Science in Arcady

The Greenwood Tree

It is a common, not to say a vulgar error, to believe that trees and plants grow out of the ground. And of course, having thus begun by calling it bad names, I will not for a moment insult the intelligence of my readers by supposing them to share so foolish a delusion. I beg to state from the outset that I write this article entirely for the benefit of Other People. You and I, O proverbially Candid and Intelligent One, it need hardly be said, are better informed. But Other People fall into such ridiculous blunders that it is just as well to put them on their guard beforehand against the insidious advance of false opinions. I have known otherwise good and estimable men, indeed, who for lack of sound early teaching on this point went to their graves with a confirmed belief in the terrestrial origin of all earthly vegetation. They were probably victims of what the Church in its succinct way describes and denounces as Invincible Ignorance.

Now, the reason why these deluded creatures supposed trees to grow out of the ground, instead of out of the air, is probably only because they saw their roots there.

Of course, when people see a wallflower rooted in the clefts of some old church tower, they don't jump at once to the inane conclusion that it is made of rock--that it derives its nourishment direct from the solid limestone; nor when they observe a barnacle hanging by its sucker to a ship's hull, do they imagine it to draw up its food incontinently from the copper bottom. But when they see that familiar pride of our country, a British oak, with its great underground buttresses spreading abroad through the soil in every direction, they infer at once that the buttresses are there, not--as is really the case--to support it and uphold it, but to drink in nutriment from the earth beneath, which is just about as capable of producing oak-wood as the copper plate on the ship's hull is capable of producing the flesh of a barnacle. Sundry familiar facts about manuring and watering, to which I will return later on, give a certain colour of reasonableness, it is true, to this mistaken inference. But how mistaken it really is for all that, a single and very familiar little experiment will easily show one.

Cut down that British oak with your Gladstonian axe; lop him of his branches; divide him into logs; pile him up into a pyramid; put a match to his base; in short, make a bonfire of him; and what becomes of robust majesty? He is reduced to ashes, you say. Ah, yes, but what proportion of him? Conduct your experiment carefully on a small scale; dry your wood well, and weigh it before burning; weigh your ash afterwards, and what will you find? Why, that the solid matter which remains after the burning is a mere infinitesimal fraction of the total weight: the greater part has gone off into the air, from whence it came, as carbonic acid. Dust to dust, ashes to ashes; but air to air, too, is the rule of nature.

It may sound startling--to Other People, I mean--but the simple truth remains, that trees and plants grow out of the atmosphere, not out of the ground. They are, in fact, solidified air; or to be more strictly correct, solidified gas--carbonic acid.

Take an ordinary soda-water syphon, with or without a wine-glassful of brandy, and empty it till only a few drops remain in the bottom. Then the bottle is full of gas; and that gas, which will rush out with a spurt when you press the knob, is the stuff that plants eat--the raw material of life, both animal and vegetable. The tree grows and lives by taking in the carbonic acid from the air, and solidifying its carbon; the animal grows and lives by taking the solidified carbon from the plant, and converting it once more into carbonic acid. That, in its ideally simple form, is the Iliad in a nutshell, the core and kernel of biology. The whole cycle of life is one eternal see-saw. First the plant collects its carbon compounds from the air in the oxidized state; it deoxidizes and rebuilds them: and then the animal proceeds to burn them up by slow combustion within his own body, and to turn them loose upon the air, once more oxidized. After which the plant starts again on the same round as before, and the animal also recommences da capo. And so on ad infinitum.

But the point which I want particularly to emphasize here is just this: that trees and plants don't grow out of the ground at all, as most people do vainly talk, but directly out of the air; and that when they die or get consumed, they return once more to the atmosphere from which they were taken. Trees undeniably eat carbon.

Of course, therefore, all the ordinary unscientific conceptions of how plants feed are absolutely erroneous. Vegetable physiology, indeed, got beyond these conceptions a good hundred years ago. But it usually takes a hundred years for the world at large to make up its leeway. Trees don't suck up their nutriment by the roots, they don't derive their food from the soil, they don't need to be fed, like babies through a tube, with terrestrial solids. The solitary instance of an orchid hung up by a string in a conservatory on a piece of bark, ought to be sufficient at once to dispel for ever this strange illusion--if people ever thought; but of course they don't think--I mean Other People. The true mouths and stomachs of plants are not to be found in the roots, but in the green leaves; their true food is not sucked up from the soil, but is inhaled through tiny channels from the air; the mass of their material is carbon, as we can all see visibly to the naked eye when a log of wood is reduced to charcoal: and that carbon the leaves themselves drink in, by a thousand small green mouths, from the atmosphere around them.

