Pursuing this line of enquiry, we may endeavour to trace the descent through the ages of our present plants from bygone types; and coming at length to the still remote time—as measured by human standards—when the plants which now grow appeared on the Earth’s surface, we may try, from a study of their present distribution and of the distribution of their remains in regions where they are no longer found living, to determine their area of origin, and to trace the date and course of the migrations by which they reached our country. In the case of the British Isles, geological considerations play a leading part in such investigations, these islands being but outlying hummocks of a great continental area, at times joined to the main land-mass by a slight upward movement of the Earth’s crust, and anon cut off from it by a movement of depression. In this connection also we may be led to investigate the means by which plants spread, and especially their capacity for crossing barriers of the various kinds indicated in our brief study of deserts in the previous pages—the serious barriers offered by water-channels, or others equally difficult to negotiate produced by areas of uncongenial soil, by mountain ranges, or by forests. This will involve especially a study of seeds and the interesting phenomena of seed-dispersal.

Again, the most popular branch of botanical study in England is Floristic Botany, which traces the distribution within our area of the various species composing its flora; and with it is necessarily associated a study of the plants themselves so far as the characters are concerned, by which they may be distinguished from each other. This last is the province of Descriptive Botany. The study of local distribution, if conducted intelligently, will greatly assist in solving problems relating to the migrations and routes by which the existing flora reached its habitats.

Once more, we have already from Farleton Fell observed that plants do not grow higgledy-piggledy over the country, but are arranged in more or less definite societies depending on similarity of climate, soil, and other external conditions. Studied from this point of view, the flora resolves itself into a series of communities, each requiring a certain set of conditions for its continued welfare. The study of these inter-relations between plants and their environment, and of the types of vegetation resulting from the grouping together of plants requiring similar conditions, is the province of Ecological Botany.

Again, the morphologist deals with the forms of the organs of plants, and the changes which these undergo in different plants, while the anatomist investigates their minuter structure.

Physiological Botany deals with the life processes of plants, and the way in which they feed and grow and move. It has a very important bearing on the distribution and grouping of plants, since this is largely governed by their food-supply and by the need of surroundings which allow them to carry on their life processes with success.

It will be seen that there are many lines of enquiry open to the student of botany. In the following pages no more can be attempted than the preliminary study of some of the more familiar phenomena of plant life as it presents itself to the holiday-maker on the hills and woods and shores of our own land.

CHAPTER II

PLANT ASSOCIATIONS

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. Much of the former woodland is now represented by “arable land,” which covers over one-third of England, and about half that proportion of the other parts of the British Isles. Permanent grassland, partly natural, partly replacing ancient woodland, bulks large in England and Wales, occupying about two-fifths of the whole country; in Scotland and Ireland the proportion is much less, but in those countries a large area is under moor, heath, or natural grass, over which wander great herds of sheep and cattle. A. G. Tansley[3] thus contrasts (in percentages) the area of cultivated land (on which natural vegetation has been to all intents destroyed), with the area on which natural or semi-natural conditions still prevail:

It will be seen how little of the original vegetation of England is left to us for purposes of study—less than one-fifth, almost the whole of which has been influenced to some degree by human operations; while in Scotland and Ireland a much larger area is more or less in its primitive condition. The Scottish mountain-sides and Irish moorlands still to a great extent retain a natural flora, save that the greater number of grazing animals which they now support, as compared with the times when wolves and other enemies roamed unchecked, leaves its impress upon the vegetation.

Viewing the plant world as a whole, its primary divisions, from the point of view of ecology, are governed by the factor of rainfall. It is true that the plants of the Tropics differ profoundly from those of the Temperate regions, and those again from the plants of the Arctic. But this is a difference in the species and families which constitute the vegetation, rather than a difference in the types of vegetation or plant formations which occur. A certain area in Siberia may not have one species in common with a certain area in India, but in both we may find the three great vegetation types of forest, grassland, and desert. A rainfall gradient, on the other hand, will cause a progressive change in vegetation type, as may be seen in crossing North America from east to west, where the forests of the New England States give way as precipitation diminishes to the prairies of the middle States, and these again to the deserts which stretch far over the west. It is only in the extreme north that temperature, apart from precipitation, becomes the dominant influence in determining the presence or absence of vegetation, or its character.

