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-à-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 chink 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.