Toxic plant

“ It probably affects the colloidal condition in some way similar to its hardening effect upon gelatin, etc.
The toxic effect of many different organic compounds is so varied in its nature and extent that it is impossible to give any satisfactory brief review of its manifestations. Recent investigations appear to indicate that organic products of decomposition [Pg 248] of plant residues in the soil may exert powerfully toxic effects upon succeeding generations of the same, or of different, plants growing on the land. ”

“ Extreme changes in environmental conditions generally result in the death of the plant; but changes which do not result in the lethal condition affect the plant by either stimulating it to more rapid physiological activity or by depressing its normal growth or functions. As has been pointed out, the same external influence, either chemical or physical, which acts as a stimulant if it differs only slightly from normal conditions, may become depressing, or positively toxic, if present to a larger extent. ”

“ The toxic conditions which cause trouble in working with pure cultures of organisms in specific cultures media do not, so far as is known, arise in the soil. All attempts to find bacterio-toxins or plant toxins in normal soils have failed. The product toxic to one organism seems to be a useful nutrient to another, and so the mixed population keeps the soil healthy for all its members. ”

“ A review of the [2] whole subject leads one to conclude that in general both favourable and unfavourable conditions of nutrition are present side by side, and only when a balance is struck in favour of the good conditions can satisfactory growth take place. As indicated above, experiments have shown that the very substances that are essential for plant food may be, in reality, poisonous in their action, exercising a decidedly depressing or toxic influence on the plant when they are presented singly to the roots. ”

“ Lower down in the scale of concentration differentiation of action may set in, and while some processes may still be hindered, others may be stimulated. If the two actions balance one another an apparent indifference may be manifested, so that it seems that such strengths of the poison have no effect on growth, either harmful or beneficial. At still lower concentrations, with certain plants and certain poisons, the stimulative action overpowers the toxic effect, so that in some respect or other improvement occurs in growth. ”

“ In studying the general question of soil fertility, the laboratories of the Bureau of Soils in the United States Department of Agriculture have obtained some instructive evidence bearing upon this problem of plant excretions. The data obtained from different lines of experimentation all go to prove the truth of the general assumption that plants do excrete substances which may have a toxic action on the species of plants producing them. ”

“ In every other case the coppered plants were inferior, root and shoot, to those grown in the pure water. With the first three plants it appears that while the toxic water has a bad effect on the roots, yet the growth of the shoots is increased. The idea suggests itself that this apparent stimulation is in reality the result of a desperate struggle against adverse circumstances. The roots are the first to respond to the action of the poison, as they are in actual contact; their growth is checked, and hence the water absorption is decreased. ”

“ The collectors of medicinal plants cut from the roadside and rubbish heaps, plants whose "operations" for good are quite well known, and have been handed down by tradition for centuries, cannot be absolutely ignorant of the other side of the picture, the toxic properties which other plants, or sometimes even the same plants, contain. Foxglove, for instance, from which digitalis used as a medicine is extracted, is a good example of these kill-or-cure plants. Every portion of the plant is poisonous, leaves, flowers, stalks, and berries. ”

“ A solution may be of such a nature and strength that at first growth is seriously checked, though later on some recovery may be made, while it is also possible that a concentration which is apparently indifferent at first may prove more or less toxic or stimulant at a later date, according to circumstances. Consequently too much stress must not be laid upon the results of the short time experiments with regard to the ultimate effect of a poison upon a particular plant. ”

“ The toxic effect of all these acids is greatly reduced in soil cultures, or if particles of sand, graphite, clay, filter paper, etc., are suspended in the solutions containing the acids, the reduction in toxic effect being probably due to the adsorption of the acids upon the solid particles. Hence, the concentrations which limit the toxic effects of these acids in water solutions cannot be taken as representing the condition with which the same plant will have to contend when growing under normal cultural conditions. ”

“ Nearly forty years later Nobbe, Baessler and Will found that, if transpiration were hindered by placing plants in a dark or moist room, it was possible to keep [60] the plants turgescent in arsenic solutions for a long time without thereby increasing the toxic effect later on. The poisonous action proceeds from the roots, of which the protoplasm is disorganised and the osmotic action hindered. Finally, in the presence of sufficient of the poison, the root dies without growth. ”

“ Their water culture experiments were carried on in glass vessels coated internally with paraffin to avoid solution of glass, and in tests with seedlings of maize, lupins, oats and soy beans it was found that such metals as copper, iron, zinc and arsenic were almost always fatal to the growth of plants. As a general rule those metals whose salts are toxic, themselves poison plants when they are present in water. ”

“ It is quite conceivable, however, that some poisons are truly indifferent in weak concentrations, as no stimulation makes itself evident under any circumstances. In these cases one is inclined to suspect that the action is somewhat more simple, in that the toxic effects gradually diminish until no poisonous action is manifest at very weak concentrations, and as no stimulation is present to bring the growth above the normal with these very weak concentrations the plant is similar to those grown without any addition of the poison. ”

“ The results obtained with the different substrata showed that it does not follow that a salt highly toxic in solution is equally so in soil, or that one which holds a relatively high toxic position in soil should occupy the same relative position in solution cultures. For instance, while in soil cultures nickel compounds were the most toxic of all the substances tried, in solution cultures silver compounds were more poisonous than nickel. ”

“ Another method of water cultures is used by some investigators, in which the experiments only last for a few hours or days, usually 24–48 hours. While such experiments may not be without value for determining the broader outlines of toxic poisoning, they fail to show the finer details. The effect of certain strengths of poison is not [14] always immediate. Too great concentrations kill the plant at once, too weak solutions fail to have any appreciable immediate action and so appear indifferent. ”

