Taxaceae

Slow-growing evergreen conifers of the northern temperate world, with flat dark needles set in two ranks and, on female trees, the feature that gives the family away: a single seed sitting in a soft scarlet cup called an aril. Yews are gymnosperms, but they make nothing resembling a woody cone — the aril is fleshy, sweet and bird-dispersed, which is a strategy the rest of the conifers gave up.

The yew family holds one of the clearest cases in pharmacognosy of a poison and a drug from the same tissue, made of different molecules.

The poison is the taxine alkaloids, taxine B chiefly, which block cardiac sodium and calcium channels. Yew poisoning is abrupt, refractory to treatment and has no proven antidote; it remains a recurring cause of livestock death and of deliberate human poisoning. Clippings are more dangerous than the living tree, because wilting concentrates the alkaloids while removing the bitterness that would otherwise deter an animal.

The drug is paclitaxel, isolated from the bark of the Pacific yew (Taxus brevifolia) and one of the most important chemotherapy agents ever developed. It stabilises microtubules — freezing them in place rather than preventing their assembly — which arrests cell division. Neither fact explains the other; the two compound classes are unrelated beyond sharing a plant.

Paclitaxel also produced a genuine conservation crisis. The original extraction required the bark of several mature trees per patient, from a slow-growing understory conifer that had been treated as logging waste. The problem was solved by semisynthesis: 10-deacetylbaccatin III is taken from the renewable needles of the common English yew (Taxus baccata) and converted chemically, so no tree is felled.

One detail is oddly specific and worth knowing. The red aril flesh is not toxic — birds eat it freely and pass the seed intact. The seed inside it is. Children eating the conspicuous red fruit is the commonest domestic exposure, and the outcome depends entirely on whether the seed was chewed.

Two unrelated systems: the taxine alkaloids (cardiotoxic, concentrated in needles, bark and seed) and the taxanes — paclitaxel and the semisynthetic precursor 10-deacetylbaccatin III. Yews also carry biflavonoids and, in Torreya, distinctive volatile oils.