Summing up, then, what has been sketched in this chapter, we must think of our plant as a very complicated and wonderful machine, of which the terrestrial Seed Plant is the highest expression. Water is the basis on which its activities are founded—the currency in which all business is transacted. The amount of water contained in a growing plant is seldom realized. Even solid timber, when growing, is half wood, half water. A fresh lettuce loses 95 per cent. of its weight if the water is driven off by drying. Living in an aerial medium which tends to deprive it of moisture continually, and which furnishes water to the soil only intermittently in the form of rain, and often in sparing quantity, the plant envelops itself from end to end of its exposed portions in a waterproof cuticle; the only openings in its surface layer are the spongy tips of the root hairs on the one hand, and in the stomata on the other. These minutest of openings—so small that the number on a square inch of leaf surface often far exceeds a hundred thousand—might prove danger-points were they not most jealously watched over. But each is provided with a pair of guard-cells ready to close the opening at any moment; and where drought threatens, the whole of the stomata are found in concealed positions. An ample pipe-system extends from root, through stem, to leaf, but it does not communicate directly with the openings at either end. All material, whether liquid or gaseous, absorbed or given out, has to run the gauntlet of the living cells, which are jealous watchmen, and allow only selected substances to pass through them. The crude building materials and food materials are assembled in the leaves, where in cells spread out to the light the chlorophyll is massed. Under the microscope, the chlorophyll is seen to be located in minute granules embedded in the semifluid contents of the cells. Well may we gaze in wonder at these tiny green specks. Each is so small that although a couple of hundred of them are often present in each cell, they occupy but a very small proportion of its volume. The cells themselves are of microscopic size. The chlorophyll itself occupies only quite a small portion of the corpuscle in which it is immersed; yet on its activity as spread in this infinitesimal quantity through the leaves the whole organic world, animal as well as vegetable, depends.[9] Utilizing the energy which comes through space from the sun, it builds up organic compounds; from the energy thus stored comes all the varied life and vital movement which fill our world—the opening of flowers, the hum of insects, the march of armies, and our own restless thought; while its work in the distant past, laid by in coal and oil, warms our houses and drives our trains, factories, and steamships.
The work of the living chlorophyll accomplished, the food materials produced by its agency are sent by the pipe-system to all parts of the plant, for present use, or to be stored in root, stem, or leaf for future requirements.
Nor is our plant the passive, motionless thing that it may appear to be in comparison with animals and their larger movements. Active motion, local and general, though usually of relatively small amount, accompanies all plant-growth. Throughout root, stem, leaf, and flower transference of material is going forward vigorously. The root hairs and stomata are working at high pressure; the chlorophyll never ceases its activities while daylight lasts. Externally, the growing branches, leaves, and flowers also display incessant movement, sweeping the air in small circles, or in the case of climbing plants in curves of considerable amplitude. Alterations of illumination or of temperature produce other movement—bendings towards or away from light, the drooping of leaves and closing of flowers at nightfall, and so on.
All these phenomena of growth and movement culminate in the production of flowers, and in the remarkable developments by which, through the agency of pollen and ovule, a new generation is produced.
CHAPTER VI
PLANTS AND MAN
The appearance of man upon the Earth is an event of very recent occurrence, not only in terrestrial history, but in the history of organic life in the world. In the life-story which began somewhere in far pre-Cambrian times, the record of the whole of human activities occupies but the last paragraph of the last chapter. For millions of years—ever since the larger animals first abandoned the aquatic haunts of their ancestors and took to a terrestrial life—creatures great and small, of myriad kinds, including huge reptiles and amphibians, and later on a crowd of birds and mammals, have fed on land plants, without effecting any profound changes in the appearance of the mantle of vegetation which covered so much of the Earth’s surface. It has been left for the human race, in the course of the few thousand years that have elapsed since it emerged from an existence comparable to that of the beasts and birds, and learned the arts of peace and war, to effect such sweeping changes in terrestrial vegetation over wide areas, that its influence in this respect requires a separate chapter for its consideration.