Within any one climatic region—say within the British Islands—the soil in which the plants grow is the controlling factor in determining the character of the plant population. And while a classification by plant form—such as woodland, grassland—is often convenient, when we come to analyze the various plant associations which colonize the ground, it will be found that similarity of form-type does not necessarily imply affinity as regards either physiological conditions or floristic constituents. Thus, a Beech wood on the Chalk has really no affinity with an Oak wood on the Coal-measures, save that they are both woods: they shelter plant groups of quite different composition, one a constituent association of the Limestone Formation, and the other of the Formation of Clays and Loams, according to modern English classification. Similarly, the Hazel copse which covers the screes of Farleton Fell has no close relation to the Hazel copses along the Westmorland becks, although the dominant plant—the Hazel—is the same in both cases: soil is the controlling factor, and the one is related to the limestone vegetation of the hill above, the other to the vegetation of the loams and peaty soils of the adjoining mountain-side. In the British Isles the leading plant formations are those of clays and loams, of sands and sandstones, of siliceous soils, of calcareous soils, of peat, of marsh, of lakes and rivers, of salt-marsh, sand dune, and shingle beach; also, governed by the climatic factor, alpine vegetation stands somewhat apart. While the vegetation of some of these, such as salt-marsh or peat, usually presents a uniform aspect, others, such as the clays, sands, and limy soils, display each a characteristic type of woodland and of grassland, as well as other variants, dependent on the composition, depth, and wetness of the soil, the degree of exposure, and so on: these form the associations which together constitute the formation. Each association, if the plants composing it be examined, will be found to consist of an assemblage of species, large and small, brought together by their superior fitness for the particular conditions which prevail. There are mostly in each association one or more dominant species—such as the trees of an Oak wood, or the Heather of a moor—which by their abundance or vigorous growth control the association. The shelter which they give may protect some of the members of the community: the shade which they cast may keep out other plants which otherwise would invade the ground. The association will include some species specially adapted to the particular conditions which prevail, and perhaps not found elsewhere in the area; these are the indicator plants of the association, which give it its special character, and which will help us to identify the association should we encounter it again; there will be others—dependent species—which are attracted by the shade, or shelter, or other advantages which the growth of the dominant plants affords: and there will be others, again—probably many—of wide distribution, which are merely as much at home here as elsewhere. But all grow here because they are better fitted for the particular conditions prevailing than are the other plants of the surrounding area. On Farleton Fell, for instance, among the most abundant species which fill the crevices of the limestone plateau are two ferns—the Limestone Polypody (Polypodium Robertianum) and the Rigid Buckler Fern (Lastrea rigida). Though there is rocky ground of many kinds in the Lake District, these two plants are never found save on similar outcrops of the Carboniferous Limestone, and they are clearly specially fitted for life in the hollows of this rock. But the same rock crevices also harbour many species which are found equally on the soils derived from the slate rocks or sandstones. To take another instance: many of our most familiar spring flowers are woodland plants—the Primrose (Primula acaulis), Wood Anemone (A. nemorosa), Wild Hyacinth (Endymion nonscriptum). These rejoice in the humus soil which is formed from the dead leaves of preceding years; they flower before the trees are in full leaf, thus securing plenty of light and air for their period of growth; and they are accustomed to have their stems and roots protected from summer heat by the leafy canopy overhead. Transplanted into an adjoining sunny pasture they will soon die out. They are characteristic members of the woodland association of one or more formations. But with them we shall find other species, such as the Wild Strawberry (Fragaria vesca), which are equally at home on dry sunny banks or even on sand dunes.

