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.

Natural grasslands, if undisturbed, possess a flora which has been built up during a long period of time, and which, like all purely natural plant associations, represents a delicate balance between its many constituents, which often include rare and shy species. If such land be once broken up, its flora will probably never again resume its former composition even if allowed to regenerate during a long series of years, for the alteration in the old substratum caused by its being turned over and mixed introduces new edaphic (_i.e._, soil) conditions which will not entirely pass away. As regards grazing, likewise, when land is pastured up to or near its full capacity, as is generally the case on enclosed areas, the weaker and often more interesting members of the flora tend to disappear. In primitive times all grasslands had, of course, their natural grazing inhabitants--in our islands deer of more than one species, sheep, and smaller creatures such as rabbits and geese--and so a total exclusion of grazing animals now would no more tend to reproduce exactly the flora of pre-husbandry days than does the excess of herbivores; but the present heavy stocking of the land is to be deplored by the botanist, even as it is rejoiced in by the economist. The more vigorous plants, and especially those which propagate themselves largely by vegetative means, survive, or even increase owing to the augmented food supplied by the manure which the animals provide; but many species fail to ripen seed, being either eaten or trampled; the rarer Orchids, strange ferns like the Adder’s Tongue (_Ophioglossum vulgatum_), and Moonwort (_Botrychium Lunaria_), and the other choicer denizens of the grasslands, tend to disappear.

Drainage is an obvious cause of loss to our flora. Whole lakes and areas of swamp, with their peculiar and to a great extent natural flora, have disappeared from parts of the country. Some of the most interesting marsh plants of the British flora--such as the two fine Ragweeds, _Senecio palustris_ and _S. paludosus_, and the Marsh Sow-thistle (_Sonchus palustris_)--have on this account almost vanished from our islands, like the Bittern and Great Bustard which are their companions.

Some lakes, again, have been ruined for the botanist by being used as reservoirs. The considerable changes of level which this involves is a thing to which plants are not adapted, and only a few can withstand it, such as the Water Bistort (_Polygonum amphibium_) and the Shore-weed (_Littorella uniflora_), which are equally at home on land or in water, being able to change rapidly their structure and mode of life to suit change of environment. As compared with a lakelet with a natural outlet, a dam with a sluice has always a much reduced and usually quite uninteresting flora.

The proximity of a large town, especially if it is a centre of manufacture, is a notorious factor in the reduction of the native flora: not only by the thoughtless and wanton destruction carried out by its inhabitants, but more subtly by the deposition of soot, and by the poisoning of the air by sulphurous and acid fumes. The higher Cryptogams, such as Mosses and Hepatics, are particularly susceptible in this respect, and vanish along with the more delicate Seed Plants. Mining centres are specially destructive of plant life, since, in addition to other drawbacks, the soil is often buried under masses of excavated material containing poisonous substances. If there is a purgatory for plants, it is surely found in such areas.

Other examples of the multitudinous ways in which human activities disturb and destroy native plant life will occur to the reader--the burning of moors in order to improve them as pasturage; in recent years the tarring of roads, which kills the pleasant wayside herbage and poisons the streams into which the road drainage is carried; and so on. The indictment is an overwhelming one, and, as said in the first chapter, the flora is now everywhere so altered that we can gain some idea of its original aspect only by a study of isolated fragments and much-adulterated samples.

But if the debit side of the account, as presented by the lover of nature, is heavy, it must not be forgotten that there are many items to man’s credit. Though our country’s vegetation has lost in scientific interest, it has gained vastly in both economic and æsthetic value by the introduction of useful and ornamental plants from all the Temperate regions of the world; and besides, a large number of species have followed in man’s footsteps, and, taking advantage of the disturbance of the native flora caused by his operations, endeavour with more or less success to establish a footing in the country. Before we trespass on the domains of arboriculture, horticulture, or agriculture, under which heads the cultivation of useful or ornamental plants divides itself, some consideration is required of those plants which, quasi-wild, are usually included in accounts of the vegetation under the head of aliens, denizens, colonists, and so forth. These constitute a quite considerable proportion of the total number of species found in any area which has felt the influence of man. For instance, in the county of Dublin, which, owing to its diversified surface--sea-cliff, sands, moorland, woodland, and cultivation--and its favourable climate--the warmest and driest in the country--possesses the largest flora of any similar area (354 square miles) in Ireland, the list of about 760 “wild” plants includes some 170, or over one-fifth of the whole, whose presence is attributable, directly or indirectly, to human activities. We may compare these figures with those drawn from a study of the flora of Kent, which faces across the Channel towards France just as county Dublin faces across the Irish Sea towards England; both are areas of early settlement and both lie in the main stream of traffic. In Kent we have to deal with a larger area (1,570 square miles), and a larger flora (1,160 species). We find that, of these 1,160 species, 146, or about one-eighth, are set down as owing their presence to man.[10] And so it is in all the more populous and highly tilled parts of our islands.