But how about the juice, the sap, the qualities of the soil, the manure required? is the incredulous cry of Other People. What is the use of the roots, and especially of the rootlets, if they are not the mouths and supply-tubes of the plants? Well, I plainly perceive I can get 'no forrarder,' like the farmer with his claret, till I've answered that question, provisionally at least; so I will say here at once, without further ado--the plant requires drink as well as food, and the roots are the mouths that supply it with water. They also suck up a few other things as well, which are necessary indeed, but far from forming the bulk of the nutriment. Many plants, however, don't need any roots at all, while none can get on without leaves as mouths and stomachs. That is to say, no true plantlike plants, for some parasitic plants are practically, to all intents and purposes, animals. To put it briefly, every plant has one set of aerial mouths to suck in carbon, and many plants have another set of subterranean mouths as well, to suck up water and mineral constituents.

Have you ever grown mustard and cress in the window on a piece of flannel? If so, that's a capital practical example of the comparative unimportance of soil, except as a means of supplying moisture. You put your flannel in a soup-plate by the dining-room window; you keep it well wet, and you lay the seeds of the cress on top of it. The young plants, being supplied with water by their roots, and with carbon by the air around, have all the little they need below, and grow and thrive in these conditions wonderfully. But if you were to cover them up with an air-tight glass case, so as to exclude fresh air, they'd shrivel up at once for want of carbon, which is their solid food, as water is their liquid.

The way the plant really eats is little known to gardeners, but very interesting. All over the lower surface of the green leaf lie scattered dozens of tiny mouths or apertures, each of them guarded by two small pursed-up lips which have a ridiculously human appearance when seen through a simple microscope. When the conditions of air and moisture are favourable, these lips open visible to admit gases; and then the tiny mouths suck in carbonic acid in abundance from the air around then. A series of pipes conveys the gaseous food thus supplied to the upper surface of the leaf, where the sunlight falls full upon it. Now, the cells of the leaf contain a peculiar green digestive material, which I regret to say has no simpler or more cheerful name than chlorophyll; and where the sunlight plays upon this mysterious chlorophyll, it severs the oxygen from the carbon in the carbonic acid, turns the free gas loose upon the atmosphere once more through the tiny mouths, and retains the severed carbon intact in its own tissues. That is the whole process of feeding in plants: they eat carbonic acid, digest it in their leaves, get rid of the oxygen with which it was formerly combined, and keep the carbon stored up for their own purposes.

Life as a whole depends entirely upon this property of chlorophyll; for every atom of organic matter in your body or mine was originally so manufactured by sunlight in the leaves of some plant from which, directly or indirectly, we derive it.

To be sure, in order to make up the various substances which compose their tissues--to build up their wood, their leaves, their fruits, their blossoms--plants require hydrogen, nitrogen, and even small quantities of oxygen as well; but these various materials are sufficiently supplied in the water which is taken up by the roots, and they really contribute very little indeed to the bulk of the tree, which consists for the most part of almost pure carbon. If you were to take a thoroughly dry piece of wood, and then drive off from it by heat these extraneous matters, you would find that the remainder, the pure charcoal, formed the bulk of the weight, the rest being for the most part very light and gaseous. Briefly put, plants are mostly carbon and water, and the carbon which forms their solid part is extracted direct from the air around them.

How does it come about then that a careless world in general, and more especially the happy-go-lucky race of gardeners and farmers in particular, who have to deal so much with plants in their practical aspect, always attach so great importance to root, soil, manure, minerals, and so little to the real gaseous food stuff of which their crops are, in fact, composed? Why does Hodge, who is so strong on grain and guano, know absolutely nothing about carbonic acid? That seems at first sight a difficult question to meet. But I think we can meet it with a simple analogy.