“ For instance, Burlingham found that the typical nutrient magnesium sulphate in concentrations above m/8192 (m = molecular weight) is toxic to most seedlings, the degree of toxicity varying with the type of seedling and the conditions under which growth takes place. It will be shown in the following pages that even such a typical poison as boric acid may, under suitable conditions, increase plant growth just as if it were a nutrient. ”

“ There seems to be no doubt that certain organic compounds which are injurious to plant life are often present in the soil, either as the normal excretions of plant roots or as products of the decomposition of preceding plant growths. A consideration of these supposedly toxic organic substances would be out of place in this discussion of mineral soil nutrients. But there seems to be no doubt that there may also be mineral substances in the soil which may sometimes exert deleterious influences upon plant growth. ”

“ Alkaloids, and even the amino-acids which are produced in the tissues of some species of plants, while not toxic to the plants or organs which elaborate them, sometimes exhibit strikingly toxic action upon other plant organs with which they are brought into contact. ”

“ Not only may such insects weaken the infested plants, but they also cause the destruction of chlorophyll, interfere with the normal functioning of the stomata, and have a toxic effect upon the tissues; further, many serious plant diseases are carried and spread by sucking insects, whilst the punctures made when feeding may allow the entry of disease spores. ”

“ The same substance is sometimes much more injurious to the tissues of one part of a plant than it is to those of another part of the same plant. Thus, the rootlets of a young growing plant are much more susceptible to injury by many mineral salts than are the vegetative parts of the same plants; while anæsthetics of various kinds generally exhibit their greatest injurious effects upon the leaves, or synthetizing cells. Again, the mycelia of fungi are much more easily killed by toxic agents used as fungicides than are the spores of the same fungi. ”

“ Examples of specific toxicity are of only minor importance in plant studies. They seem to be generally explainable on the basis of some unusual lack of resistance or failure of the susceptible plants to be able to exclude the entrance of these injurious substances into the protoplasm by "selective adsorption," or to convert the injurious substances into insoluble and non-injurious forms, as is done by other plants which are not susceptible to injury by these "specific" poisons. Hence, particular attention need not be given to this type of toxins. ”

“ Stimulation may express itself in various ways—the green weight and the general appearance of the fresh plant may be improved, the dry weight may be increased, the transpiration current may be hurried up, entailing increased absorption of water and food substances by the roots, assimilation processes may be encouraged. But these benefits are [3] not of necessity correlated with one another, e.g. a plant treated with a dilute solution of poison may look much healthier and weigh far more in the green state than an untreated plant, whereas the latter may prove the heavier in the dry state. ”

“ Unfertilised eggs of insects have been started into growth by treatment with acids and those of frogs with mercury salts, and the germination of beans quickened by various poisonous alkaloids. In other words, graduated doses of [131] poison may alter the physiological activity of living cells, inducing pathological phenomena, while larger doses kill them.
Now we know at least one parasitic fungus which poisons the cells of its host, and kills them, with similar symptoms to those resulting from excessive doses of the above-named toxic agents. ”

“ It is a well-known fact that fruits and seeds are frequently poisonous. Therefore Metchnikoff supposed that the death of the plant may be due to an auto-intoxication by poisons manufactured by it in order to defend its seeds and ensure the next generation; in Nature, the individual does not count, but the species. Once the survival of this is ensured the individual may disappear.
A similar phenomenon of auto-intoxication is manifested by lower vegetables, yeasts, and microbes. ”

“ Plants contain very often inorganic elements in a form in which they cannot be utilized. It is therefore quite easy to mistake their presence either as a toxicity symptom or as a high requirement, when as a matter of fact these elements are present due to conditions unfavorable to metabolism, and they remained in bark and leaves as end products, in an inert form. Rather than being transmitted from soils to plant, their functions may consist of the formation of enzymes, proteins, hormones, chlorophyll, antibodies, vitamins, in carbon assimilation. ”

“ All parts are poisonous, particularly roots, shoots, and unripe berries. As the berries ripen, their poisonous properties lessen but, nevertheless, if many are eaten, they too can be harmful. Toxic materials include a resinous material and a water-soluble saponin. ”

“ A consideration of the experimental work that has been done on this subject of poisoning and stimulation leads one to the inevitable conclusion that it is not true to maintain the hypothesis that all inorganic plant poisons act as stimulants when they are present in very small quantities, for while some poisons do increase plant growth under such conditions, others fail to do so in any circumstances. ”

“ The proximity of a large town, especially if it is a centre of manufacture, is a notorious factor in the reduction of the native flora: not only by the thoughtless and wanton destruction carried out by its inhabitants, but more subtly by the deposition of soot, and by the poisoning of the air by sulphurous and acid fumes. The higher Cryptogams, such as Mosses and Hepatics, are particularly susceptible in this respect, and vanish along with the more delicate Seed Plants. ”

“ An honest herb abounds in good green coloring matter (chlorophyll) , that serves as a light screen to the cellular juices of leaf and stem. It also forms part of its digestive apparatus, aiding a plant in the manufacture of its own food out of the soil, water, and gases; whereas a plant that lives by piracy - a parasite - or a saprophyte, that sucks up the already assimilated products of another's decay, loses its useless chlorophyll as surely as if it had been kept in a cellar. In time its equally useless leaves dwindle to bracts, or disappear. Nature wastes no energy. ”