Popular Science Monthly/Volume 35/August 1889/The Defensive Armor of Plants

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WHILE, as Darwin and his successors have established, plants are dependent to a considerable extent upon insects for the means of securing the fertilization of their seed, they are also liable to be eaten by them, and are in great danger from the voracious appetites of other animals. They are not, however, wholly without defense against these attacks, but are provided with armors of various kinds, by the aid of which they offer a more or less effective resistance to them. These methods of defense have been the subject of special investigation by Prof. E. Stahl,[1] of the University of Jena, whose work, "Pflanzen und Schnecken" (June, 1888), presents a most interesting chapter in the history of the vegetable struggle for existence.

While every plant has its enemies more or less numerous and dangerous, the number as a whole is not generally considerable. Some attack the young plant, others the adult; some one part of it, some another. They would, perhaps, be more numerous were it not for the effectiveness of the means of defense that the plant can present against them. These means are various, but without them vegetable species would disappear very quickly. The protection conferred by them is evident, but an enemy more or less is much for a plant. It is sometimes a question of life or death. The phylloxera alone has been competent to destroy the vine in France; and, if ruminants should add their attacks to those of insects against the thyme or euphorbia, those kinds would soon disappear. In some cases, as of thorns or nettles, the armor is easily discovered; in other cases it is internal, chemical, or toxic. The protection is evident, whatever its nature may be. The question arises whether it is fortuitous or the result of a selection among plants. We can hardly doubt what the answer should be. Selection has certainly played a considerable part in the matter.

M. Stahl's experiments were made in his own garden and in the woods in the neighborhood, and bore direct reference to the attitude of snails toward the plants. The questions were asked. What plants do snails prefer; what ones do they avoid, and why do they avoid them? The results of his study may be verified by almost any one. Several species of snails were observed; including special feeders, those which live wholly on mushrooms, and omnivorous snails, which, while preferring certain species, eat more or less of all kinds of plants, and sometimes accommodate themselves to animal food.

Pieces of mushroom were offered to the snails, a part of them fresh, others after having been macerated in alcohol, dried by evaporation, and washed. The different species varied in their behavior toward the food. The omnivorous snails would not eat, or would only touch the fresh pieces, but readily devoured those which had been treated with alcohol; but a special feeder ate the fresh pieces and left the others. Hence the author concluded that there exists in the fresh mushroom a substance soluble in alcohol that attracts some animals and repels others. It must not, however, be believed that the special feeders can only live on particular food, for they are capable of accommodating themselves to other kinds when it is necessary. That the ingredient soluble in alcohol was the essential element of the food was proved by the special feeders, which avoided the macerated and dried food, but returned to it when it had been soaked again in the alcohol by which that ingredient had been abstracted.

Some light is cast upon the bearing of this experiment by reflecting on the enormous quantities of food which the omnivorous snails in a state of nature require. A vine-snail or a slug will eat a quarter or a third of its weight of carrot or potato in twelve or twenty-four hours. Although their needs are but slight, they can hardly find enough to assuage their hunger, on account of the mechanical or chemical defenses which most plants offer against them. Thus, the garden snail causes immense destruction of the filbert-leaves in the spring; but it would cause more if these leaves did not contain certain chemical substances, for it eats them more greedily after they have been treated with alcohol. Though this sort of protection is only relative, it will appear very considerable when we reflect upon the abundance and fertility of some species of snail.

Examining a garden near Jena after a warm rain in April, of forty-four snails of the species hortensis, fruticum, and arbustorum, ten were found upon living plants, while the thirty-four others were eating dead leaves. These three species, therefore, most usually attack dead plants. Helix pomatia, on the other hand, was observed almost exclusively upon living species. Experiments in which this species, with Helix hortensis and Limax agrestis, a voracious all-feeder, were put in presence of several plants having strong odors and pronounced flavors, showed that their tastes as toward living plants were very different. These experiments tend to show that the living plants are protected to a greater or less extent by the presence of some constituent disagreeable to the snails, which we may regard as a defensive armor to them. The dead parts of the plants were preferred, although as a rule dried vegetable is less alimentary than fresh, because the disagreeable substance had been removed or weakened by evaporation. Other experiments show that this kind of armor is, as a rule, the most effective.

