Stinging nettles at home and abroad—The use of the nettle—Sham nettles—Sensitive plants—Mechanism—Plants alive, under chloroform and ether—Telegraph plant—Woodsorrel—Have plants nerves?—Electricity in the Polar regions—Plants under electric shocks—Currents of electricity in plants—The singing of trees to the electro-magnetic ear—Experiments—Electrocution of vegetables.

THE common nettle is one of our most interesting British plants. It is exposed to great danger; one sees it growing not only in pastures and parks, but in waste places, along roadsides, and near cultivated ground. Yet it is very seldom either eaten or even touched. Cattle do occasionally eat the young shoots. But this is exceptional, for even in fields where there are plenty of cattle great clumps of nettle luxuriate and increase in size every year.

The stinging hairs are hollow and shaped rather like a narrow bulb or flask; the tip is slightly bent over and rounded (not sharp); the hairs contain formic acid. If one grasps the nettle or strokes it in a particular way (from below upwards) the hairs are pressed flat against the stem or broken, so that no wound is made by them in the skin and consequently they do no harm. But if the point of the hair pierces the skin, the well-known irritation is set up. That is because formic acid is poured into the wound. Besides the stinging hairs which keep off all the larger animals (including man) there are others, shorter and thickly set, which do not sting at all, but are intended to keep off snails.[96]

The pain produced by our common nettle is, however, a very trifling matter compared with that produced by some of the foreign species. One of the Indian kinds was used to excite and irritate bulls when they were intended to fight with tigers in the games which used to be held at some Indian Courts. Another found in Timor is called the Devil's Leaf; the effect of its sting may last for twelve months and may even produce death. But a still more dangerous stinging plant is a handsome tree (Laportea moroides) found in Australia. It is often 120-140 feet high, and has fine dark-green leaves often one foot in length. The sting is so powerful that even horses are killed by touching its leaves. The sting of Jatropha urens is so strong that people become unconscious. In Java also the sting of Urtica stimulans continues to smart for twenty-four hours, and may produce a fever which is very difficult to shake off.[97]

Yet our common nettle is the favourite food-plant of the caterpillars of the Small Tortoiseshell, Red Admiral, Peacock, Camberwell Beauty, and other butterflies.[98] These caterpillars are possibly more intelligent than many of our country folk, who do not know that the nettle is a very useful plant, as the following statements most clearly prove. Its young leaves make an excellent spinach, and it was, according to Sir Walter Scott, formerly cultivated in Scotland as a pot-herb. Pigs, turkeys, geese, and fowls like the leaves when they are chopped up. It is said that the dried leaves and seeds will make hens lay in winter time. The seeds, under pressure, yield quite a good oil. A yellow dye can be obtained by boiling the roots with alum. An excellent string can also be made from the inner bark of the stems, which has, in fact, been used to make twine and even clothing. The nettle is also valuable as an external stimulant in cases of paralysis.

A plant with so many wonderful properties would not be so common as it is, or so little disturbed, if it were not for its powerful stings.

There are one or two plants which are extremely like the nettle at first sight. Lord Avebury has an illustration in his excellent little book[99] in which it is most difficult to tell which are White Deadnettles and which are stinging nettles. No doubt the harmless deadnettle is helped to escape injury by this resemblance. The Hemp Deadnettle and some Campanulas are also very like it when growing. These also are sham nettles and may escape in the same way.

There are several common greenhouse Primulas which also produce irritation of the skin. When handled by gardeners a painful smart is set up which lasts for some time. Primula obconica is the worst of these, but P. sinensis, P. cortusoides, and P. Sieboldii sometimes have the same effect. In all these cases it is due to a peculiar secretion of certain glandular hairs.[100]

The methods of protection against grazing animals so far described, such as stinging hairs, thorns, spines, etc. (see page 190), are obvious enough, but perhaps the most ingenious system of defence is that exhibited by the Sensitive Plant and a few others. When man or any heavy animal is approaching certain Indian plants, their leaves suddenly drop, and the leaflets close together. The mere shaking of the ground or of the air produces these extraordinary movements in the sensitive Woodsorrel (Oxalis sensitiva), in two Leguminous plants (Smithia sensitiva and Aeschynomene indica), and in several Mimosas.

When one leaf-tip of Mimosa pudica, the Sensitive Plant (par excellence), is touched or injured, a series of changes begin. All the little leaflets shut up one after the other; then the secondary stalks drop; after this the main stalk of the leaf suddenly droops downwards. After a short interval, the next leaf above goes through identically the same movements. If the shaking or injury is severe, every leaf from below upwards moves in the same way.

One probable advantage of these movements can be understood from the behaviour of flies, which alight upon the leaves and make them drop. The flies are startled and go away. Grazing animals will consider such behaviour in a vegetable as very uncanny, and will probably go to some other less ingeniously protected plant.

Of course such extraordinary behaviour has been a challenge to the botanical world, and there is an overwhelming mass of speculation, and observations about the Sensitive Plant.

It has been proved that the movements are caused by the thickened part at the base of the main stalk of the leaf. This is swollen, and full of water, and much thicker than the stalk itself. It is by this thickened portion that the leaf is kept at its proper angle. When the tip of the leaf is shaken or injured, the cells on the under side of this swollen part allow their water to exude into the spaces between them, and in consequence down comes the leaf-stalk. This is not, by any means, a full or even a sufficient explanation. There is certainly some peculiar sending of messages from the tip of the leaf to the swollen part itself. It is not safe to say that it is a nerve message, but the process resembles the way in which messages are sent by the nerves in animals. Not only so, but the contraction of the under side and a corresponding expansion on the upper side, resembles the muscular movements of contraction and expansion in animals.

