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What is a Poisonous Plant? As will be shown later, so-called “poisonous” plants differ widely in “degree of harmfulness,” and it is highly probable that under ordinary conditions many of the plants commonly reputed to be poisonous are really almost or quite harmless. It is possible, however, that a plant usually unsuspected may on occasion prove noxious—for example, Nepeta Glechoma (p. 96), included as suspected of poisoning horses. For these reasons, no line of demarcation can be drawn to separate actually poisonous plants from those which are suspected or are almost certainly quite harmless; and a large number of species is included in Chapter VII as suspected, many of them, however, being almost certainly more or less poisonous in certain circumstances. In many cases it is practically impossible to come to any conclusion as to the degree of toxicity of a plant, owing to the want of exact information. Many plants are quite harmless except when affected by fungi, moulds, etc.

A really poisonous plant may be defined as one a small quantity of which when eaten induces some form of indisposition with irritant, narcotic, or nervous symptoms, with serious or even fatal consequences either immediately or by reason of cumulative action of the toxic property.

Harm done by Poisonous Plants. A perusal of the following pages will afford convincing proof that the question of the general “wholesomeness” of wild plants is worthy of serious consideration by all who are interested in the practice of agriculture. Still more important is a satisfactory knowledge of the extent to which plants are actually poisonous—that is, sufficiently injurious when eaten in small or large quantities to induce more or less severe indisposition, illness or death, with the consequent losses which such bring in their train—loss of milk and meat production in the case of cattle, of meat and wool production in sheep, of power in the horse, of expenditure in attendance and veterinary treatment generally, and possibly total loss by death of the animals concerned.

The losses due to Poisonous Plants in Great Britain happily afford no comparison whatever with the immense losses sustained in some other countries, such as the cases of lupine poisoning mentioned at p. 29, but deaths are sufficiently numerous to make it certain that financial losses are in the aggregate very heavy. In this connection it may suffice to refer to the many cases of yew poisoning, the losses due to Umbellifers (pp. 36–42), and the instance reported in the Staffordshire Weekly Sentinel in relation to meadow saffron and water hemlock (p. 80). Further, it appears to be extremely likely that many losses due to unascertained causes are really due to plant poisoning. For this reason veterinary surgeons will be well advised always to consider this possibility and, if need be, to obtain the services of a trained botanist to survey the farm or field involved, with the object of deciding whether poisonous plants are present.

Circumstances in which Poisoning occurs. It may be assumed that many plants are to a considerable extent protected from animals by the fact that they have an unpleasant odour, are acrid or bitter to the taste, or are actually toxic in character, just as others assume such protective devices as spines. In a state of nature animals appear to avoid instinctively such plants as are toxic or “unwholesome,” and to be less readily poisoned than are domesticated animals living under artificial conditions. Indeed, it has been remarked that farm stock reared in a locality where certain poisonous plants abound are much less likely to be injured by these plants than animals imported from a district where they do not occur.

The individuality of stock is also a factor which may be responsible for poisoning, some animals having what may be described as a depraved appetite for unusual and unappetising food plants. It would appear that animals are often tempted to eat dark-green plants of luxuriant growth which are soft and succulent. This is especially true when the plants are young and tender, particularly as regards sheep, which, however, usually avoid tall, old rank-growing and coarse herbage—unless absolutely pressed by hunger. Cattle, however, are not so particular, and will commonly eat large coarse-growing plants.

Sheep have been observed to be particularly variable in their choice of food plants, not only individually in the flock, but from day to day. Chesnut and Wilcox remark^[1] that “there seems to be no way of accounting for the appetite or taste of stock. This statement is perhaps especially true of sheep. We have often observed sheep eating greedily on one day plants which they could scarcely be persuaded to eat on the following day on the same range.” In the case of one flock of sheep on a foothill range at an altitude of 4,600 ft. “a few of the sheep were observed eating large quantities of wild sunflower (Balsamorhiza sagittata), a few ate freely of false lupine (Thermopsis rhombifolia), some confined their attention largely to the wild geranium, while others ate false esparcet (Astragalus bisulcatus) almost exclusively. Two sheep were seen eating the leaves of lupine, and about fifty ate a greater or less quantity of Zygadenus venenosus, while the majority of sheep in the band fed exclusively upon the native grasses on the range.”

1. “The Stock-Poisoning Plants of Montana,” V. K. Chesnut and E. V. Wilcox. Bul. No. 26. U.S. Dept. Agric., Div. Bot., 1901.