When a drop of the juice of sorrel, garlic, saxifrage, or nasturtion is put upon the tegument of a snail, the animal manifests pain and exudes abundance of its mucous secretion; yet it is not thus affected by a drop of water. When snails avoid plants marked by such juices, we have a right to regard the plants as defended by a chemical armor. The offensive substance may also be important to the nutrition of the plant, but that is not the question we are dealing with here. Many plants are evidently lacking in this means of defense; for, of some plants, all the animals experimented upon have been found to prefer fresh to dead parts. Others are never touched by them, whether living or dead. Hence we may conceive that an infinite variety may exist in the degrees of chemical armoring between total absence of protection and complete protection.

Plants containing perceptible tannin are disagreeable to nearly all animals. Only swine will eat acorns as if they regard them as food. Other animals reject them, except when they can not get anything else. Leguminous plants containing tannin in weak proportions are eaten by horses and cattle, but snails are not fond of them. But the garden snail, which lets fresh clover alone, will eat it freely after the tannin has been extracted with alcohol. It is also probably tannin that inspires snails with respect for vetches, saxifrage, and stone-crop. Many water-plants, likewise, strong in tannin, are respected by water-snails, while the treatment with alcohol converts them into savory dishes for the same animals. Other plants, like dock, sorrel, and begonia, contain oxalic acid in notable quantities, and are obnoxious to them when too freely mixed with their food. It is worthy of remark that if carrot, of which snails are fond, is soaked in solutions of tannin or oxalic acid, they will avoid it in proportion as it is strongly impregnated with the offensive substance.

Strongly acid substances are often found on the surface of the leaves of plants. M. Stahl casually perceived that a leaf of Œnothera caused a very pronounced acid sensation on contact with the tongue, which was due to the presence of a superficial acid. On examination he found the same property present in other plants of the Onograceæ and Papilionaceæ. The acid is secreted by numerous one-celled cylindrical hairs. It consists of a mixture of oxalic, acetic, and malic acids, and, being very disagreeable to slugs and snails, constitutes an efficacious protection against their ravages. A simple contact of its tentacles or teguments with the secretory hairs is enough to cause the animal to draw back and go somewhere else to indulge its cravings. But if the leaves are washed, and the hairs cleansed of the acid secretion, they will be eaten at once.

Many plants are furnished with strong and pungent ethereal oils or similar substances. Prof. Tyndall thinks that these essences help to protect the plant against excessive heat. Without disputing this, M. Stahl finds that they are also efficient in defense against animals. This was proved with respect to rue, calamus, peppermint, dictamnus, and crane's-bill; and snails would at once turn out of the way to avoid a crushed leaf of the latter when placed in their road. Bitter leaves were avoided when fresh; when dead, even those of the gentian were relished, although the fresh ones were rejected by very hungry animals. The expressed juices were very disagreeable to them. The bitter was evidently the unpleasant quality, for the plants in question were free from tannin.[2] The liverworts, according to W. Pfeffer's researches, contain fat substances, the function of which is unknown, but to which Mr. Stahl ascribes a protecting agency. It is certain that, though they are easily accessible to all animals, they very rarely present any traces of having been attacked by them; and land snails respect them in a very marked manner. Even after fourteen days of fasting, Helix hortensis could not resolve to eat the thallus of Pellia. But there are genera (Lunaria and Marchantia) of which the less delicate snails will consent to eat a little. When the thalluses are treated with alcohol, the mollusks accept them readily; and there are some, like Plagiochila, that they will even eat fresh, in spite of their disagreeable smell, because of the much sugar that is in them. But most plants of the order are avoided, because of the unpleasant taste and smell given them by their fats. It is not always easy to determine of what other uses disagreeable or toxic chemical constituents may be to the plant; the point that concerns us in this discussion is, that they protect it from being eaten, and of this there can be hardly any doubt.

