There are four species of this large, primarily tropical genus that occur in Texas; Zanthoxylum clava-herculis is the one in our area of north Texas.  The genus Zanthoxylum is in the Family Rutaceae, which includes the citrus trees, which are obviously of economic importance. Most if not all members of this family produce "secondary compounds", which are metabolic products that are not part of the primary metabolic pathways of the plant (e.g. are not involved in photosynthesis or respiration, nor are structural parts of the plant). The most common type of secondary compound in the Rutaceae is alkaloids, which are generally toxic to animals. Obviously the function of these secondary compounds is to deter herbivores from eating the plant and its parts.

Many plant alkaloids have desirable (although mildly toxic) properties, and we humans ingest them intentionally to take advantage of those properties. Caffeine, for instance is a plant alkaloid, as are nicotine and cocaine. Obviously these compounds have psychoactive properties. Other alkaloids have medicinal properties. Quinine is an alkaloid that has toxic effects on the parasite that causes malaria. The British in colonial times occupied areas where malaria is common, and they consumed a daily dose of quinine to reduce the probability of contracting malaria. Quinine is an ingredient in tonic water, and gin was added to the tonic water to counteract the bitter taste of the quinine. Finally a little twist of lime juice was added to the mix to help prevent scurvy.

Toothache tree was used in frontier times and by native Americans as a remedy for toothache, and also externally for joint pain. As many of you experienced in class, a little piece of bark contains enough of the secondary compound to produce a tingling sensation followed by numbness in the lips or gums. The leaves work as well as the bark. In fact, if you hold the leaves up to the light, you can see the small glands that produce the secondary compounds scattered around in the leaf tissue. Obviously a compound that causes numbness in vertebrate tissues would be an effective deterrent to herbivores, because it would cause numbness and loss of function in the digestive tract of any animal that ate the plant's leaves or other tissues.

Tickletongue has another defense in addition to its secondary compounds. The bark of young stems has small but hard and sharp thorns that act to mechanically deter herbivores that would attempt to get at the leaves. In older larger stems (and on the trunk of larger plants) these thorns are modified to form large sharp nodules, which again would serve to deter herbivores. It is thought that the common name "Hercules' club" is a reference to these nodules, in that only a Hercules could use a stem of this plant as a club.