If we ask why the plants group themselves into the associations which we may study any day in the country, in many cases the answer is not obvious. It is clear that while many species accommodate themselves easily to different soils or different degrees of light or of moisture, others have small powers of accommodation, and are in consequence restricted in their range. By long usage many plants have acquired special characters enabling them to live under special conditions—some examples will be discussed a little later—and in some such cases it is easy to correlate the peculiar characters of the plant with those of the habitat. But in many other cases the relation is not obvious. For instance, we cannot tell, by examining a plant, whether it is partial to a limy or to a non-limy soil; yet many plants are poisoned by lime, while others, though generally capable of growing in a soil devoid of lime (if planted in a garden), are nevertheless absent from the non-calcareous areas adjoining their limestone habitat; in other words, they can hold their own on limestone, but are unable to do so elsewhere. The two ferns already mentioned (Polypodium Robertianum and Lastrea rigida) are cases of the latter kind; while some of the most familiar of our hillside plants, such as Foxglove (Digitalis purpurea) and Broom (Sarothamnus scoparius), are instances of the former.

If, however, we consider some of the formations or associations which are the result of extreme conditions of environment, we get more light on the relations between the plants and the factors which control the vegetation. Take the case of the plants inhabiting desert regions such as were discussed in Chapter I. Here the outstanding feature is scarcity of water, and the plants display various remarkable adaptations which fit them for a thirsty life. There are three ways to meet scarcity of water—facilities for gathering it, arrangements for storing it, and economy in using it; and arrangements for all three are familiar features of desert plants. To effect the first, the root-system is extended, and is often enormously developed in proportion to the aerial parts. This adaptation may be studied in the flora of dry places in our own country, such as shingle beaches and sand dunes, which are characteristic semi-deserts. Take such plants as the Sea Holly (Eryngium maritimum), the Sea Convolvulus (C. Soldanella), or the Sea Sedge (Carex arenaria), and compare the extent of the root-system or underground stems with that of the aboveground portions. Fig. 4 represents the Wild Carrot (Daucus Carota) as found growing under extreme exposure on the west coast of Ireland. To meet the conditions the tall branched stem has been entirely dispensed with, and the terminal umbel is seated on the ground in the middle of a ring of leaves. In this way the plant prepares to resist both drought and wind. Water storage is often developed in different parts of xerophytes (drought-resisting plants)—in roots, or stems, or leaves, which become much enlarged, and at the same time covered with a

highly impervious skin, so that they act as veritable cisterns. In plants like the Cacti water storage in the stems is carried very far indeed; while in such genera as the Stonecrops (Sedum) the leaves are often so swollen and charged with water that they lose up to 98 per cent. of their weight if they are dried. Prevention of excessive loss of water by transpiration is effected in plants of dry places mainly by reduction in the size of the leaf and by protection of its surface. Leaf reduction is very marked in many dry countries. If we compare the flora of the Mediterranean region (a dry area) with that of Middle Europe or of England, we shall be struck with the prevalence in the former of small-leaved twiggy plants—Lavender (Lavandula) and Rosemary (Rosmarinus officinalis) will serve as examples. Often leaf-reduction is carried much farther, and we need not go beyond our own commons to find a good example, for in the Gorse (Ulex) flat leaves are entirely absent and the branches are shortened and converted into prickles, thus largely reducing the surface exposed to the sun and wind. The seedling Gorse has little trifoliate leaves, which remind us of its affinity to the Trefoils and Brooms, but they are discarded almost at once, to fit the plant better for life in the dry, breezy localities which it favours. Reverting to the Mediterranean flora, a characteristic of its plants is the prevalence of a grey hue in their stems and leaves, such as we see in the Pinks and Achilleas of our rock gardens. This is due to a coat of wax, as in the Pinks (Dianthus), or a felt of hairs, as in the Achilleas, designed to check excessive transpiration. The coatings of hairs are often of great beauty and complexity, and form an almost impenetrable covering to the leaf surface, protecting the upper side from the fierce rays of the sun, and on the underside sheltering the stomata, or minute openings through which the plant exhales the surplus water drawn up from the roots and inhales carbon dioxide. Another very beautiful device for protecting the underside of the leaf, and one which may be studied in many of our commonest plants, consists of the inrolling of the edges, often combined with a wrinkling or ridging of the underside, so that the stomata are set in deep hollows, communicating with the open air only through narrow openings. The leaves of some of our common grasses show these characteristics to great advantage. And again the stomata are often sunk in little pits, by which device they obtain further protection. If we now examine the plants composing the sand-dune or shingle-beach associations in the light of these facts, we shall find them full of interest. The plants are well equipped to meet the adverse conditions of a very porous soil, drying winds, and scorching sun. Note the grey felt of hairs which protects the leaves of the Horned