M. Stahl's study of the mechanical defenses of plants is no less interesting than that of their chemical armor. Many of the weapons of this character are obvious and well known; but some of them are more difficult of discovery, while a great variety prevails among them. In the large majority of cases the mechanical defense consists of a hardening of some parts of the plants, which may be general, so as to form a kind of carapace, or local, in the production of hard special organs, such as hairs, thorns, or needles, making it harder for animals to reach the plants. Sometimes the mechanical weapons are associated with chemical qualities, as in the nettle, crane's-bill. Primula sinensis, blessed thistle, etc. They either serve to prevent or impede the access of snails and slugs, to make it harder for them to take hold of the alimentary part, or to cause pain during the eating.

Hairy plants certainly offer more obstacles to snails going about on them than do glabrous plants. If we place a snail upon a comfrey-plant, it will find itself very uncomfortable, unable to get any hold on the leaves, and continually brought to a stop by the disagreeable contact of the hairs with its tentacles; and a free snail or slug will be hard to find on this plant. Other hairy plants possess immunity in less marked degrees; and M. Stahl's conclusions from his experiments as a whole are that chemical armor is more efficient than hairs. In some cases downy plants were preferred, while chemically armored species were always respected. So, when glabrous and downy species of the same family were tested, downy ones were eaten, while smooth ones were left alone. Hence, the hairs afford only an inefficient defense. M. Stahl accounts for this by supposing that, while the smooth plants are protected by disagreeable chemical constituents, the hairy plants are without this armor, or else present attractive qualities of odor or taste, against which their hairs are only an imperfect set-off.

Some plants are defended by the calcification of their superficial cells. The snails would not eat the leaves of Erysimum cheirantoides (treacle-mustard) when fresh, or even when treated with alcohol, but attacked them readily after the carbonate of lime had been dissolved out by acetic acid. The same was observed with other plants having a similar property. The grasses are protected against attacks from many animals by the silicification of the walls of their cells, without which the new enemies that would be added to supplement the assaults of their present foes would make an end of the whole family. This may be tested by offering to snails full-grown leaves and young, tender ones of the same grass. The latter will be taken and the others left. But if, by a method of cultivation proposed by Sachs, we make a normally siliciferous plant grow where it can get no silica, it will be devoured at once.

Some plants, that were avoided after treatment with alcohol as well as before it, were found to contain a gum which the alcohol failed to remove, and which stood between the snails and the edible substance. Among these were linden, althea, cactuses, and gummy roots. Another series of plants, including an Arum, narcissus, leucojum, and the balsam touch-me-not, which contain no tannin or gum or substances of disagreeable taste or smell, appeared to be protected by raphides. Tabernæmontanus recognized in 1587 that the leaves of these plants produce a violent sensation of burning in the bronchial tubes, and that it is not due to soluble products or juices, but to the raphides, which are abundant in their tissues. This is proved by the fact that the filtered juice of the pounded leaves does not produce the burning sensation, while the residue on the walls of the filter, and the pounded leaves themselves, produce the characteristic sensation that is felt after chewing the fresh leaves. It is also confirmed by the fact that if the leaves of Arum maculatum, for example, are treated with dilute hydrochloric acid, which dissolves the raphides, animals will readily eat them, while they let alone leaves treated with alcohol, even when they have been steeped in sugar-water. In the case of the squill, snails avoid the outside of the scales, which are rich in raphides, and eat the inner sides, which are free from them. So in the narcissus and orchids, and various other plants, there are parts protected by raphides which are objectionable to snails, and other parts free from them that they eat. But, while raphides protect against some animals, they do not against all. Birds and ruminants do not object to the plants containing them; and even snails manifest different degrees of aversion to them. In a similar manner to these plants with raphides, some species of iris are protected by crystals of oxalic acid. It is very probable that the kinds of armor that we have named are available for protection against other animals than snails. But investigation on this subject has not been sufficiently advanced to permit of definite conclusions or generalizations.

Of the kinds of defense named, a minority of the plants studied by M. Stahl possess but one; many are endowed with two; and some with three—as, for instance, Oxalis (oxalic acid, tannin, and hairs); Circæa (bitter hairs, tannin, and raphides); Smilax (thorns, raphides, and poisons); Aloe (leaf-teeth, raphides, and bitter substance); and Pontederia (crystals of oxalate of lime, raphides, and tannin). In fact, considering the number of enemies against which a plant has to contend to maintain its existence, their defenses are more numerous than we would suspect, and more important than we might at first believe.

