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CHAPTER XV
ON NETTLES, SENSITIVE PLANTS, ETC

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.⁹⁷

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.⁹⁸

Yet our common nettle is the favourite food-plant of the caterpillars of the Small Tortoiseshell, Red Admiral, Peacock, Camberwell Beauty, and other butterflies.⁹⁹ 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 book¹⁰⁰ 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.¹⁰¹

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."¹⁰²

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.

CHAPTER XVI
ON FLOWERS OF THE WATER

The first plant – Seaweeds in hot baths – Breaking of the meres – Gory Dew – Plants driven back to the water – Marsh plants – Fleur-de-lis – Reeds and rushes – Floating islands – Water-lilies —Victoria regia– Plants 180 feet deep – Life in a pond, as seen by an inhabitant – Fish-farming – The useful Diatom – Willows and Alders – Polluted streams – The Hornwort – The Florida Hyacinth – Reeds and Grass-reeds – The richest lands in the world – Papyrus of Egypt – Birds and hippopotami – Fever and ague.

WHAT was the first green plant? When was the surface of the earth first covered with flowers? Such questions are quite impossible to answer. We cannot even tell how plants ever came to exist on the earth at all. Wonderful as are the stories of the hardihood of bacteria, of spores, and of seeds, it is not possible to imagine that they could have been whirled or drifted through infinite space to this particular planet.

Yet it is at least probable that the first real plant on this world was a seaweed or alga.

In Germany and Austria there are certain springs in which the water coming from immense depths is at an exceedingly high temperature. These hot springs are used as natural hot baths, and have many interesting peculiarities. Amongst others there is the fact that certain seaweeds or algæ are found luxuriating in the hot water. Some of these can even live in springs with a temperature of 176° F.!

Such algæ may have remained living in exceedingly hot water ever since that long distant time, the very first of all the geological periods, when there was no distinct separation betwixt land and water, and when the waters which were below the firmament had not been separated from those which were above it. Then the world seems to have been all fog and mist at a very high temperature.

But all theories on the origin of the world might be briefly summarized by the last nine words!

At any rate, the first plant was almost certainly a seaweed or alga not unlike those which produce the so-called "breaking of the meres."

At some seasons the water of certain lakes, usually quite clear and pure, becomes discoloured, turbid, and everywhere crowded with multitudes of tiny, bright, verdigris-green specks. The fish at once begin to sulk, refuse to take the fly, and live torpid at the bottom of the water. The minute green particles consist of a certain seaweed or alga. Mr. Phillips put the head of a common pin in the water so as to obtain a very small drop. When placed under a microscope, this minute amount of water was found to contain 300 individual algæ.¹⁰³ This was in Newton Mere (Shropshire), and as this lake extends over 115 acres, it is possible to imagine the millions upon millions of algæ which must have existed in it. The names of these seaweeds are many thousand times longer than the algæ themselves, and it is not really necessary to give them. One of them, however, Aphanizomenon flos-aquæ, has been noticed "tingeing with its delicate green hue the margin of the smallest of the Lochs Maben, in Dumfriesshire."¹⁰⁴ Yet it is not so big as the dot on the i in its name. Many other cases have been recorded of lakes that were coloured sometimes a "pea-green," or even brown or red by similar tiny little seaweeds. As we shall see, the water of such lakes generally contains a very large amount of suspended or floating vegetable life.

Another curious appearance is Gory Dew. Patches of a deep blood-red or purple colour are found on the ground or on walls. They have just the appearance of recently-shed blood. This also is due to an alga (Porphyridium cruentum). Dr. Cooke quotes from Drayton as follows: "In the plain, near Hastings, where the Norman William, after his victory found King Harold slain, he built Battle Abbey, which at last, as divers other monasteries, grew to a town enough populous. Thereabout is a place which, after rain, always looks red, which some have attributed to a very bloody sweat of the earth, as crying to Heaven for vengeance of so great a slaughter."

The ordinary "Rain of Blood" which appears on not too fresh meat, and looks like minute specks of red-currant jelly, is due to one of the Bacteria (Micrococcus prodigiosus).

The original algæ or seaweeds probably had descendants which migrated to the land and eventually after many geological periods became our flowering plants and ferns. But the earth has become so richly supplied with plants of all sorts and kinds that it is now by no means easy for any plant to find a roothold for its existence. So that a considerable number have been forced back to the water, and have accustomed themselves to live in or even under water in company with their lowly cousins, the seaweeds, who remained below its surface.

These water plants are very interesting. They are always competing with one another. There is a perpetual struggle going on round every pond and loch, and by every river side.

