The Yew Tree
Taxus baccata
Few
of our old churches can be without an ancient Yew tree close by, possibly
planted to epitomise immortality. This
tree is very slow growing and lives for a long time, some dating back several
thousand years. The wood is very hard
and dense, and is resistant to decay and insect attack. Some of the ancient artefacts found in peat
bogs were made of Yew wood. In the
past, the wood has found many uses ; in
making long bows, in carving, wood turning and for chair arms. The sapwood is yellow and the heartwood is
red-brown. The best bows were made from
wood taken at the junction of sapwood and heartwood, and contained both. Many of the Yew bows were imported from
Europe during the Middle Ages, mainly from Spain and Italy, due to the poorer
quality of the wood of the English Yew. Our oldest Yew trees, those over 500
years old, usually become hollow, as may be seen in the churchyard of Portbury
church, though they will endure for many years in this state. The woods around Nailsea have many Yew
trees, and in some, like those in Bourton Combe, this tree becomes dominant.
Very few insects feed on the Yew, since it contains a range of toxins.
However, a small fly (Taxomyia taxi) is dependent on the Yew, forming
small galls (artichoke galls) at the tips of the growing shoots in some
localities, e.g. Brockley Combe and Towerhouse Wood.
The origin of the
scientific name for the genus Taxus is debatable. Some believe that it is derived from the
Latin 'texere' to weave (hence the word textile), from the arrangement of the
leaves, which are said to be distichous, that is - in two rows. Others suggest that it is derived from the
Greek word toxon (toxin) for poison.
Yet others associate it with the word for archery, toxophily.
The Yew is dioecious,
having separate male and female trees, only the female tree having the
conspicuous soft, bright red 'berries'; the species name 'baccata' meaning
'having berries'. The correct name for
these is ‘aril, the soft part of which is edible. However the seed that it contains is said to be extremely
poisonous and should never be eaten.
Many birds appear to be unaffected by this toxin, probably because they
do not crush the seed, which is in a thick case. Although deer appear to be immune to the cardio-active taxines,
cattle may be killed within a few minutes if they eat the leaves, and this may
have been another good reason.for growing Yew in churchyards where it would
have been surrounded by a stock proof fence or wall. As little as 50 g is lethal to humans, slowing the heart rate to
30 beats per minute. Leaves that have
been cut and are slightly wilted are thought to be particularly toxic.
In 1971 it was shown that an extract from the bark of the North American
Yew (Taxus brevifolia) contains a cytotoxic drug, which had potential
for the treatment of several forms of cancer.
It achieves this by blocking the de-polymerisation of tubulin, a protein
important in cell division. Dividing
cells killed in this way become filled with tubulin fibrils. This therefore has the opposite effect of
colchicine, a drug found in the Colchicum crocus, which prevents the
polymerisation. Unfortunately, each
treatment required the bark from an entire tree since the concentration is only
0.01%, and it was realized that the species would soon be made extinct. However, further investigation of the
complex chemistry of the alkaloids showed that a precursor
(10-deacetylbaccatin) could be easily obtained from the leaves of the English
Yew. Limehurst Ltd, a company based in Chichester (tel 01243 545455,
www.limehurst.com) or Friendship Estates based in Doncaster
(tel 01302 700 220) will
purchase Yew hedge clippings. The
prescription of taxol (right) for ovarian cancer and taxotere for breast cancer
is now accepted by the National Institute for Clinical Excellence, even though
the cost is still high and had until recently been subject to 'postcode
prescribing'. These cytotoxic compounds are also being used successfully for
the treatment of lung cancer.
I am grateful to Tim Hills for his comments on my draft.
This article is reproduced from Nature in Avon, the journal
of the Bristol Naturalists’ Society, volume 61, page 58 (2001).
Terry Smith: tel. 01275 854317: email t.a.smith@blueyonder.co.uk Home