This question of alien plants, their past history and present standing, is one of the most puzzling with which the student of our flora has to deal. In the first place, most of them have been in the country for a long time, and the record of their introduction is lost. Next, while many of them are confined to ground disturbed by man, and thus clearly exist under man’s protection--however unwillingly that protection may be afforded--others have mixed with the indigenous flora, won a place in the closed native vegetation, and might be ranked as true natives were it not that a study of their general distribution raises doubts as to the possibility of their having arrived in our islands unaided--doubts which their known occurrence in gardens tends to confirm. Take the case of the Yellow Monkey-flower (_Mimulus Langsdorfii_). This has quite established itself in our native flora, in some places ascending mountain streams far into the hills, in others mingling with the rank flora of muddy estuaries. It _looks_ as aboriginal as any of the plants among which it grows: but the facts that the genus to which it belongs is American (with a few species in Australia and New Zealand), that it itself is found native in the western States and not in the eastern, and that it has been long cultivated in gardens, furnish convincing proof that it is really an alien. But it is seldom that the evidence is so satisfactory as in this case. More usually the range of the doubtful members of the flora is continuous, extending from regions where they are truly native to others where they are undoubtedly exotic. For instance, many annual plants of the Mediterranean region have followed the spread of agriculture across the former forest areas of Central and Western Europe into our own islands. Plants native in France have been transported into England, and English natives into Ireland; east Irish plants have spread westward--sixty years ago, save for a single record of _P. hybridum_, _Papaver dubium_ was the only Poppy known west of the Shannon; now all four British species occur, several of them in many places. The flora of Europe, as pointed out already, diminishes in variety as we pass westward into the outlying areas. Those species whose aboriginal distribution stopped short of the western limit of the land had no doubt a fluctuating western or northern or southern boundary to their range, dependent on temporary conditions. Thus, a hard winter might kill back a plant already at the limit of its natural range, or a warm summer, by ripening abundance of its seed, might result in its slight advance. The general effect of human operations has been to lessen competition and increase suitable habitats by the destruction of the native vegetation which occupied them, and this has resulted in a general advance of a large number of species. What renders the study of this advance so difficult is the fact that on all disturbed land the truly native plants which have been ousted are striving side by side with the immigrants to regain their former territories; and it is now often very difficult to disentangle them: to separate the sheep from the goats. If only we could have had a Watson’s “Topographical Botany” written five thousand years ago, before our restless race began to mess up the vegetation!

However, as has been said, what we have lost on one side we have gained on another. On every side bright immigrants meet the eye. Our old buildings and quarries often blaze with the Red Valerian (_Kentranthus ruber_) and Wallflower (_Cheiranthus Cheiri_); in fields Poppies of various kinds, Corn Cockle (_Lychnis Githago_), and Corn Blue-bottle (_Centaurea Cyanus_) add a glory to the rich green or gold of the cereals; dry banks and gravelly places are decorated with species of Melilot (_Melilotus_), Chamomile (_Anthemis_), Knapweed (_Centaurea_), and many others. The flora of harbours and docksides is often as cosmopolitan as the sailors of the ships by whose agency it came there; and the unfamiliar weeds--the gipsies and tramps of the plant world--which we encounter on roadsides, rubbish-heaps, and railway stations lend an additional interest to our botanical rambles.

Turning now to the plants which are used by man, it may be pointed out in the first place that the human race obtains much more, whether of profit or of pleasure, from the vegetable than from the animal kingdom. Flesh, whether derived from mammals, birds, or fishes; wool, silk, leather, oils, and so on, bulk much less than the grains, vegetables, fruits, timber, fibres, fodder plants, and other vegetable products which we use in our daily life. On the æsthetic side, again, while the beauty of birds and insects is a source of frequent delight, flowers play a part in daily life that the more delicate and sensitive animals can never do. Again, in the number of different species used, whether for profit or pleasure, the plant world takes precedence. This is especially the case as regards our farms and pleasure grounds, plants lending themselves much more readily to domestication than animals do. And so a suburban house may have a hundred or a thousand different plants in kitchen garden and flower plot, orchard, and shrubbery, while its animal dependents consist of a horse, a couple of dogs, a cat, some fowl, and a canary. So again a Botanic Garden may easily possess as many thousands of different species as a Zoological Garden contains hundreds.

