The Yew Tree
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 Ďaril, 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 Naturalistsí Society, volume 61, page 58 (2001).
Terry Smith: tel. 01275 854317: email firstname.lastname@example.org†††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††††† Home