Poison Ivy
2 of the 6 encyclopedias on this shelf carry an entry for Poison Ivy. Both are reproduced below, so you can see where they agree and where they differ.
Aiton's Encyclopedia (1910)
or Poison Oak, a poisonous plant of the sumac family. It is a low, erect shrub, or else climbing by rootlets over stumps and stones and ascending trees. It has a three-foliate leaf. Greenish flowers are borne in a loose cluster and are followed by globular, whitish fruit like that of its relative, the swamp sumac or poison elder. The sap of the poison ivy is milky and acrid. The plant may be handled with the callous palm of the hand ordinarily without evil effect, but if it touch a tender spot between the fingers, or the arm, or the leg of a barefoot boy, irritation and painful blisters are apt to occur. The effect is ascribed to an oil produced by the plant. It is most virulent at the flowering season. There is a theory that this inflammation is due not to the acrid juice of the plant, as is the case with the nettle, but to certain parasitic plant germs that reside on the plant. If this theory be true, these germs must fly in the air, for at certain seasons a person working amid poison ivy or passing to leeward of a swamp of sumac is likely to be affected by a painful inflammation of the face, even though there be no actual contact. The poison ivy may be known from the American ivy, an unrelated and harmless climber, for the former has leaflets in threes and white berries, while the latter has leaflets in fives and has bluish black berries. This distinction, and the fact that poison sumac has white berries and not the red berries like its harmless relative, may be held in mind by a clever jingle perpetrated by Gibson: Berries red, Have no dread! Berries white, Poisonous sight! Leaves three, Quickly flee!
Collier's New Encyclopedia (1921)
a climbing plant which closely resembles the woodbine and which is very irritating to sensitive skins. It may be distinguished from the woodbine by its having only three leaves on a twig, while all other harmless vines have five leaves, one for each finger of the hand. A simple remedy for the poison is a weak solution of common baking soda and water, applied immediately after exposure.
What Has Changed in the Last Century
Aiton's 1910 entry hedged between two explanations, floating "a theory that this inflammation is due... to certain parasitic plant germs that reside on the plant" which "must fly in the air." There are no germs: the agent is urushiol, an oily catechol resin named for the Japanese lacquer tree and worked out chemically by Rikou Majima around 1920, and it causes a delayed allergic reaction (type IV hypersensitivity) rather than a chemical burn like the nettle's. That also explains the face inflammation without contact -- urushiol travels on smoke particles from burning brush, on tools, clothing and dog fur, not through the air by itself. Two other corrections: the plant was moved out of Rhus into the genus Toxicodendron (T. radicans), and Collier's 1921 baking-soda remedy has given way to washing the oil off with soap and water within minutes, then topical steroids or, for severe cases, oral prednisone. The old "leaves three" jingle, however, is still taught almost word for word.
Written for this edition, 2026.