Poppy (Glaucium flavum), the tough, waxy skin which covers the Sea Holly (Eryngium maritimum), the extensive underground stem-systems of the fleshy-leaved Sea Convolvulus (C. Soldanella) and Sea Purslane (Honkenya peploides). Even the annual plants display similar characters. In the great desert regions the annuals are often quite normal in structure: that is because they appear during the brief rainy season, and pass away before the fierce heat of summer sets in. But on our shingle beaches the annuals grow throughout the summer, and need protection against drought: so the Sea Rocket (Cakile maritima), the Sea Whin (Salsola kali), and others are very fleshy plants; their leaves are small, with an impervious skin, their root-systems are better developed than in most annuals. The grasses and sedges of these places, such as the Bent (Ammophila arenaria), Sea Wheatgrass (Triticum junceum), Sea Sedge (Carex arenaria) have underground stems which burrow widely through the sand, with an extensive root-system, and tufts of inrolled leaves beautifully protected against over-transpiration and well worth microscopical examination.

If we turn from the shingle beach to the salt-marsh, where water is very abundant, we shall be struck by the peculiar fact that its vegetation displays characters quite similar to those we have just been studying. How can we reconcile this with the theory that the peculiar characters of the shingle-beach plants are correlated with lack of moisture? The explanation is to be found in the fact that plants have difficulty in absorbing water if it is highly charged with mineral substances in solution. In the salt-marsh the heavy muddy soil is impregnated with common salt (chloride of sodium): the plants absorb it with difficulty; and in consequence they are faced with the same main problem which confronts the Sea Holly and Sea Whin, and they meet it in the same way. Indeed, the salt-marsh plants appear to be more highly specialized, for very few intruders from outside can venture in, while on the beach we may meet with many plants which belong to other formations growing successfully, at least for a time. The salt-marsh flora is very exclusive, and contains but few species which we encounter in other situations. Some of them are also found on dry sea-rocks—the Sea Pink (Statice Armeria), Scurvy-grass (Cochlearia officinalis), Sea Aster (A. Tripolium), and so on; showing that soaking soil is in no way essential to their growth. (The first two reappear among alpine plants on some of our higher mountains, pointing again to an analogy of conditions not altogether understood.) But the salt-marsh formation as a whole is perhaps the most distinctive as regards its composition of any of the plant-groups of our country. It is dominated by such species as the grey leathery-leaved Obione portulacoides, the small-leaved, thick-stemmed Sea Pink, the Sea Wormwood (Artemisia maritima), which is all covered with a silky coat; the pools are fringed with Scirpus Tabernæmontani, a dwarf greyish copy of the Common Bulrush of our lakes, and filled with the narrow-leaved Ruppia and Zannichellia; and in the muddiest places are little forests of Glasswort, leafless, very fleshy, the flowers reduced to mere essentials and buried in the fleshy stems (Fig. 2, p. 18).

