R. Lloyd Praeger

“ Thus the very existence of plants upon the Earth depends on the adequacy of seed-dispersal. This being so, the imaginative mind, viewing the marvellous and infinitely varied contrivances of Nature, will possibly be struck more by the want of special provision for dispersal shown by the majority of the higher plants—their helplessness in this respect—than by the beautiful devices exhibited by the few. In the first place, seeds are inert, devoid of any power of locomotion—though in some instances the last act of the parent is to discharge them with an explosive action into the air. ”

“ In the inky darkness of the ocean depths, amid absolute stillness and a temperature little above freezing, plant life of any sort is unknown. Only the flinty skeletons of diatoms and other minute forms of vegetable life which inhabit the surface layers, raining slowly down throughout the ages, tell that plant life exists in the sea at all. ”

“ Thus far, then, as regards some of those peculiar members of our flora which make their living by the unusual method of stealing their neighbour’s goods, or which eke out their existence by the capture of animal food. Let us now take another line of exploration and consider the conditions which prevail on the loftiest portions of our islands, and how these affect the vegetation. Mountain-tops are always attractive and interesting places—the keen rarefied air, the freedom and openness of the summits, fill us with exhilaration. ”

“ SOME INTER-RELATIONS OF PLANTS AND ANIMALS The most important and fundamental difference between the animal and plant worlds is this: plants possess the power of manufacturing their food out of the inorganic materials of which it is composed, while animals cannot do this. Give an ordinary plant access to water with a pinch of mineral salts in it, to the air, and to sunlight, and by the agency of chlorophyll—the green colouring-matter of the leaves—the miracle will be accomplished, and dead materials transformed into living substance. ”

“ Before setting about discussing the various types of vegetation which our own country presents, it will be well to have a general idea of the extent to which the main types are developed, and of the amount to which agriculture has interfered with the native flora. We have seen that the natural vegetation of the greater part of the British Isles is woodland: yet so profoundly has human industry altered the face of the country that woodland, natural or planted, occupies only about one-twentieth of the surface of England, rather less of Scotland and Wales, and about one-seventieth of Ireland. ”

“ 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 ”

“ How vegetable life originated, from what inorganic forms it was evolved, is a secret which science has not yet discovered; but since those dim first beginnings it has never been absent from the Earth, so far as we know, and has increased and multiplied, and passed through a thousand changes to higher and higher forms, till it has attained to the beautiful and bountiful and varied plant world which we know, covering with a green mantle most of the land surface of the globe and filling the shallower lakes and seas ”

“ And now, just as a point is reached when at length we think we shall see our present British flora emerging fully from the obscurity of the ages, a dramatic interruption occurs, which confuses the record and brings us into difficulties of many sorts, giving rise to controversies which are still far from being settled. The climate becomes suddenly colder, and Europe is plunged into the rigours of the Ice Age. ”

“ In our familiar wild flowers, for instance, the young plant, at an early stage of its existence, while it is still minute, becomes covered with a coat often of very resistant qualities, and is then cast loose by the parent in the form of seed, mostly in great numbers, to achieve what travels it can before it takes root and settles down, like its parent before it, to a humdrum existence. ”

“ A porpoise may live on herrings, herrings on small fry, fry in turn on minuter organisms, and so on down the scale; but their ultimate source of food is the tiny Algæ which swarm in the water—the Plankton in Hensen’s original sense—which, alone in this chain, can build up their bodies out of the sea and air. That these minute plants can sustain the enormous drain upon them due to their use as a food-supply by myriads of larger organisms is due to their vast numbers and rapid increase. ”

“ The question of the Lusitanian and American elements in our flora has been treated at some length both because it offers one of the most interesting problems in British botany, and because it affords a good illustration of the far-reaching nature of the questions which may lie behind the occurrence on our hills or in our valleys of even the humblest plant or animal. Each organism has a long record behind it, stretching far beyond the earliest periods of human history ”

“ Human foreign intercourse is not so old but that the introductions which it effected are still easily discernible to the student: the plants which have come to us thus bear the imprint of their origin; they spread outwards from centres of human activity, and are absent from undisturbed areas; they cannot in most cases compete with the indigenous vegetation, and only exist by confining their attempts at colonization to places where man has ousted the native flora—such as tilled land, roadsides, railway tracks. ”

“ But before we pass to the consideration of the conditions where favourable environment results in a closed vegetation, we may suggest for consideration the following point of view: that for any plant, or group of plants with similar requirements, much of the world is a desert—that is, a place where conditions are such that it cannot live. ”

“ A plant, living on one spot and drawing, from that portion of the soil which its roots can reach, certain mineral salts essential for its continued growth, tends to exhaust the available supply of these materials, and the succeeding generation{50} needs to reach fresh ground if it in turn is to attain healthy development. And it is undoubtedly of advantage to plants, if they are to continue to exist on the Earth, to be able to jump barriers and to colonize fresh suitable habitats which may arise in the course of natural changes, which sooner or later may render old habitats untenable. ”