If you look carefully round the edge of a loch or pond which lies in a grass field, certain series of plants are generally found to follow one another in quite a definite way. The first sign of water in grass is generally the presence of moss or "fog" between the grass-stems and the appearance of what farmers call the "Blue Carnation Grass." It is not a grass but a sedge (Carex glauca or C. panicea) with leaves rather like those of a carnation. A little nearer the border of the pond, there may be a tall coarse grass (Aira caespitosa or Festuca elatior). Next there is almost certain to be a fringe of Rushes. Where the Rushes begin to find the ground too wet for them, all sorts of marsh plants flourish, such as Water Plantain, Cuckoo-flower, the Spearwort Buttercup, Woundwort, and the like. As soon as the actual water begins, one finds, whilst it is still shallow, the Flag series of yellow or purple Irises, Bogbeans, Marsh Cinquefoil, Mare's Tail, and Sedges of various kinds. In this part the water ranges from an inch or two to about eighteen inches deep.

The Flag or Iris is a very common and yet interesting plant. It has a stout, fleshy stem lying flat on the mud, and anchored to it by hundreds of little roots. The flower is the original of the Fleur-de-lis, or Lily of France, which took the fancy of the King of France as he rode through the marshes towards Paris. (It is true that there are some unromantic authors who hold that the emblem was really intended to represent a frog or toad!)

The flower consists of three upright petals and three hollow sepals, which make so many canals leading down to the honey, and roofed over by an arched and coloured style. As the bee hurries down the canal to its nectar, its back is first brushed by a narrow lip-like stigma and then dusted with pollen. The leaves overlap in a curious way, and, when they have withered, their stringy remains serve to protect the fleshy stem. Orris root, which is used in perfumery, is the stem of the Iris florentina.

Most of the other plants in this Flag series will be found to have prostrate main stems growing under the water, but giving off flowering and foliage stems which stand up above it, so that the leaves and flowers are above the surface.

In the next part of the pond, where the water is from eighteen inches to nine feet deep, masses of reeds will be found usually swaying, sighing, and whispering in the wind. There are many kinds, such as Bulrushes, Phragmites, Horsetail, Scirpus, etc. It seems to be the depth, the exposure to wind, the character of the soil, and other unknown factors, that determine which of those will be present. All of them are tall, standing well above the water; their main stem is usually flat on the bottom of the pond, or floating horizontally in the water, but giving off many upright branches.

Floating islands are often formed by some of these horizontal main stems breaking off and being carried away. Those Chinese who possess no land make floating islands of such reeds for themselves, and grow crops on them. There are hundreds of such islands in the Canton River.

Beyond the reeds, one sees the large flat, floating leaves and beautiful cup-like white or yellow flowers of the Water-lilies. They grow in water which is not more than fifteen feet deep. Their long stalks and leaf-stalks are flexible and yield readily, so as to keep the flowers and leaves floating. There are narrow submerged leaves as well. The actual stem of the White Water-lily is about three inches in diameter, and stout and fleshy. It is full of starchy material, and lies upon the mud deep down at the bottom of the pond. There are many advantages in the position of the flowers, for bees, flies, and other useful insects can reach them easily, but slugs, snails, and other enemies cannot do so. The little seeds have a curious lifebelt-like cup, which enables them to float on the surface.

Of course, our own British water-lilies cannot compare with the magnificent Victoria regia of the tropics. Its petals are white or pink on the inside, and its gigantic leaves, six feet or more in diameter, can support a retriever dog or a child. There used to be some of them at Kew Gardens. A curious point about these enormous floating leaves is that they are covered with little spiny points on the under side and at the margin; that is probably to keep some sort of fish from nibbling at the edges.

But to return to our pond. Beyond the water-lily region and so long as the water is from twelve to twenty-four feet deep, Pondweeds are able to grow, and their leaves may be seen in the water, whilst their stalks stand up above the surface so as to allow wind to scatter the pollen.

This depth of twenty-four feet seems at first sight very great, but it is a mere nothing compared with the regions entirely below the water, where certain Stoneworts (Chara) and Mosses have been found flourishing. The former has been dredged up from depths of ninety feet, and a little moss was discovered in the Lake of Geneva growing quite comfortably at a depth of 180 feet below the surface.

But it is quite impossible to appreciate the wonder and beauty of the life in a pond unless by a strong effort of the imagination.