Again, it is easy to trace the relationship existing between plant form and soil conditions in the bogland flora; and these relations, unexpectedly enough, turn out to be analogous to those obtaining in the case of the salt-marsh. The sodden peat, sour and badly aerated, and poor in mineral salts, is poor also in the bacteria which feed upon and destroy dead vegetable matter, with the consequence that acid humus compounds collect in the half-decayed vegetable mass; water charged with these substances is as unsuitable for plants as is the water of the salt-marsh. In spite of the wetness of the peat, water is in this case also a desideratum; and the moorland plants, like those of the sea fringe, possess special adaptations for economizing it. This usually takes prominently the form of a reduction of leaf-surface. The dominant plants, such as the Ling (Calluna vulgaris) and Purple Heather (Erica cinerea), have minute leaves with reflexed edges and special structure to protect the stomata. The grasses and sedges which abound have similar characteristics; the whole vegetation tends to be small-leaved and long-rooted. A few of the plants, such as the Eyebright (Euphrasia), eke out the scanty food-supply by a semi-parisitism, robbing their neighbours of portions of their hardly-won sustenance; one or two others, such as the Bladderwort (Utricularia), which floats in the bog-pools, and the Sundew (Drosera), which fringes their edges, entrap insects and digest their juices, helping out their scanty rations with an animal diet. On the moors the peculiar soil conditions determine definitely the type of vegetation, which, over large areas, is as uniform and monotonous as that of the salt-marsh.

We see, then, that the peculiar character of several of the most marked of native plant formations—those of shingle, of salt-marsh, and of moor—are due primarily to scarcity of water. They are drought formations, produced either by physical drought, as in the case of shingle, which fails to retain water, or by physiological drought, as in the salt-marsh or bog, where, though water is present in abundance, it is not in a condition in which plants can readily make use of it.

Let us now go to the opposite extreme, and consider the plant formation which characterizes lowland lakes and rivers, where water suitable for plant use is

superabundant. In such places we are faced with a vegetation exhibiting a great number of species and a marked variety of form, and by no means so easy to correlate with its environment as those which we have been considering. In a wide sense, the nature of the vegetation is largely dependent on the degree of aeration of the water and the amount of dissolved mineral salts which it contains, an increase of either (within limits) resulting in a richer flora. But in any one area it is clear that depth of water is the controlling factor: the plants are arranged in zones, one succeeding another as the bottom shelves. Two main zones are conspicuous: (1) A zone of tall reed-like plants near the margins, which farther out is succeeded by (2) a zone of lax floating plants which either have leaves resting on the surface or grow entirely submerged. Above the former a belt of marsh plants links the reed zone with the vegetation of the soils of normal moisture; below the latter, should the water increase in depth, we reach an aquatic desert region, where the reduction of light renders plant growth difficult, and eventually inhibits it. Let us consider the conditions prevailing in the reed zone. Here the plants are essentially aerial, and though they have their feet in water, the stems and leaves rise far above it. Water-level is variable in lakes and rivers; the plants are usually tall, so that even in case of flood the leaves and flowers will not be drowned. Wave action on lake-shores is somewhat violent, and in flooded rivers a strong current may sweep through the vegetation; we see the advantage of the slender elastic stems and narrow leaves that characterize the plants: compare Reed (Phragmites), Reed-mace (Typha), Flag (Iris), Bur-reed (Spargarium), Bulrush (Scirpus); and these characters also fit them for the windy nature of their habitat. The denuding effect of wave or current action is countered by the network of creeping stems and abundant roots which the plants possess, forming a tough felt which floats, and by its growth and decay helps materially to form fresh land. Another effect of the creeping and branching stem-systems is the production of extensive and dense groves of many of the species.