“ Clambering through a wilderness of stony screes we emerge at length on a bare grey tableland on which, in contrast to the rich country below, vegetation is strangely sparse, and bare rock is everywhere in evidence. ”

“ It is clear, therefore, that, as in the case of wind dispersal, we must look to exceptional circumstances, not normal conditions, to provide opportunities for long journeys on the part of seeds. But for the transfer of seeds from France to England, for instance, or from England to Ireland, it is clear that birds furnish a far more efficient medium than wind or water. In one important particular, dispersal by animals has a great advantage over dispersal by wind—that it is practically independent of the weight of the seeds. ”

“ The root of a young plant grows downward into the soil under the influence of gravity. Its tip, which has to force its way through the rough material of sand and clay, is beautifully protected by a special root-cap, which covers the growing point as with a cushion. The surface of the root-cap is slimy, to aid it in slipping forward, and its cells, which are being worn away constantly, are replaced by the growth of the interior. ”

“ But while the dispersal obtained by such means may often aid in spreading a species over a tract of land, it does not generally aid in the crossing of barriers, such as mountains or sea, on account of the limitations to the movements of such animals. To arrive at a true estimate of the importance of the animal kingdom in regard to plant migration, we have to study the movements, habits, and food of birds, to whose wanderings neither mountains nor seas set a barrier. ”

“ It must be pointed out that the insects which pollinate these specialized flowers have in many cases acquired modifications in their structure corresponding to the modifications in the flowers which they frequent. In the more specialized forms, indeed, plant and animal have become entirely dependent on each other; the plants would become extinct in the absence of the special insects through whose agency they are able by pollination to produce fertile seed; and the insects would likewise die out if the flowers to whose nectar and pollen they look for food were not available. ”

“ 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. ”

“ Vegetation as we know it on our world requires certain physical and chemical conditions for its existence. For instance, a temperature which, at least during the growing season, is well above the freezing-point of water is requisite; yet the temperature must remain a long way below the boiling-point of water; neither could plants as we know them exist in the absence of an atmosphere containing oxygen, carbon dioxide, and water vapour, and incidentally, by its capacity for retaining heat, warding off violent extremes of temperature which otherwise would be a daily and nightly occurrence. ”

“ If the seed on liberation is carried along by the wind, it will acquire almost immediately the horizontal velocity of the air-current, but it will at the same time move downward through the air with the same velocity as if the air was still—just as a body dropped in a railway carriage will fall at the same rate whether the train is moving or standing still. If we measure the speed of fall of a seed in still air, then we can easily deduce the distance to which it will be carried by a horizontal air-current of given velocity if liberated at any given height above the ground. ”

“ Most plants are more adaptable than these, and occupy a variety of habitats. The little Tormentil (Potentilla silvestris) , for instance, flourishes equally on hot banks by the sea, in woods, and on mountain-tops. The more accommodating a plant is as regards habitat, the wider its distribution tends to be, both locally and in a broader sense. But wide range does not follow of necessity from adaptability to a variety of conditions: the problem of plant distribution is not so simple as that. One species may be spread right round the world, yet be always found in a special habitat ”

“ Many of the high alpine plants are frozen solid every night only to be roasted alive by day; it seems amazing that any living organisms can endure under such circumstances. Yet it is not only species confined to areas where such extremes exist, and specially adapted thereto, which can resist them successfully. In Central Europe the Common Chickweed and Common Daisy are often frozen solid, so that leaves and stems snap between the fingers like sealing-wax, yet with a rise of temperature they continue growth quite unperturbed, just as they do in areas where frost is unknown. ”

“ The Seed Plants, then, achieved their wonderful abundance and variety owing to the highly stimulating conditions offered by a terrestrial existence; we must assign to all the existing types a long terrestrial ancestry. How, then, about the water plants whose leaves and flowers so decorate our lakes? There seems no doubt [13] that they are species which have left the land to resume the aquatic habits of their remote ancestors. With few exceptions they retain the aerial mode of pollination which is the pride of the specialized land plants. ”

“ These sedentary creatures, of which barnacles, sea-anemones, and zoophytes will serve as examples, once attached, do not move from the spot where they have settled down; but it is important to note that not only are their eggs or young mostly liberated into the water, and by it transported to new homes, but in their juvenile stages they often swim vigorously, and thus achieve a wide dispersal.{49} In the plant world, the higher forms, with very few exceptions, spend their lives attached to one spot, like sea-anemones, deriving their food-supply from the air and from the soil ”

“ 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. ”

“ Among plants, as distinguished generally from animals by the production and abundant use of chlorophyll and of cellulose, we have still existing on the Earth a range of forms extending from almost the most primitive organism that we can imagine up to the splendid Seed Plant, specialized in a hundred ways. ”

“ Every pool, every soil, swarms with bacteria, the lowliest form of life—organisms exceedingly minute, exceedingly simple, and capable of existing under highly diverse conditions both physical and chemical. Thence we can trace an irregular ascending scale through the Fungi, the Algæ, Mosses, Horsetails, Ferns, and Club-mosses, to the Conifers, and on to the highest of the Seed Plants, which exceed in their beauty of structure and complicated life anything that has gone before them. ”