Horses also have been known to acquire in America a depraved appetite for horsetail and loco-weed.

The different species of live stock are often quite differently affected by poisonous plants, some being very susceptible to a given plant while others may be little or not at all susceptible. One species (e.g. the pig) may readily vomit the poison of a plant which is emetic, while another (e.g. the horse) may be unable to do so and hence be the more seriously injured. The variability of the different classes of live stock in this respect is frequently brought out in Chapters II to VI. Poisonous effects may also vary with the individuality and age of animals of the same species.

At certain periods of the year—e.g. in early spring, and during dry summers,—there may be a scarcity of green herbage, and this may induce animals to eat any green plants which are especially early, including poisonous ones, which they would otherwise refuse.

In some cases poisonous plants which do not lose their toxic properties on drying (e.g. meadow saffron) may be included in hay, and hence find their way to stock in such a form that they may not be distinguished. It has been found, however, that some poisonous plants or parts of them are refused by stock when mixed with good herbage in hay. Care should be exercised that poisonous plants are not included with hay or green fodder, and in cases of poisoning all forage should be examined.

Animals may also be poisoned by certain toxic seeds (e.g. corn cockle) fed to them with cereal grains, in feeding stuffs generally, or in the refuse seeds from the sources mentioned. Here again judgment is necessary, and it is probably advisable on all counts to burn the weed seeds and similar refuse from the sources mentioned. Poisonous seeds may occur in low quality feeding stuffs, and poisonous seeds of foreign origin are occasionally sold for food purposes owing to the mistaken idea that they are a valuable addition to the ration (e.g. the poisonous “Java” beans). In any case in which an animal is believed to have been poisoned purchased feeding stuffs should always be considered as a possible source of injury and be submitted to examination.

Clippings and trimmings from gardens and shrubberies have proved a more or less common cause of live stock poisoning, such material being too often carelessly thrown out for animals to pick over. In such circumstances it may quite easily happen that the animals get yew, daphne, privet, rhododendron, azalea, solanums, and other plants of a poisonous character. For this reason it is better to destroy such trimmings, etc., by burning them, or by adding them to the compost heap as the case may be.

A further source of poisoning must be noted here—fleshy and parasitic fungi (toadstools, rust fungi), moulds and similar organisms. Many toadstools are directly poisonous when eaten, but the microscopic organisms are probably in themselves harmless, though taken with food which they are responsible for injuring (bad hay, cakes, etc.), the poisoning being due to the changed and damaged feeding stuffs, or possibly to poisonous principles directly elaborated by the microscopic fungi. Fungi and related organisms cannot be dealt with in this volume, but it may at least be said that the use of mouldy hay and similarly affected feeding stuffs is attended with some danger, which is not yet very clearly defined.

Effect of Soil, Climate and Cultivation on the Toxic Properties of Plants. In general, wild poisonous plants are richer in either alkaloids or glucosides than the same species when cultivated, though there are exceptions. In many cases it is found that plants vary considerably in toxicity, or the percentage content of the poisonous principle, according to soil, light, moisture, etc. Solanaceous plants in particular vary in this way, and one or two instances may be given as examples. Solanum nigrum varies so much that it has been regarded as harmless in one country and quite poisonous in another (p. 52).

Experiments conducted at the Arlington Experimental Farm, Virginia, showed that in 24 first-year plants of Atropa Belladonna grown in 1910 the alkaloid contents of the leaves varied from 0·334 to 0·700 per cent., and averaged 0·547 per cent. In 1911 the alkaloid contents (usually the average of five pickings) of the leaves of 59 plants varied from 0·306 to 0·766 per cent., and averaged 0·532 per cent. In 1912 the alkaloid contents (commonly the average of 5 pickings) of the leaves of 57 plants varied from 0·352 to 0·768 per cent., and averaged 0·545 per cent. In individual plants at a single picking the highest alkaloid content in 1911 was 0·925 and the lowest 0·200, and in 1912 the highest was 0·882 and the lowest 0·292. (Jour. Agric. Res., I. 2, Nov., 1913.)

