As regards (2) the results may or may not be satisfactory according as the plan pursued is sound or otherwise. For example, it cannot be considered altogether reasonable and satisfactory to extract the principles present in the plant, inject them into the blood stream, and conclude from any ill effects that the animal may exhibit that the plant is poisonous, since the substance extracted may be poisonous under such conditions but little or not at all harmful when the plant is eaten in the small quantities commonly taken by animals. Again, it cannot be held satisfactory to feed an animal on a heavy and exclusive diet of the suspected plant for a considerable period. The real test would, in general, consist in a feeding trial in which the suspected plant occupied a place in the ration in reasonable quantity—such a quantity as might well be taken in natural circumstances, in view of its relative abundance in regard to other food available; and if considered likely that the plant would be eaten daily it may be fed regularly for some days. Should such a test prove negative it may generally be held that the plant is not poisonous, or only so in exceptional circumstances.
Legal Aspect of Plant Poisoning. There is clearly some legal liability in regard to poisonous plants which may, by hanging over a boundary, cause injury to a neighbour’s stock. The only cases known to the author are in relation to the yew. In the case of Crowhurst v. Amersham Burial Board (48 L. J., Ex. 109; 4 Ex. D., 5) a Burial Board was held liable for the loss of a horse poisoned by eating leaves of a yew tree planted in the cemetery owned by the Board, the tree having grown through and over their fence and projected on to the meadow occupied by the plaintiff. In Ponting v. Noakes (63 L. J. B. 549; (1894) 2 Q.B., 281) the defendant was not held liable for the death of a horse which ate off the branches of a yew tree, because the tree did not extend up to or over the plaintiff’s boundary, though it overhung a ditch, the edge of which was the boundary, and was hence accessible to the plaintiff’s stock.
The Toxic Principles of Plants. The poisonous substances in plants may be grouped either (1) according to their physiological effects on certain organs; (2) according to the principal outward and perceptible symptoms caused; or (3) in accordance with their chemical relationships.
In regard to (1) the poisons may be nerve, heart, blood-poisons, etc. (2) The poisons may be acrid, narcotic, or both. Acrid poisons are those which cause irritation or inflammation of the digestive tract (Euphorbia, Ranunculus, Daphne, etc.); narcotic poisons affect only or chiefly the brain (Agrostemma, Papaver, Atropa) or chiefly the spinal cord (Claviceps purpurea, Lolium temulentum); acrid-narcotic poisons induce to a greater or less extent the symptoms of both the foregoing groups (Taxus, Colchicum, Cicuta, Solanum, etc.).
(3) As the accounts of the individual plants will show, the toxic principles of many plants are not yet well understood, either as regards chemical constitution or symptoms caused. Many poisons may be driven off by heat (boiling, drying), and some plants may thus be rendered harmless.
Most of the toxic principles of our native poisonous plants are Alkaloids or Glucosides. The former all contain nitrogen, differ considerably in molecular constitution, and are usually combined with widely distributed organic acids. In the pure state they are colourless and usually stable, crystalline or amorphous solids, or readily volatile liquids; they usually have a burning taste. In general the same base is confined to species of the same order—e.g. Solanine to the Solanaceæ. The alkaloids include the most powerful poisons.
The term “Glucoside” indicates a group of substances which by the action of an acid or enzyme are split up into a sugar (grape sugar, galactose or rhamnose) and other substances (alcohol, aldehydes, acids). They have a bitter taste and are generally readily soluble in water. Related to the glucosides are also the Saponins, remarkable for the fact that they induce an exceedingly frothy condition in water; the prussic acid yielding compounds or cyanogenetic glucosides (e.g. Amygdalin and Phaseolunatin) also belong to this group. Other substances will be mentioned in the succeeding chapters.
CHAPTER II
RANUNCULACEÆ.
Traveller’s Joy (Clematis Vitalba L.). The extent to which this species is poisonous is not clear, though all parts are stated to be poisonous, acrid and narcotic, while the juice tends to blister the skin. It is remarked by Cornevin that it is less poisonous in spring, when the ass and goat browse on it to a considerable extent without serious trouble, than later, when it cannot be eaten without danger.
Toxic Principle. Traveller’s Joy appears to contain strongly poisonous substances which have not been closely investigated. Greshoff found a Saponin in the leaves[[2]]. The poison is dissipated by heat.