It must always be remembered that plants are alive; their living matter is not in any way (so far as we know) essentially different from that of animals or of man. Their living matter (protoplasm) in leaf-stalks and leaves is cut up into boxes or cells, each enclosed in a case or wall of its own. Yet these are not entirely independent and unconnected, for thin living threads run from cell to cell, so that there is an uninterrupted chain of protoplasm all along the leaf, leaf-stalk, and stem.

In this particular case of the Sensitive Plant, the leaves at night regularly take up the position which they adopt when injured or shaken during the daytime.

The easiest way to produce the shrinking of the leaves is, as has been mentioned, to hold a lighted match a little below the leaf-tip. Severe shaking, a strong electric shock, or a railway journey will also produce closing of the leaves.

Under chloroform or ether, or if the atmospheric pressure is suddenly diminished, the leaves will also fall. In some respects they are very lifelike, for if too often stimulated they become "fatigued," and will not react unless a sufficient interval of rest is allowed them.

The reaction occurs very soon if the plant is in good condition: in less than one second it begins, and the leaf-stalk may fall in two to five seconds, but the recovery is very slow.

Vivisection is a cruel sort of proceeding, although it may sometimes be necessary. The most curious vivisections have been performed on Mimosa. When the leaflets are cut off, it is possible, on a stimulus being applied, to see water oozing out of the cut surface of the stalk. This would go to show that it is the water being discharged from the leaf-base that produces the movement.

There are, however, many points in the behaviour of the Sensitive Plant which have not yet been explained.

Possibly the curious Semaphore or Telegraph Plants, whose leaflets suddenly and without any obvious reason move with a jerk through an angle of several degrees, may also be protected from animals by this uncanny and unusual behaviour.

But though the Sensitive Plant is certainly protected from grazing animals by these movements, other advantages may be derived. Heavy rain, for instance, such as occurs in the tropics, will not injure its delicate leaves. Dust-storms will not damage it, and at night there will be no loss of heat by radiation. The "shrunk" or folded condition of the leaflets will decrease any chance of injury by raindrops, for the rain will not fall on the broad surface of the leaflets. A nearly vertical leaf also will not suffer the loss of heat which a horizontal one would endure.

Besides the plants mentioned above, there are several others in which by a rather severe shaking the leaves can be made to fold up. This is the case with the common Woodsorrel (Oxalis acetosella), with the False Acacia (Robinia), and a few others.

The former has a peculiarly delicate leaf. In cold, wet weather its leaflets hang limp and numb from the leaf-stalk all day. In fine weather they are spread out horizontally. On a fine sunny afternoon its leaflets may sometimes take a mid-day sleep, for they hang loosely down in the same way that they do in cold, wet weather or at night.

But in the Woodsorrel these movements are not for protection against grazing animals.

There are other examples amongst plants of a distinct sudden movement which begins whenever part of the plant is touched. The movements of tendrils have been already referred to. The Venus' Fly Trap and the Sundew will be mentioned when we are discussing Insectivorous Plants. There are also several flowers in which the stamens suddenly spring up when they are touched by an insect (Barberry, Centaurea, and Sparmannia), and in Mimulus the style-flaps close when touched (see p. 70).

All these cases seem to involve some sort of mechanism which replaces the nervous system of animals.

No very definite laws have yet been discovered as to the way in which plants are affected by electricity, but enough is known to show that there are many interesting discoveries in prospect.

Professor LemstrÖm has made some interesting experiments in the Polar regions which go to show that the rich development of plant life in that desolate region may be connected with the peculiar electrical conditions of the Polar atmosphere; the aurora borealis, which is a common phenomenon there, being also produced by those conditions.

Several writers have claimed that slight electric shocks given at frequent intervals help the growth of plants and especially quicken the germination of seeds, but it can scarcely be said that this has been proved. When a branch or leaf-stalk is wounded or injured by being tightly clamped in a vice, then it will be found that a current of electricity passes from the injured spot to the part that is untouched, and then in the reverse direction.

Changes of current are also produced when a leaf is suddenly exposed to light for a short time and then shaded.

One of the most interesting observations is that made by Major Squiers near Lorin Station, in America, where the California Gas and Electric Corporation of San Francisco has a long-distance transmission telegraph line. The power is transmitted at a voltage of 56,000 with a frequency of sixty cycles per second (three-phase). Major Squiers, from previous experiments, thought that a note corresponding to this frequency might be heard in a telephone receiver. The following was the result:—

"Upon connecting the telephone between two nails driven in any growing tree along the route of the line, and at a reasonable distance therefrom, the telephone responded to this note with great clearness, and when the distance was not more than 100 feet, the sound was very loud. For this experiment no microphone need be used, nor any source of electromotive force other than that induced in the tree itself, the telephone being connected directly between two nails driven into the tree....

"Several kinds of trees of various sizes and forms were examined along this power transmission line, and all were found to be singing with a loud voice the fundamental note characteristic of the line current. Indeed, the strip of vegetation along this line has thus been singing continuously, day and night, for several years, since the operation of the line began; it needed only the electro-magnetic ear to make the sound apparent.... "The general appearance of vegetation along this route is certainly vigorous."[101]

An interesting little experiment was carried out by the author in Glasgow, with the kind help of Professor Blyth, at the Glasgow and West of Scotland Technical College. By attaching one wire to the upper part of the stem of a young pot-plant whilst the other wire was inserted in the base of the stem, it was easy to show that an electric current was passing—at any rate, during the daytime. In the evening, however, this was not at all distinct. That such currents do occur in living trees seems to be admitted. A similar current was not found in a stick of dead-wood. The mere passage of the water through the plant in transpiration might, however, cause such a current, for the water is evaporated at the leaves.

A strong electric shock may of course electrocute a plant by killing the cells. It is possible to cause the Mimosa leaves to close by means of an electric shock.