When we pass beyond the reed zone, a completely different type of vegetation prevails. Here the plants are essentially aquatic. They make no effort to raise their stems and leaves above the water surface; but almost all of them raise their flowers into the air, though the seed is often ripened below the surface by a downward curving of the stem. These plants, surrounded by water, use their roots chiefly as anchors, and absorb through their stems and leaves the water from which they obtain the necessary mineral salts. As regards the supply of oxygen and carbon dioxide which the air supplies to them, those with floating leaves absorb it from the atmosphere, while those whose leaves are submerged have to subsist on the small quantity of these gases which is dissolved in the water—no wonder that such plants are rare in stagnant waters where aeration is poor. To assist respiration and transpiration, abundant and often comparatively gigantic air-spaces are provided in roots or stems or leaves, giving them a cellular appearance, and making them singularly light and spongy in texture. The leaf system of those plants which possess floating leaves—such as Water Lily (Castalia and Nymphæa) or Common Pondweed (Potamogeton natans), are well worth study. They are tough, to withstand battering by waves; the stomata are situated, not on the lower side of the leaf, as in land plants, but on the upper side, where they are in contact with the atmosphere; and the upper surface is waxy or oily, so that it is not wetted and the stomata are not blocked. Changes of water-level are met by means of long flexible stems, rising not vertically from the root, but at an angle, so that the leaves can rise with a rise of water-level. But not all the plants are anchored to the bottom. Some, which favour especially ditches and quiet waters, float freely with roots hanging down in the water—the Frog-bit (Hydrocharis) and Duckweeds (Lemna) are familiar examples. In the Duckweeds true leaves are absent, but the tiny stems are flattened and green and serve the same purpose, the minute flowers being borne on their edges. A few plants, such as the smallest of the Duckweeds (Wolffia arrhiza) and the Bladderworts (Utricularia), have gone farther still, and have dispensed with roots altogether. In Wolffia, indeed, the degeneracy of structure which results from the simplification of life problems in plants which live thus floating freely in water, is carried to its extreme limit. Leafless, rootless, and almost flowerless, it maintains itself by the budding of its tiny green fronds, a life-history as primitive as that of the lowly Algæ among which it lives. In the Bladderworts, the long flaccid stems, clothed with much-divided leaves converted in part into ingenious insect-traps (see p. 188), hang limply in the water, sending up boldly into the air their flowering shoots with yellow Snapdragon-like blossoms. In most of such free-floating plants, compact buds are formed at the tips of the shoots in autumn, and while the rest of the stem dies away these sink to the bottom and remain there safe from frost and storm until the spring, when they rise to the surface and produce a new crop of plants.

We have now glanced at the most distinctive of the plant formations which we meet with in our own country, and find that they accompany extreme conditions relating to water and soil: it remains to return to the consideration of the vegetation which develops under conditions of a more normal character—on ordinary soils, in fact, which are neither very wet nor very dry. Such conditions are precisely those which are required for agricultural purposes; and over the wide areas where they prevail, we find, as pointed out already, mere fragments of the native associations remaining in an undisturbed condition. This renders their study more difficult, and the difficulty is heightened by the fact that while the physical conditions show no contrasts so marked as those which we have been considering, the formations which can be distinguished are several, and each contains several associations—often a woodland, a scrub, and a grassland type. Thus, the formation which occupies calcareous soils exhibits characteristic woodlands—woods of Ash (Fraxinus excelsior), for instance, and on the downs peculiar woods or scrub of Box (Buxus sempervirens), Juniper (Juniperus communis), Yew (Taxus baccata), or Hazel, as on Farleton Fell. It also bears some very marked types of grassland, as on the chalk downs; and the limestone pavement of Farleton Fell is a special variant of this. Similarly, clays and loams, sands, and siliceous soils possess similar characteristic types of vegetation. But the consideration of these would occupy more space and lead us into more technical detail than the scope of this book warrants. For an account of these associations, written by botanists who have made a special study of them, the reader is referred to Tansley’s “Types of British Vegetation.”