In analyzing M. Stahl's results, we perceive that some families possess, as a whole, similar methods of protection; the grasses, sedges, and horse-tails, silicification; the rough-leaved orders, hairs; the Amaryllidece, Asparageæ, orchids, and Onagraceæ, raphides; the gentians, bitter substance; the rose family, geraniums, legumes, and heaths, tannic acid; the nightshades, alkaloids; the labiates, ethereal oils; mosses, mechanical means (by silicification); and liverworts, chemical means, and one genus of them, Riccia, mechanical means also.

Different genera in the same family sometimes present quite diverse means. Among the lilies are genera (Scilla and Ornithogallus) having raphides; others, alliaceous compounds; lilies, tulips, and crown imperial, poisons. There are also differences between the species of the same genus; thus, one species of Sedum is protected by tannin, and another by an alkaloid. And in the same plant there are often very notable differences between the leaves, fruit, and root.

M. Stahl asserts that he has not found a single phanerogamous species, living in a wild condition, that is not armed in some way against slugs and snails. Such armor is wanting only among cultivated plants, or, rather, among some of them. It appears as if at the moment when man cultivates a species of plant, or takes it under his protection, using all possible means to facilitate its existence and remove its enemies, the plant gives up its own means of maintaining the struggle, surrendering its defensive armor at man's invitation. The common lettuce is a striking example of this fact. It is a favorite viand, as all know, of the Gasteropods of the garden. Nothing protects it against their attacks, and its smooth, tender, and succulent leaves make it a ready prey to them; yet it is the descendant, modified by cultivation, of the Lactuca scariola, which has chemical constituents so distasteful to snails, and so constant, that they will not eat it even after it has been treated with alcohol.

The defensive armor of plants is most frequently situated upon their surface, or where the attack begins. This is particularly the case with the mechanical weapons and such chemical ones as tannin, special juices, etc.

When we consider how varied are these armors of plants and how generally spread they are among all the orders, and that without them some species would not be able to exist, it is hard to deny that there is some special adaptation in them, or to suppose that they are merely accidental. The case is undoubtedly one of natural selection; and the fact that the protection is gained sometimes in one way and sometimes in another, is not unfavorable to this hypothesis. It is with plants as with animals. One animal endures by means of his agility, another by his thick skin; another by this kind of defense, and another by that. The field opened by M. Stahl is one that has as yet been but little explored. It promises much that is novel, and bids fair to afford a new and most interesting chapter in the history of natural selection.—Translated for the Popular Science Monthly from the Revue Scientifique.

  1. Prof. Stahl's study is not the only one that has been made in this line, although it is perhaps the only experimental one. M. L. Erréra, of Brussels, presented a short memoir to the Royal Botanical Society of Belgium in 1886, in which he pointed out how experiments and observations could be carried on in reference to the subject. He drew up a table in which he classified the means of defense presented by plants as follows:—Biological characters: Plants at stations not easily accessible or with organs difficult of access, social plants, vassal plants, bullying plants (simulating dangerous species). Anatomical characters: Hard, cutting, or piercing organs, calcification, silicification, nettle-hair?, thorns, etc. Chemical characters: Acids, tannins, volatile oils, bitter properties, alkaloids, and glycosoids. M. Erréra adds a table of plants known to him which present one or another of the characteristics thus described. But his design was simply to show how great an interest the study might be made to afford. His views are confirmed by Prof. Stahl's researches.
  2. M. Stahl did not particularly concern himself with alkaloid?, although they must have played a considerable part in defense in some of the plants that he experimented with. On this point we may refer to some of M. Erréra's conclusions, as given in the paper of himself and Maistrian and Clautrian (Brussels, 1887) on the "Localization and Importance of Alkaloids in Plants": "The alkaloids can hardly be regarded as other than the waste of protoplasmic activity. In fact, it has been proved by experiment that they can not serve as nitrogenous food to plants, and are toxic even to the plant that produces them. . . . The recent researches of Armand Gautier in the animal kingdom bring a strong confirmation to these views. It may be said that a few grammes of an alkaloid protect a plant against the devastations of animals as effectually as the strongest thorns."