can make the crown
too heavy. However, often such precautions are not necessary, since these hollow stems are often
stronger than the solid stems of youthful trees. They are wide and so hard to bend, they are lighter
and so less likely to collapse under their own weight when bent by winds, and most of the strains from
these bending forces occur on the outside of the trunk, where the wood is still solid and strong. I once
saw an oak of about 400 years of age pollarded by tree surgeons, though I never understood why, its
trunk was solid and the branches were sound too.
The oak above is heavily smothered by ivy. The oak's foliage is golden-brown (usual for this time of year)
and the ivy's foliage is the deep green. The ivy is not feeding on the tree, since it does not penetrate the
bark and manufactures its own food and obtains nutrients from the soil through its own roots, but it is
simply clinging to the tree, using it for support and it will drape over walls in similar fashion. The ivy is
Britain's only native evergreen liana or climber, and will also creep across the floor in the absence of
support and can stand on its own, unsupported, when mature. The stems of the ivy can grow very thick
and woody. I have seen ivy trunks as thick or thicker than a man's thigh (though typically somewhat
flattened) attached to old oak trees. Eventually, the ivy's foliage may smother the oak and prevent it from
getting enough light and carbon dioxide and cause it to slowly whither and die. The ivy also adds to the
weight that the tree must support, making it more prone to damage in high winds. Presumably, the ivy
also competes for water and nutrients from the soil around the tree. However, in a natural ecosystem,
climbers and trees coexist side-by-side, so the tendency to cut down ivy at first sight is probably not
entirely necessary.

The ivy clings on by putting out numerous adhesive roots. Climbing plants have touch sensors that direct
growth around nearby obstacles. In many climbers these sensors are born on tendrils, as in the passion
flower (
Pasiflora), and when the tendril touches a solid object it will slowly wrap around the object and
contract, pulling the climber in.

See also
sensitive plants to see how climbers find their way by touch!
oak and ivy 2
hollow oak
burnt oak 1
burnt oak 2
burnt oak 3
oak and ivy 1
Technically, the common ivy Hedera helix is technically a liana (liane) - a woody climber rooted in
woodland soils, but with its leaves reaching full sunlight. The term vine has a broad sense and a narrower
sense. In the narrower sense it is a thin-stemmed herbaceous climbing plant, such as dodder (
Cuscuta) -
a chlorophyll-lacking
parasitic plant. However, ivy is sometimes described as an herbaceous vine, the
formation of wood being variable. Rarely it will form a small tree. The grapevine,
Vitis, is another example
of a liana found in temperate zones.

Scrambling plants like
Rubus (bramble) are also abundant in temperate forests. Examples include the
various blackberries. These put out arching branches which slowly sway and/or branches which slowly
snake across the floor. Both do this to find other plants to lean on for support. Brambles are capable of
prolific growth, for example, a branch of
Rubus armeniacus will grow several centimetres a day, and be
over one cm thick at its base, growing 4-10 metres in one year. This rapid growth is in part possible due
to their remarkable construction. Despite being  tough and flexible (and protected by large prickles) their
central tissues consist largely of light woody pith which reduces weight and can be manufactured more
rapidly than dense wood.

Hedera helix

Plant form. The common or English ivy, Hedera helix, is a good example of a temperate liana. This plant
prefers moist clay-rich and fertile soils. It climbs to 30 meters in height or else is prostrate or creeping
along the woodland floor.  The stems reach 25 cm in diameter. It is evergreen with the individual leaves
living for 3-4 years. The leaves borne on the juvenile vegetative stems are alternately arranged, palmate
in contour, 4-10 cm in length with 3-5 triangular lobes. These are shade leaves. On the upright flowering
stems, the leaves are thicker, 6-10 cm long but undivided with entire lobes and ovate contour and
arranged in a spiral around the stem. (Ovate - loosely 'egg-shaped' that is broader towards the base).
The switch from shaded vegetative shoot to mature sunlit flowering shoot is under hormonal control. The
plant hormone auxin stimulates development of the mature flowering form, gibberellin of the immature
shade-tolerant form. (See
phytohormones). Low light levels and high temperatures stimulate shoots to
maintain their juvenile form. The switch from juvenile to mature shoot involves selective DNA replication to
produce multiple copies of the required genes. Adventitious roots also occur on the juvenile shoots.

Flowers. The flowers of common ivy are actinomorphic (radially symmetric about the floral axis) and in
groups of about 20 in terminal globose
umbels (rounded umbrella-shapes) or sometimes arranged in
panicles of 1-6 umbels (
panicle - an alternately branching arrangement). There are five very small
sepals and 5, sometimes 6, petals which are yellowish-green and only 3-4 mm and not fused together.
The flowers are hermaphrodite, bearing 5 stamens and an inferior 5-chambered ovary (inferior -
positioned beneath the stamens on the receptacle). The five styles unite to form a column. The nectaries
form a domed disk surrounding the styles.

Fruit. Ivy fruit are black to red-yellow to white berries, 6-8 mm in diameter. Each contains 1-5 seeds.
Usually only the terminal umbel on a panicle produced fruit.

Pollination. The anthers mature first in the hermaphrodite flowers, and are shed before the stigma
matures and nectar is produced. The flowers are insect pollinated, particularly by flies and
cross-pollination is the norm.

Toxicity. Ivy is renown for its toxicity. The leaves are toxic, containing alpha-hederin which is insecticidal
and appetite-suppressing.  The seeds are especially toxic and contain beta-hederin. Molluscicidal
compounds (triterpenoid saponins) are also found in ivy (molluscicidal - kills molluscs such as snails and
slugs). The sap of the common ivy may cause dermatitis.

Annual cycle (Northern temperate). New leaves appear in March to October, main leaf-fall of old leaves
occurring in late spring. The leaves store more anthocyanins in cold temperatures, often appearing
tinged with purple in winter. Flowers open in August to November and fruit ripen in March and April.