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.