This army of plants which human beings collect about themselves may be grouped under two categories--useful and ornamental. On a previous page (p. 136) a suggestion has been made as to how the cultivation of useful plants may have arisen. As now practised, this industry is the largest in the world, and with the growth of means of transport has ceased to be only or even mainly of local importance: we use every day wheat from Australia, rice from China, tea from India, cotton from the United States, timber from Norway. In some cases, as in the last, these materials are harvested as they occur in the wild state, but in the majority of instances the plants are not merely conserved, but cultivated; cultivation has led to selection of the best varieties; and continued selection has resulted in the production of forms often very different in appearance from the wild plants from which they originated. We cannot _create_ new forms; but by taking advantage of the innate tendency to vary which all plants display--some to a much greater degree than others--and by raising, generation after generation, the seeds of those individuals in which a certain abnormal feature is best displayed, we can produce an artificial race in which the selected character may be developed to an extraordinary degree. But we have not by this means produced a new species. Seedlings of such plants will tend to “throw back” towards the original form; we can preserve or improve the special characters only by continued selection; if allowed to grow and seed unchecked, most of such plants will revert to the natural type in a few generations. Often this reversion is so rapid that seeds are useless for cultural purposes, and it is only by cuttings or graftings--that is, by growing parts of the original possessor of the required characters--that constancy can be maintained; this is what is usually done in the case of fruit-trees, Roses, Pansies, and so on.

Equally efficient in the hands of the cultivator has been another method of producing new forms--namely, hybridization. If the pollen of a plant be transferred to the stigma of a related species, offspring is often produced; and the product is a batch of plants intermediate in characters between the two parents, and generally uniform in appearance. Should these be crossed again, a heterogeneous offspring is the result, displaying a variety of characters inherited from one or other original parent. The crossing of varieties, native or cultivated, has the same result. Hybrids occur in nature, but not very frequently. Insects visiting flowers are well known to confine their attention to a great extent to one species at a time, so as agents of hybridization they are not efficient. Again, many hybrids do not produce fertile seed, so that if they arise by natural means they are not perpetuated. In the garden, hybridizing has been resorted to largely; but its practice is not so ancient as the method of producing improved breeds by selection.

The cultivation of specially selected forms is certainly of remote origin, and probably goes back to the earliest days of agriculture: of early date, too, is the introduction into regions where they do not occur naturally of plants desirable for their use or beauty. The records of the cultivation of the Vine, for instance, go back for five or six thousand years in Egypt. Two thousand years ago Pliny writes that ninety-one principal forms could be reckoned in his day, though “the varieties are very nearly as numberless as the districts in which they grow.” Theophrastus, three hundred years earlier, discourses learnedly of the different kinds of cultivated Figs, etc., and their superiority over the wild kinds. These and other authors make frequent mention of plants introduced into Greece or Italy from the East for their usefulness or their pleasing qualities. Nowadays, the number of species cultivated, the innumerable forms of these which are grown, and the wide distribution which these forms have attained, have resulted in the cultivated flora of a country like England being, so far as the higher plants are concerned, much larger than the native flora, even when all the plants which are grown under glass are left out of consideration.

In the case of plants of economic importance, the usual aim of selection has been increase of size or productiveness of the parts which are useful. In some instances selection has taken several directions inside the limits of a single species, as in the forms of Cabbage, which are all the offspring of _Brassica oleracea_ (Fig. 25), a seaside plant of Western and Southern Europe, and are mostly creations of comparatively recent date. The Cauliflower has been produced by increasing the size of the inflorescence; White Cabbage by promoting leaf production; Brussels Sprouts by encouraging the development of axillary shoots; while a form with a tall and woody stem is made into walking sticks. More often we find a species developed along a single line. For instance, the tendency to store food materials in a fleshy taproot has been developed in the case of Turnip, Beet, Carrot; the fleshy scale-leaves which form bulbs have been exploited in the case of the Onion; increased stem-growth is promoted in Asparagus; increased leaf-growth in Spinach and Lettuce; while by the development of seeds and fruits of many kinds artificial selection has supplied us with the foods on which the human race mainly subsists. The most important of all these last are, of course, the different grains, which are the seeds of grasses of various genera--_Triticum_ (Wheat), _Hordeum_ (Barley), _Secale_ (Rye), _Avena_ (Oat), _Panicum_ (Millet), _Oryza_ (Rice),

_Zea_ (Maize). The value of these to the human race is incalculable, and some of them have been in cultivation for at least five thousand years. In some of them, indeed, the native form is now unknown, the improved varieties alone having been preserved by the care of man. The Wheats are a case in point. While a wild grass growing in Palestine has been quite recently identified as the probable source of the Hard Wheats, the native parent of the Soft Wheats is unknown. That productiveness has in all cases been much increased by long selection there can be no doubt; it may be pointed out that several species of _Triticum_, _Hordeum_, and _Avena_, allies of the Wheat, Barley, and Oat, are included in the native British flora, but they are useless as producers of grain.