Oxygen is an absolute necessary of human life. Even food itself is hardly so important an element in our daily existence; for Succi, Dr. Tanner, the prophet Elijah, and other adventurous souls too numerous to mention, have abundantly shown us that a man can do without food altogether for forty days at a stretch, while he can't do without oxygen for a single minute. Cut off his supply of that life-supporting gas, choke him, or suffocate him, or place him in an atmosphere of pure carbonic acid, or hold his head in a bucket of water, and he dies at once. Yet, except in mines or submarine tunnels, nobody ever takes into account practically this most important factor in human and animal life. We toil for bread, but we ignore the supply of oxygen. And why? Simply because oxygen is universally diffused everywhere. It costs nothing. Only in the Black Hole of Calcutta or in a broken tunnel shaft do men ever begin to find themselves practically short of that life-sustaining gas, and then they know the want of it far sooner and far more sharply than they know the want of food on a shipwreck raft, or the want of water in the thirsty desert. Yet antiquity never even heard of oxygen. A prime necessary of life passed unnoticed for ages in human history, only because there was abundance of it to be had everywhere.

Now it isn't quite the same, I admit, with the carbonaceous food of plants. Carbonic acid isn't quite so universally distributed as oxygen, nor can every plant always get as much as it wants of it. I shall show by-and-by that a real struggle for food takes place between plants, exactly as it takes place between animals; and that certain plants, like Oliver Twist in the workhouse, never practically get enough to eat. Still, carbonic acid is present in very large quantities in the air in most situations, and is freely brought by the wind to all the open spaces which alone man uses for his crops and his gardening. The most important element in the food of plants is thus in effect almost everywhere available, especially from the point of view of the mere practical everyday human agriculturist. The wind that bloweth where it listeth brings fresh supplies of carbon on its wings with every breeze to the mouths and throats of the greedy and eager plants that long to absorb it.

It is quite otherwise, however, with the soil and its constituents. Land, we all know--or if we don't, it isn't the fault of Mr. George and Mr. A.R. Wallace--land is 'naturally limited in quantity.' Every plant therefore struggles for a foothold in the soil far more fiercely and far more tenaciously than it struggles for its share in the free air of heaven. Your plant is a land-grabber of Rob Roy proclivities; it believes in a fair fight and no favour. A sufficient supply of food it almost takes for granted, if only it can once gain a sufficient ground-space. But other plants are competing with it, tooth and nail (if plants may be permitted by courtesy those metaphorical adjuncts), for their share of the soil, like crofters or socialists; every spare inch of earth is permeated and pervaded with matted fibres; and each is striving to withdraw from each the small modicum of moisture, mineral matter, and manure for which all alike are eagerly battling.

Now, what the plant wants from the soil is three things. First and foremost it wants support; like all the rest of us it must have its pou sto, its pied--terre, its locus standi. It can't hang aloft, like Mahomet's coffin, miraculously suspended on an aerial perch between earth and heaven. Secondly, it wants water, and this it can take in, as a rule, only or mainly by means of the rootlets, though there are some peculiar plants which grow (not parasitically) on the branches of trees, and absorb all the moisture they need by pores on their surface. And thirdly, it wants small quantities of nitrogenous matter--in the simpler language of everyday life called manure--as well as of mineral matter--in the simpler language of everyday life called ashes. It is mainly the first of these three, support, that the farmer thinks of when he calculates crops and acreage; for the second, he depends upon rainfall or irrigation; but the third, manure, he can supply artificially; and as manure makes a great deal of incidental difference to some of his crops, especially corn--which requires abundant phosphates--he is apt to over-estimate vastly its importance from a theoretical point of view.

Besides, look at it in another light. Over large areas together, the conditions of air, climate, and rainfall are practically identical. But soil differs greatly from place to place. Here it's black; there it's yellow; here it's rich loam; there it's boggy mould or sandy gravel. And some soils are better adapted to growing certain plants than others. Rich lowlands and oolites suit the cereals; red marl produces wonderful grazing grass; bare uplands are best for gorse and heather. Hence everything favours for the practical man the mistaken idea that plants and trees grow mainly out of the soil. His own eyes tell him so; he sees them growing, he sees the visible result undeniable before his face; while the real act of feeding off the carbon in the air is wholly unknown to him, being realizable only by the aid of the microscope, aided by the most delicate and difficult chemical analysis.