CHAPTER III

PLANT MIGRATION

All organisms, animal as well as vegetable, are at some period of their existence provided with an opportunity of migration. In the animal world, most land creatures have legs or wings, which allow them to roam about freely—a freedom which is of special importance as enabling them to obtain nourishment and to avoid disadvantageous conditions. Aquatic animals are likewise to a great extent possessed of powers of locomotion, but such powers are not so essential to them as to terrestrial creatures, since the water itself is full of small organisms, both animal and vegetable, on which they can feed; hence a large variety of water creatures are content to remain during much of their lives fixed to one spot, extracting from the water as it passes by both the supply of organic food and the inorganic substances, such as oxygen or carbonate of lime, which they require for their life processes. 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. 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; but they similarly are given the opportunity, after birth, of migrating. 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. In the Cryptogams, or so-called Flowerless Plants, this temporary compression of the organism into very narrow limits suitable for easy dispersal takes place at a different period in the life cycle, but for mechanical purposes the results are similar. Minute bodies, or spores (much smaller than the seeds of the Seed Plants), are cast loose by the parent often in vast numbers, and eventually settle down and reproduce the species. In many of the lower aquatic plants these spores are provided with means of locomotion in the form of a tail-like appendage, which by its movement propels the germs through the water, giving them the same advantage which is possessed by the young of many of the sedentary animals.

The opportunity for migration thus offered to sedentary plants once at least in each cycle is of very great importance. 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 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. 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. They are dependent on the movements of external media—air, or water, or wandering animals—for transportation of any magnitude, and while many possess very beautiful devices for enabling them to take advantage of opportunities in this regard, the majority are devoid of any special structures. They are as inert as pebbles or grains of sand: but they possess two attributes which form important assets—namely, numbers and vitality. The amount of seed produced annually is hundreds, or more usually thousands, sometimes hundreds of thousands, for each parent. What matter if myriads perish? If one in so many thousands takes root and grows, the species will not diminish in numbers. Vitality also largely affects the problem. The seed can endure extremes of heat and cold which would be fatal to the parent; it can be drowned, or scorched, or dashed about, or in many cases eaten by animals without injury; it can lie buried in the soil for a long period of years, yet if turned up again and placed within reach of the requisite amount of air and heat, will spring up vigorously.

As a matter of fact, investigation soon shows that absence of special devices for dispersal provides no measure of the breadth of a plant’s distribution, nor is profuse seed-production necessarily related to abundance of offspring. Many factors come into play, and conclusions of this obvious kind will generally only lead us astray. But that does not render the study of each one of the factors less interesting.

This matter of seed-dispersal is of prime importance in our study of familiar British plantscapes, for our vegetation is the expression of the past and present efficiency of its particular rôle in the ever-changing drama of Nature. We shall do well to spend a little time in considering it.

First of all, as to the nature of the seeds with which we have to deal. These are, as already pointed out, young plants, already a long way advanced from the egg stage, neatly tucked up and enclosed, in most cases along with a supply of food material, in a tight, strong skin, which is mostly of a particularly impervious character, protecting the young plant from injury by bruising, from attacks of small animal enemies, from extremes of heat and cold, of moisture and dryness. The young plant, too, is in a peculiarly resistant physiological condition. For instance, its breathing—or absorption of oxygen—is exceedingly slow, and it is not suffocated by burial, sometimes even for years, in the soil. And while the mature plant is killed instantly by immersion in boiling water or by exposure to a very low temperature, some seeds, if boiled for a quarter of an hour, are quite uninjured, while others, subjected experimentally to even the temperature of liquid hydrogen (-260° C., or 436 degrees of frost on our more familiar Fahrenheit scale), remain unaffected. Many seeds are liberated from the parent plant enclosed by or attached to appendages of various sorts (when they are called by the botanist fruits) which sometimes greatly aid dispersal, as in the Dandelion (Taraxacum), and sometimes appear to hinder it; in any case, while the young plant itself is usually quite small, it may, when surrounded by its food-supply and enclosed in its wrappings, be a bulky object—as is seen in the Cocoanut or Horse Chestnut. In the British flora, to which we may confine our attention, a crab-apple (containing a number of seeds), a hazelnut, and an acorn (each containing a single seed), are the largest units of dispersal with which we have to deal. But these are quite exceptional in size, and the average seed (using that term in its original sense of the natural unit of dispersal) in the British flora does not exceed the size of a pin’s head. This remarkable reduction of size alone aids dispersal greatly.