The changes referred to are largely—though by no means wholly—due to the requirements of the art of husbandry; and to the history of agriculture we may look for information as to the time and place and nature of man’s conquest of the surface of the globe. At the period of the earliest human civilizations, such as those of Egypt and Mesopotamia, the domestication of plants and animals had already reached an advanced stage. Its origin lies far behind the historic period. We can picture in imagination the time when in all inhabited parts of the globe man wandered with no fixed abode, seeking food when he was hungry, and making no provision for the morrow. Residence in a spot which afforded a valued supply of food, such as an abundance of buckwheat or millet or dates or bread-fruit, might lead to a desire to encourage the growth of such useful plants by protecting them and their offspring; following on which might arise the practice of assisting their growth, and thus eventually of cultivating them. Selection of the most productive strains would gradually follow, and barter would cause the spread of useful plants over wider and wider areas. We can picture development from such rude beginnings into the regular cultivation of the soil and the enclosing of the cultivated areas for their protection. It is clear that such practices would not readily arise among nomadic tribes, nor among those inhabiting forest regions where the ground was densely covered by trees. An abundance of animal food would produce a race of hunters rather than of tillers of the soil; and as for forest regions, they are unsuitable for human development; forest races have never been pioneers of civilization. Before agriculture—indeed, before civilization in any form—could make much progress, a settled life was necessary, free from migrations in search of food or for the avoidance of enemies. Hence the earliest civilizations tended to arise in areas which were protected by natural ramparts from the irruption of rival tribes. Egypt had the desert on three sides, and the sea—an impassable barrier to early peoples—on the fourth. The valleys of the Euphrates and Tigris presented similar features. In both areas rich alluvial soil offered a full reward to attempts at agriculture, and the alternation of summer and winter encouraged the making of provision for the non-productive period by the taking advantage of the period of growth: conditions not present under the “endless summer skies” of Tropical lands, where an easy and perennial food-supply tended against the development of industry.
The basin of the Mediterranean—the cradle of the earlier Western civilizations from the time of Egypt down to Rome—was, then, also the cradle of European agriculture. These lands, with their wet winters and dry summers, the latter inimical to the development of tree growth, lent themselves to cultivation more readily than the great forest-belt which lay to the northward, sweeping across Europe from Britain to the Urals. Although there is clear evidence that grain was cultivated in Europe as far back as the Neolithic Period (say 7,000 to 5,000 B.C.), it seems established that when Roman agriculture stood at its perfection the peoples to the north were still mainly nomads, dependent for their food-supply on their flocks or on the chase. In Britain, Cæsar found corn grown in Southern England, but the centre and north were largely forest land tenanted by tribes living on flesh and milk, and clothed in skins. The vigorous colonization of the Romans may well have been accountable for the introduction into Britain of many of the farm plants still grown there. The wars of the next fifteen hundred years on the one hand, and the spread of agriculture on the other, caused the steady destruction of the forests, till at length England and Central Europe began to assume their present appearance. The draining of marshes and fens, the enclosing of land, went on steadily, and to a slight extent is going on still; within recent years, the European War has resulted in the disappearance of many of the remaining woods, and in the breaking up of fresh land.
From the point of view of the botanist, agriculture consists of the destruction of the plant associations which for some thousands of years have occupied the ground, and their replacement by other plants which are useful to man. The natural plant associations being the result of the survival of the fittest through a long period of time, while the farmer’s crops represent plants which do not grow naturally on the ground, nor often indeed in the country (while they are frequently artificial forms unable to reproduce themselves), it follows that the latter cannot compete with the former, and can be maintained only by the most careful protection. The native plants are always striving to reoccupy their legitimate territory, and the farmer is incessantly engaged in trying to keep them out. Agriculture, indeed, has been defined as “a controversy with weeds.” Incidentally, the suppression of the natural flora allows many weaker plants an opportunity of which they are not slow to take advantage. These may be natives, but are often annuals which have followed the spread of farming operations, or which are directly—though unintentionally—introduced by man as impurities in the seed which he sows.
Let us look a little more closely into the question of profit and loss in our flora resulting from agriculture. In the first place, whether the ground is tilled or grazed, the woodland which primitively occupied so much of it disappears. The plough and the scythe are fatal to all seedling trees. Little less fatal is the browsing of cattle and sheep, and even in rough pasture only thorny plants like Whitethorn and Gorse may be found battling successfully for a lodgment. Where woodland is used for pasturage, the delicate shade plants—Anemones, Wild Hyacinths, Primroses—soon die out. No young trees appear on the grazed surface, though hundreds of thousands of seeds may be shed annually over the ground. In the course of time the present trees will die, and only grass remain. How different is it where cattle are excluded and the scythe unused! Among the grass young trees spring up everywhere, and in the woods a dense undergrowth of saplings sheltering a varied shade flora makes its appearance; regeneration of the natural woodland proceeds apace.