“ The tens of thousands of feet of sedimentary rocks, laid down in slow succession on the floors of ancient seas and lakes, and still reposing layer upon layer, and no less the great gaps in the series produced when, raised into the air, deposition ceased, and thousands of feet of rock were slowly worn away and washed down again into the sea by the action of frost and wind and water, point to periods incalculably remote as measured by the standards which we apply to human history. A few thousands of years measures the span which separates us from the Neolithic Period ”

“ On mountain-ranges situated within the Temperate Zone, such as the European Alps, and much more on those nearer the Equator, the day temperature in summer is very high wherever the sun strikes, and while plants may have to withstand at night a temperature comparable to that borne by the Arctic flora, they must endure by day the most intense insolation.
Neither in the Arctic nor on the high hills does plant life cease merely on account of low temperature. Species belonging to many families venture even beyond the limit of perpetual snow. ”

“ The British Isles are geographically and biologically by no means a separate area, and they have derived their population, both plant and animal, by immigration at various periods of time from the great land area to the eastward. Our present flora proves the truth of this as a general assertion; a study of its constituents shows that it is essentially a reduced continental European flora. As we step from France across to England we lose a number of plants familiar on the French side. As we step again from England into Ireland a further number of plants disappears ”

“ Why, indeed, should there be plants at all? This great globe, with its whole land surface covered, save at the Poles and in desert regions, with green plants in ten thousand forms, is indeed something to be wondered at. ”

“ 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{139} 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. ”

“ Plants are not so abundantly preserved in the rocks as the skeletons and shells of animals, on account of their softer nature; but in the oldest known plants it is again clear that we are dealing with forms by no means primordial. It is the more interesting, then, to note that many very lowly forms of life have come down to us from times immensely remote, and are still present on the earth in abundance, swarming in every sea and in every pond, or nestling in damp crevices of the land ”

“ Animals, on the other hand, are dependent for their food-supply on organic material—that is, on either plant or animal substances; and since they cannot live by taking in each other’s washing—in other words, by eating each other—it follows that the animal world is dependent on the plant world for its continued existence. ”

“ PREFACE In the following chapters an attempt is made to deal, in a quite elementary way, with some of the wider aspects of plant life—to discuss questions which arise in the mind from a contemplation of the vegetation which clothes with a green mantle the surface of our own country. No essay is made to enumerate or define the plants to be met with in the different types of ground, or in the different geographical areas, which go to make up the British Isles: there are already plenty of excellent handbooks and local floras in which that aspect of native plant life is treated. ”

“ Although the world offers many different aspects of closed vegetation, they may all in a broad sense be reduced to two general types—namely, grasslands and woodlands, the former the result of a lighter, the latter of a heavier, rainfall: grasses and their associates requiring for their life-processes a much less amount of water than a tree vegetation. The British Isles lie within a broad belt that sweeps east and west across Europe, characterized by a prevalence of south-west winds laden with moisture, and yielding a tolerably heavy rainfall distributed throughout the year. ”

“ For another cause—the diminution of light—the deep woods are a desert for many plants of the sunny pastures, and vice versa. Plants vary very much as to their degree of adaptability to different soils and different climatic conditions. Some are highly specialized. Our salt-marsh flora, for instance, is, as regards most of its species, confined to its peculiar habitat. If on a map of Europe we coloured in its distribution we should find it formed a ribbon round the coast, except for a few dots where the plants have discovered inland salt springs or salt lakes, and have found their way to them. ”

“ Should it come in contact with a living stem of a suitable plant, it attaches itself to it by means of disc-like suckers, penetrates the tissues of its victim, draws out nourishment, and, growing rapidly, spreads from plant to plant, taking a couple of close turns round each stem after the manner of a lasso, and then sending in rootlets from the attaching disc, and sucking the life out of each as it goes. It has no roots, no leaves, no chlorophyll, being of a red or yellow tint, and is entirely dependent for its nourishment on the plants which it attacks. ”

“ It is generally conceded that life on our globe began in the water, and thence spread to the land. Very significant in this regard is the fact that all but the highest plants require the presence of external water for the act of fertilization, as the male cell swims through water to the ovum. Only the most recently evolved groups have shaken off this ancestral trait; and as regards the whole economy of plants the water relation remains, throughout the entire vegetable kingdom, the most obvious and universally important of the different relations existing between plants and their environment. ”

“ 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. ”

“ Let us turn now to the relations existing between flowers and insects, which form one of the most picturesque and romantic features of field life, and of which the materials for study and observation are ever at our own doors. What is a flower? A flower is a group of modified leaves set apart for the business of sexual reproduction. The essential parts or sporophylls are of two kinds, which may be borne on the same flower or on separate flowers on one plant, or on separate plants. These are the stamens, bearing pollen grains (or microspores) , from which male cells arise ”

“ 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. ”