Nevertheless French chemists have amply proved by actual experiment that plants can grow and produce excellent results without any aid from the soil at all. You have only to suspend the seeds freely in the air by a string, and supply the rootlets of the sprouting seedlings with a little water, containing in solution small quantities of manure-stuffs, and the plants will grow as well as on their native heath, or even better. Indeed, nature has tried the same experiment on a larger scale in many cases, as with the cliff-side plants that root themselves in the naked clefts of granite rocks; the tropical orchids that fasten lightly on the bark of huge forest trees; and the mosses that spread even over the bare face of hard brick walls, with scarcely a c***k or cranny in which to fasten their minute rootlets. The insect-eating plants are also interesting examples in their way of the curious means which nature takes for keeping up the manure supply under trying circumstances. These uncanny things are all denizens of loose, peaty soil, where they can root themselves sufficiently for purposes of foothold and drink, but where the water rapidly washes away all animal matter. Under such conditions the cunning sundews and the ruthless pitcher-plants set deceptive honey traps for unsuspecting insects, which they catch and kill, absorbing and using up the protoplasmic contents of their bodies, by way of manure, to supply their quota of nitrogenous material.

It is the literal fact, then, that plants really eat and live off carbon, just as truly as sheep eat grass or lions eat antelopes; and that the green leaves are the mouths and stomachs with which they eat and digest it. From this it naturally results that the growth and spread of the leaves must largely depend upon the supply of carbon, as the growth and fatness of sheep depends upon the supply of pasturage. Under most circumstances, to be sure, there is carbon enough and to spare lying about loose for every one of them; but conditions do now and again occur where we can clearly see the importance of the carbon supply. Water, for example, contains practically much less carbonic acid than atmospheric air, especially when the water is stagnant, and therefore not supplied fresh to the plant from moment to moment. As a consequence, almost all water-plants have submerged leaves very narrow and waving, while floating plants, like the water-lilies, have them large and round, owing to the absence of competition from other kinds about, which enables them to spread freely in every direction from the central stalk. Moreover, these leaves, lolling on the water as they do, have their mouths on the upper instead of the under surface. But the most remarkable fact of all is that many water plants have two entirely different types of leaves, one submerged and hair-like, the other floating and broad or circular. Our own English water-crowfoot, for example, has the leaves that spring from its stem, below the surface, divided into endless long waving filaments, which look about in the water for the stray particles of carbon; but the moment it reaches the top of its native pond the foliage expands at once into broad lily-like lobes, that recline on the water like oriental beauties, and absorb carbon from the air to their heart's content, The one type may be likened to gills, that similarly catch the dissolved oxygen diffused in water; the other type may be likened to lungs, that drink in the free and open air of heaven.

Equally important to the plant, however, with the supply of carbonic acid, is the supply of sunshine by whose aid to digest it. The carbon alone is no good to the tree if it can't get something which will separate it from the oxygen, locked in close embrace with it. That thing is sunshine. There is nothing, therefore, for which herbs, trees, and shrubs compete more eagerly than for their fair share of solar energy. In their anxiety for this they jostle one another down most mercilessly, in the native condition, grasses struggling up with their hollow stems above the prone low herbs, shrubs overtopping the grasses in turn, and trees once more killing out the overshadowed undershrubs. One must remember that wherever nature has free play, instead of being controlled by the hand of man, dense forest covers every acre of ground where the soil is deep enough; gorse, whins, and heather, or their equivalents grow wherever the forest fails; and herbs can only hold their own in the rare intervals where these domineering lords of the vegetable creation can find no foothold. Meadows or prairies occur nowhere in nature, except in places where the liability to destructive fires over wide areas together crushes out forest trees, or else where goats, bison, deer, and other large herbivores browse them ceaselessly down in the stage of seedlings. Competition for sunlight is thus even keener perhaps than competition for foodstuffs. Alike on trees, shrubs, and herbs, accordingly the arrangement of the leaves is always exactly calculated so as to allow the largest possible horizontal surface, and the greatest exposure of the blade to the open sunshine. In trees this arrangement can often be very well observed, all the leaves being placed at the extremities of the branches, and forming a great dome-shaped or umbrella-shaped mass, every part of which stands an even chance of catching its fair share of carbonic acid and solar energy.