The variation in the percentage of poisonous principle was well shown in several papers read at the International Congress of Applied Chemistry held at Washington and New York in 1912 (see Chemist and Druggist reports). For example, Carr stated that at the Wellcome Materia Medica Farm, Dartford, Kent, the effect of manuring on medicinal plants has been tested for some years, and the effect of the more common fertilisers on Atropa Belladonna was shown by the following table:—

	Percentage of Alkaloid in Dry Stem and Leaf
Fertiliser	Time of Application	Per acre	1906 3rd year’s plants	1907 4th year’s plants	1910 1st year’s plants	1911 2nd year’s plants	1912 3rd year’s plants
Main crop			0·54	0·34	0·61	0·59	0·68
Farmyard manure	March	50 loads	0·54	0·34	0·61	0·53	0·71
Nitrate	March & April	2 cwt.	0·52	0·23	0·54	0·46	0·64
Calcium cyanamide	Do.	1 cwt.			0·69	0·49	0·75
Basic slag	Do.	2 cwt.	0·61		0·65	0·56	0·84
Superphosphate	Do.	5 cwt.	0·46		0·81	0·49	0·76
Potash	Do.	5 cwt.	0·61	0·40	0·75	0·53	0·69

In considering these results it must be remembered that the soil is naturally suited to the plant, and the percentage of alkaloid obtained without added fertilisers is already high. The low figures obtained in 1907 were probably due to the seasonal conditions. Atmospheric conditions have a modifying influence.

It was also shown that the Belladonna root of commerce varies greatly in alkaloid strength. “In a number of analyses made of commercial roots, variations from 0·27 to 0·69 per cent. have occurred. The average of twenty-one analyses of German and Austrian commercial roots was 0·40 per cent. Other observers have recorded similar results. Chevalier (Compt. Rend., 1910, 150, 344) gives the following figures for Continental roots: French, 0·300 to 0·450 per cent.; Austrian, 0·251 to 0·372 per cent.; Italian, 0·107 to 0·187 per cent. Henderson has shown the average of thirty samples of foreign root to be 0·3 per cent. It is interesting to observe that the average of nine samples of root grown at Darenth is 0·54 per cent. In order to determine whether this variation was due to collecting at different times of the year, roots from the same plot, derived from second year’s plants, which were sown at the same time, were dug up at intervals and dried. The following is a record of the analysis of these samples:—

March, 1911	0·56	per cent.
May, 1911	0·59	„ „
June, 1911	0·53	„ „
August, 1911	0·50	„ „
December, 1911	0·59	„ „

“The amount of variation throughout the year is thus seen to be very small.”

Dunstan (Bul. Imp. Inst., 1905) has shown that Hyoscyamus muticus grown in India yielded 0·3 to 0·4 per cent. of hyoscyamine, but that the same species grown in Egypt produced 0·6 to 1·2 per cent.

According to Esser no coniine is found in Conium maculatum growing in the far north. The same authority says that the root of Hyoscyamus niger is quite free from toxic properties in winter.

Variation in the Poisonous Parts of Plants. As will be shown in succeeding chapters, many plants (e.g. meadow saffron) are poisonous in all their parts, though the amount of the toxic substance may differ according as to whether the seeds, leaves, stem or roots are severally considered. In other cases one part of the plant alone is toxic (e.g. the seeds, as in corn cockle). Further, some species vary in the percentage of the toxic substance in the leaves before and after flowering. Frequently the root is the most toxic portion of the plant. This point may be usefully recollected in relation to poisoning of live stock, which are in general affected by the foliage or ripe seeds.

Eradication of Poisonous Plants. Wherever poisonous plants are found, particularly in quantity, where they are liable to be eaten by live stock, an attempt should be made to eradicate them. They may be simply dealt with as weeds as may be necessary according to the species concerned. When any difficulty is experienced in regard either to determination of the species or to methods of eradication the advice of the Board of Agriculture and Fisheries (Whitehall Place, London, S.W.) should be requested. Stock should at once be removed from a suspected pasture, which should be immediately and thoroughly examined by a competent botanist. Harmful plants should be eradicated and their place filled by better herbage.

Treatment of Poisoned Animals. This volume makes no pretensions to deal with the veterinary treatment of cases of plant poisoning. Whenever poisoning is suspected the services of a veterinary surgeon should be sought without delay; the text books at his disposal will aid him in the treatment of any case which presents unusual difficulties. The symptoms indicated in the following pages, together with the possible discovery of a suspected plant, may be utilised in diagnosis.

Tests with Suspected Plants. The action of plants on animals may be ascertained (1) by observing the effects in cases in which it has been established that the plants have been eaten, or (2) by direct experimental feeding of animals with the plant. In the first case the results may be accurate and satisfactory if observations have been made from the outset.

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.