The migrations of plants are effected mainly during the seed stage, these tiny, tightly packed portmanteaux being much better fitted for travel than the bulky and fragile organisms to which they give rise. But before we consider the adventures of seeds it must be pointed out that a considerable, if slow, migration of plants takes place by mere vegetative growth. The stems of many species are not erect, but prostrate; creeping upon or below the ground, they may in time cause a plant to spread far beyond its place of origin. A whole field, or for that matter a whole hillside, of Bracken (Pteris Aquilina) may quite possibly have originated from a single wind-borne spore. Among Sedges and Grasses this mode of growth is common—as we know to our cost in the case of the Couch-grass (Triticum repens)—and it is found in varying form in many kinds of plants, as in the suckers of trees, the offsets of bulbs, the runners of the Strawberry (Fragaria); it is especially characteristic of marsh and water plants. Its effect is to produce large colonies, such as the great beds of Reeds (Phragmites) or Reed-mace (Typha) which fringe our lakes, the groves of Bent (Ammophila) on sand dunes, and the beds of Anemones (A. nemorosa) or Broad-leaved Garlic (Allium ursinum) of our spring woods. In all these cases the whole colony may be the result of the continued growth of a single individual. It should be noted, however, that such migration is possible only so far as favourable soil conditions extend. A slight barrier—a streamlet, a patch of ground too wet or too dry, will arrest further progress, and the plant must fall back on seed-dispersal in order to conquer further territory.

A vegetative device which, so far as its method and value in dispersal are concerned, approaches those of seeds, is found in the bulbils with which some plants are furnished. These are small buds—congested shoots—borne on stems, or on leaves as in the Lady’s Smock (Cardamine pratensis), or among the

flowers as in many Leeks (Allium spp.). These usually fall from the parent when mature, and being comparatively small and possessed of considerable vitality, they may achieve a considerable dispersal before they send out roots and fasten themselves to the soil. An example is figured (Fig. 7). In this plant (Dentaria bulbifera, the Coral Root, a rather rare native of England) the bulbils resemble not the smooth flower-stems of which they are axillary branches, but the curiously knobby underground stems from which the leaves and flowering shoots arise.

Since seeds themselves possess, as already stated, no power of locomotion, they have to rely on external agents for their dispersal. These may in general be summed up as (1) Action of the parent plant, (2) water, (3) wind, (4) animals.

1. Action of the Parent.—The Ivy-leaved Toad-flax, or Mother-of-Thousands (Linaria Cymbalaria), is a pretty little plant, native in central and southern Europe, naturalized and common on old walls in this country. Its Snapdragon-shaped purple flowers are borne on short stalks which curve towards the light, placing the blossoms in a conspicuous position, where they may be the more readily visited by insects, and thus pollinated. But when flowering is over, and the little round fruit is ripening, the stalk twists so that the fruit is turned towards the wall and finally pushed into any convenient crevice: when the capsule opens, the seeds, instead of dropping to the base of the wall where on germination the young plants would be smothered among stronger growths, find themselves lodged in niches in which the young plants may develop successfully. Many water plants have flowers which rise into the air, following on which the flower-stem curves and the seed is ripened below the surface, free from the dangers of weather, of feeding water birds, and so on.

A very common type is that in which the seed-vessel opens at the top when the seed is mature. Gusts of wind, or passing animals, bending the stem, cause the latter to spring back, casting the seeds out. When the seed-vessel opens widely, as in the Columbine (Aquilegia), the seeds may be cast to some small distance. The efficacy of the arrangement is not so obvious when, as in the Poppies (Papaver) or Bell-flowers (Campanula), the openings are small (Fig. 8), but it is clear that these plants do not suffer from lack of dispersal, in view of their abundance and wide range.