The shapes of the leaves themselves are also largely due to the same cause, every leaf being so designed in form and outline as to interfere as little as possible with the other leaves on the same stem, as regards supply both of light and of carbonaceous foodstuffs. It is only in rare cases, like that of the water-lily, that perfectly round leaves occur, because the conditions are seldom equal all round, and the incidence of light and the supply of carbon are seldom unlimited. But wherever leaves rise free and solitary into the air, without mutual interference, they are always circular, as may be well seen in the common nasturtium and the English pennywort. On the other hand, among dense hedgerows and thickets, where the silent, invisible struggle for life is fierce indeed, and where sunlight and carbonic acid are intercepted by a thousand competing mouths and arms, the prevailing types of leaf are extremely cut up and minutely subdivided into small lace-like fragments. The plant in such cases can't afford material to fill up the interstices between the veins and ribs which determine its underlying architectural structure. Often indeed species which grow under these hard conditions produce leaves which are, as it were, but skeleton representatives of their large and well filled-out compeers in the open meadows.

It is only by bearing vividly in mind this ceaseless and noiseless struggle between plants for their gaseous food and the sunshine which enables them to digest it that we can ever fully understand the varying forms and habits of the vegetable kingdom. To most people, no doubt, it sounds like pure metaphor to talk of an internecine struggle between rooted beings which cannot budge one inch from their places, nor fight with horns, hoofs, or teeth, nor devour one another bodily, nor tread one another down with ruthless footsteps. But that is only because we habitually forget that competition is just as really a struggle for life as open warfare. The men who try against one another for a clerkship in the City, or a post in a gang of builder's workmen, are just as surely taking away bread and butter out of their fellows' mouths for their own advantage, as if they fought for it openly with fists or six-shooters. The white man who encloses the hunting grounds of the Indian, and plants them with corn, is just as surely dooming that Indian to death as if he scalped or tomahawked him. And so too with the unconscious warfare of plants. The daisy or the plantain that spreads its rosette of leaves flat against the ground is just as truly monopolizing a definite space of land as the noble owner of a Highland deer forest. No blade of grass can spring beneath the shadow of those tightly pressed little mats of foliage; no fragment of carbon, no ray of sunshine can ever penetrate below that close fence of living greenstuff.

Plants, in fact, compete with one another all round for everything they stand in need of. They compete for their food--carbonic acid. They compete for their energy--their fair share of sunlight. They compete for water, and their foothold in the soil. They compete for the favours of the insects that fertilize their flowers. They compete for the good services of the birds or mammals that disseminate their seeds in proper spots for germination. And how real this competition is we can see in a moment, if we think of the difficulties of human cultivation. There, weeds are always battling manfully with our crops or our flowers for mastery over the field or garden. We are obliged to root up with ceaseless toil these intrusive competitors, if we wish to enjoy the kindly fruits of the earth in due season. When we leave a garden to itself for a few short years, we realize at once what effect the competition of hardy natives has upon our carefully tended and unstable exotics. In a very brief time the dahlias and phloxes and lilies have all disappeared, and in their place the coarse-growing docks and nettles and thistles have raised their heads aloft to monopolize air and space and sunshine.

Exactly the same struggle is always taking place in the fields and woods and moors around us, and especially in the spots made over to pure nature. There, the greenwood tree raises its huge umbrella of foliage to the skies, and allows hardly a ray of sunlight to struggle through to the low woodland vegetation of orchid or wintergreen underneath. Where the soil is not deep enough for trees to root securely, bushes and heathers overgrow the ground, and compete with their bell-shaped blossoms for the coveted favour of bees and butterflies. And in open glades, where for some reason or other the forest fails, tall grasses and other aspiring herbs run up apace towards the free air of heaven. Elsewhere, creepers struggle up to the sun over the stems and branches of stronger bushes or trees, which they often choke and starve by monopolizing at last all the available carbon and sunlight. And so throughout; the struggle for life goes on just as ceaselessly and truly among these unconscious combatants as among the lions and tigers of the tropical jungle, or among the human serfs of the overstocked market.

An ounce of example, they say, is worth a pound of precept. So a single concrete case of a fierce vegetable campaign now actually in progress over all Northern Europe may help to make my meaning a trifle clearer. Till very lately the forests of the north were largely composed in places of the light and airy silver birches. But with the gradual amelioration of the climate of our continent, which has been going on for several centuries, the beech, a more southern type of tree, has begun to spread slowly though surely northward. Now, beeches are greedy trees, of very dense and compact foliage; nothing else can grow beneath their thick shade, where once they have gained a foothold; and the seedlings of the silver birch stand no chance at all in the struggle for life against the serried leaves of their formidable rivals. The beech literally eats them out of house and home; and the consequence is that the thick and ruthless southern tree is at this very moment gradually superseding over vast tracts of country its more graceful and beautiful, but far less voracious competitor.