Nowhere is the effect on plants of selection and cultivation seen better than in our native fruit-trees. We have only to compare the size, flavour, and almost endless variety of apples and pears with the fruit of the wild stock of these two species--the Crab (_Pyrus Malus_) and Wild Pear (_Pyrus communis_) of our hedgerows--to realize how much has been accomplished. In garden flowers, also, we see most striking results of continuous selection. By taking advantage of the tendency of stamens and carpels to change occasionally into petals, and of petals to increase in number, “double flowers” have been effected. When “doubling” is complete--that is, when the conversion into petals is thorough--no seed can of course be produced, and the plants must be propagated by cuttings. Different other slight natural variations, exaggerated by selection and cultivation, have been the source of innumerable “varieties” in our gardens.

Sometimes the natural variation is by no means slight, but of a striking character which the efforts of gardeners have not succeeded in developing further. Take, for instance, the case of fastigiate trees, such as the Lombardy Poplar (_Populus nigra_, var. _italica_) or the Irish Yew (_Taxus baccata_, var. _fastigiata_). These are freaks or sports, the character being that _all_ the branches, not only the leader, tend to assume a vertical position. The Irish Yew originated as a wild “female” (pistillate) seedling found on the hills of Co. Fermanagh about 1780 and never rediscovered. It appears to be a juvenile form, preserving throughout life its seedling characters--a kind of Peter Pan among plants. Of the Lombardy Poplar the origin is not known, but it was no doubt similar. Seedlings of the Irish Yew revert to the ordinary type, and all the Irish Yews in cultivation are pieces of the original plant grown as cuttings. Poplars, like the Yew, bear the “male” (staminate) and “female” (pistillate) flowers on different trees, and the original Lombardy Poplar having been a “male” it also can be propagated only by cuttings--probably seedlings would in any case revert to the usual form.

The reverse of this abnormal erect habit is seen in weeping trees, where the branches for unexplained reasons seek to grow downward. In nature this results in a creeping habit. If planted on a height the branches will deliberately grow downwards towards the ground. Cultivators graft such forms on the top of a tall stem of a normal specimen, with the result that we see in the Weeping Ash and similar gardeners’ productions.

Another large group of casual abnormalities is concerned with the colour of leaves. The Purple Beech is a case in point. It was not produced by selection, but arose naturally, no doubt as a chance seedling. In this instance the character is usually passed on to the offspring, most seedlings having similar purple leaves, though some individuals are green. The peculiar colour is due in this case to a pigment in the epidermis of the leaf; the green chlorophyll is duly present, though its colour is masked by the purple leaf-skin. To a different category belong the “gold” and “silver” variegations which are so much exploited in shrubberies and borders and greenhouses. These spots or stripes or tintings of pale colour on the leaves are due to the lack of chlorophyll in the chromatophores (chlorophyll corpuscles); sometimes to an absence of the chromatophores themselves; and this omission appears to be caused by an enfeebled condition of the plant. Variegated plants are weaker than normal ones, and hence do not tend to survive in nature. But gardeners have protected and propagated a large number of them. When the variegation arises, as it often does, on a branch of an otherwise normal plant, it usually is not reproducible from seed, and must be perpetuated by cuttings. But where it happens with seedlings, it is often more or less fixed, and may be reproduced generation after generation, as in the Golden Elder, Golder Feather, and the marginal-variegated form of Winter Cress (_Barbarea vulgaris_).

Flower colour is not so fixed as leaf colour, for obvious reasons, the green colour of leaves being due to chlorophyll, which is an absolutely necessary ingredient of the leaf if plant food is to be manufactured; whereas flower colour is merely for advertisement, and any pigment can be made to serve. In nature most flowers vary in tint, and some in a marked degree--take the little native Milkwort (_Polygala_), which may be blue or purple or white. Flowers offer great opportunities, therefore, to the gardener, and by selecting on the one hand and hybridizing on the other every known tint has been reproduced in some blossom. Adding to this the variability in size and shape of petals, and the tendency to “doubling,” the flower in the hands of skilful cultivators has been altered almost beyond recognition. Take the Roses, for example, with their infinite variety of form and colour. The bulk of them are derived from a dozen wild species, possessing comparatively small single flowers, white, yellow, or red--_Rosa centifolia_, _damascena_, _gallica_ (the source of the older Roses), _indica_, _moschata_, _odorata_, _rugosa_, _Wichuræiana_, with our native _arvensis_ and _spinosissima_. By selecting for colour, shape, and “doubleness,” both from the species themselves and from the offspring produced by hybridizing one of these with another, what a wealth of beauty has been developed! More than any other flowers, the Roses are the crown and glory of the gardener’s art. Well has the Rose been called the Queen of Flowers; but it owes its royal prerogative to man. Nature provided blossoms--elegant, but of no special promise--and a tendency to vary, of priceless value; human skill and industry have done the rest.