Suppose yourself to be a fish two or three inches long, and accustomed to the dim, mysterious light which filters down through the water from the sky above. Every here and there great olive-brown leaf-stalks and stems cross and, branching, intercept the light. Everything, the surface of the mud, the stems and branches of the submerged water-plants, is covered by an exquisite golden-brown powder, which consists of hundreds and thousands of "Diatoms." Here and there from the Pondweed and other stems hang festoons or wreaths or threads of beautiful green Algæ. Little branching sprays of them, or perhaps of the brown kind, are attached here and there to the thick stems.

Even the very water is full of small, floating, vivid green stars or crescents or three-cornered pieces which are free floating Algæ or Desmids. Other diatoms are also free or swim with a cork-screwing motion through the water. Great snails and slugs crawl upon the plants, and weird large-eyed creatures, with a superfluity of legs and an entire absence of reserve as to what is going on inside their bodies, skirmish around. So that such a pond is full of vegetable activity. The free-swimming diatoms and desmids make up the food of the snails and crustaceans. These latter in turn are the food of fishes.

It is even possible to-day by carefully stocking an artificial pond with water plants, by then introducing Mollusca and Crustacea, and finally by the introduction of "eyed ova" or fry of the trout, carp, or other fishes, to produce a regular population of fishes which can be made more or less profitable, and the process can be spoken of as "fish-farming." Unfortunately there are a great many gaps in our knowledge as to what fish actually feed on, and we know even less about what the Mollusca and Crustacea require.

There is, however, a distinct annual harvest of these minute seaweeds, of which different sorts appear to develop one after the other, just as flowering plants do. The two months January and February, which are almost without flowers, are also those in which most of these minute vegetables take their repose in the form of cysts or spores.

But these diatoms are too important and too interesting to be dismissed in such a cursory manner. Each consists of a tiny speck of living matter with a drop or two of oil enclosed in a variously sculptured flinty shell. They have, in fact, been compared to little protected cruisers which pass to and fro in the water and multiply with the most extraordinary rapidity.

If you (1) use dynamite to blast a rock, (2) if you employ a microscope or telescope, (3) if you paint an oil picture, (4) if you make a sound-proof partition in a set of offices, the probability is that it has been necessary to use the substance diatomite in each case. This consists of the accumulated shells of myriads of diatoms.

Nor does that represent by any means the whole of the usefulness of these tiny seaweeds. The oil shales, such as occur in Linlithgowshire and elsewhere, are supposed to be the muddy, oily deposits of such ponds as we have endeavoured to describe. The oil found in the shales was probably worked up by these diatoms in long-past geological ages. It may be used to-day either (1) to drive motors, (2) to light lamps, (3) to burn as so-called "wax" candles, (4) to eat (as an inferior sort of chocolate cream).

Interesting as these diatoms are, it is not really possible to understand their structure without the use of a microscope, so that we must pass on to another side of the activity of water plants.

Let us, for instance, notice some of the ordinary plants to be found along a riverside. Willows and Alders are the ordinary trees, because they are specially fitted to stand the danger of being regularly overflowed. They easily take root, so that branches broken off and floated down are enabled to form new trees without much difficulty. In the United States, it has become a custom to plant Willows along the banks, because they are then not so liable to be broken down and worn away. Yet when a big Willow tree has become undermined, the weight of the trunk may cause it to fall over towards the water, so that a large section of the bank may be loosened and serious damage may be done if it is torn away by a heavy flood.

Amongst such Willows, should be mentioned the "cricket bat" kind, which has to be grown with the very greatest care, and of which a single tree may be worth £28.

Many of our rivers are, alas, sadly polluted by artificial and other impurities which kill the fishes and destroy the natural vegetation. When this happens a horrible-looking whitish fungus (Apodytes lactea) coats the stones and banks under water and the water swarms with bacteria. This fungus and the bacteria are really purifying the water, for they break up the decaying matter in it.

The oily or slimy character of the outside skin of all submerged plants is of very great importance to them. It allows the water to glide or slip over them without any friction.

Still keeping to our river bank, let us look for submerged plants. What is that dark green feathery plume? It is the Hornwort (Ceratophyllum) gently wriggling or moving from side to side. It has probably never been still for a moment since it first began to grow. Take it out of the water, and it collapses into a moist, unpleasant little body, but as soon as it is put in its natural element again it is seen to have a thin flexible stem along which there are circles of curved, finely divided leaves. Watch it in the water and one is filled with astonishment at the perfection of the shape, arrangement, and character of the leaves, which enables them to hold their place even when a flood may cover them with an extra twenty feet of water! The same sort of leaf, but with great difference in detail, is found in the submerged Water Crowfoot, Water Milfoil, Potamogetons, and others which live under the same conditions.