Conium is a genus of 2-3 species of perennial herbaceous plants in the family Apiaceae. The most familiar species is Conium maculatum (also known as Poison Hemlock), the most common of several species of hemlock noted for their toxicity. It is a biennial herb which grows up to 10 feet tall, has a smooth (sometimes purple) spotted stem and finely divided lacy leaves. The new leaves and root, when crushed emit a rank, unpleasant odour often compared to mice or parsnips. The flowers are small, white and clustered in the umbrella shape so familiar to this family. The plant is often mistaken for fennel, parsley or wild carrot although the characteristic stem hairs of the wild carrots are missing. The Conium root is fleshy, white and often unbranched and can be mistaken for parsnip.

Conium contains the alkaloids Coniine, N-methylconiine , conhydrine , pseudoconhydrine and g-coniceïne . The most important and toxic of these is Coniine. Coniine is a neurotoxin, which disrupts the workings of the Central nervous system and is toxic to all classes of livestock and humans. A lethal dose for a horse is 4 to 5 pounds (1.8 - 2.2kg) of leaves, cattle; 1 to 2 pounds (0.45 - 0.9 kg), and sheep; half pound (226g) or less.

Conium maculatum is a native of Europe and Asia, but has been introduced and naturalised in many other areas, including much of North America and Australia. Poison hemlock is often found on poorly drained soils, particularly near streams, ditches, and other surface water.

A useful trick to determine whether a plant is poison hemlock rather than fennel, which it resembles, is to crush some leaves and smell the result. Fennel smells like anise or licorice, whereas the smell of poison hemlock is often described as mouse-like or musty. The plant should be discarded if there is any doubt. As the two can cross-polinate to give you fenlock it is wise to avoid consumption of all plants of the apiaceae family in an area where poison hemlock has been identified.

Socrates

The most famous victim of hemlock poisoning is probably Socrates. After being condemned to death for impiety in 399 BC, Socrates was given a potent solution of the hemlock plant.

Plato described Socrates' death in Euthyphro, Apology, Crito, Phaedo

He walked about and, when he said his legs were heavy, lay down on his back, for such was the advice of the attendant. The man who had administered the poison laid his hands on him and after a while examined his feet and legs, then pinched his foot hard and asked if he felt it. He said "No"; then after that, his thighs; and passing upwards in this way he showed us that he was growing cold and rigid. And again he touched him and said that when it reached his heart, he would be gone. ... To this question he made no reply, but after a little while he moved; the attendant uncovered him; his eyes were fixed. -- Plato, Euthyphro, Apology, Crito, Phaedo

Medicinal Uses

Poison hemlock has been used as a sedative and for its antispasmodic properties. It was also used by Greek and Arab physicians for a variety of problems such as arthritis. However, it wasn't always effective as the difference between a therapeutic and a toxic amount is very slight. Overdoses can produce paralysis and loss of speech being followed by depression of the respiratory function and then death.