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.

But the assistance which the parent plant gives is often of a more active and even dramatic character, though in these cases it is usually effected not by a movement of living tissue as in the last case, but by mechanical changes taking place in tissues already dead or dying. If we stand by a bank of Gorse (_Ulex_) on a warm day we may become aware of a snapping sound, and may possibly feel on our faces the impact of small bodies. These are gorse seeds in process of being distributed by the parent. In this shrub the fragrant flowers are succeeded by short tough, hairy pods, formed of two valves joined together by their edges. (In reality the pod is a modified leaf folded down the middle, the two edges thus brought together being joined--see p. 129.) When the seed is ripe the pod dries, and owing to unequal shrinkage of the valves stresses are set up which at last tear the pod suddenly asunder along its edges, flinging the seeds violently out into new ground, where they will have a better chance of life than if merely dropped into the middle of the parent bush. A similar arrangement is found in the Vetches and many other Leguminosæ. In the Cranesbills (_Geranium_) a very ingenious catapult device may be examined. The fruit is of peculiar structure. We might make a rough model of it by taking five single-sticks and tying them to a broom-handle--firmly at the points, less securely elsewhere--and slipping a tennis-ball into each basketwork handguard before turning its open side in against the broom-stick, so that the ball cannot fall out. Imagine now that unequal drying on the part of the sticks tends to make each bend into a semicircular form, which is hindered by the fastenings at either end. The stress will eventually tear the weak fastenings at the base: the lower end will fly up, bearing with it the ball (representing the seed), which will be projected

out through the open side. In the Cranesbills the jerk is so violent that seeds may be flung to a distance of twenty feet. One of the most efficient of all devices of this kind is found in the Sand-box Tree (_Hura crepitans_), a native of South America. By sudden rupture and twisting of the carpels of the woody sub-globular fruit, the large seeds of this plant are thrown to a distance of thirty yards, the explosion being accompanied by a report like that of a pistol-shot. In the common Dog Violet (_Viola Riviniana_) (Fig. 10) the fruit is a three-valved capsule, which on ripening divides; each valve assumes a horizontal position and its edges contract till it is shaped like an open boat, the seeds lying in a row down the middle. The sides as they dry close in tighter and tighter on the seeds, which are in turn pinched out, and fly off with a little snap to a distance of many feet. It is an interesting experience to watch these tricks of Nature--much more interesting than merely to read about them. If plants of Vetch, Gorse, Dog Violet, Storksbill, Wood Sorrel, Touch-me-not (to name a few), bearing unripe fruit, be brought home and placed in water in a sitting-room, the click of the bursting fruits will be distinctly audible, and by spreading a white sheet the efficiency of the devices may be tested.

A very interesting case, in which the seed is actually buried in the soil by movements of its appendages (portions of the parent plant which remain attached to it), may be watched in the case of the Storksbills (_Erodium_), Several species of which are British plants of frequent occurrence. Here the young fruit much resembles that of its allies the Cranesbills. The long rod-like axis at the lower end of which the seed is enclosed contracts unequally in drying, so that the upper half assumes a position at right angles to that of the

lower half, which when dry is much twisted, like a rope (Fig. 11). The covering of the seed itself is furnished with stiff short hairs pointing upwards. The whole structure when mature is cast off by the parent. The curiously twisted appendage is hygroscopic, and readily responds to wetness by untwisting and to dryness by twisting. Should it be thus caused to untwist when the upper end is free from obstruction the latter will revolve slowly like the hand of a clock. But should it meet with an obstacle in the course of its revolutions, such as a blade of grass, the motion is transferred to the lower end, which revolves like an auger, and, lengthening as it untwists, forces the seed into the ground. Should dryness supervene, the backward-pointing hairs on the seed-envelope prevent its being drawn out again when retwisting and consequent shortening take place. These _Erodium_ fruits are among the most interesting in the British flora, and are well worth experimenting with.

2. _Water._--Water, which forms the most frequent and the most serious barrier to plant migration, under certain circumstances is a very efficient agent of dispersal. At the same time, its powers in the latter direction are strictly circumscribed. As regards fresh water, seeds which float may be wafted across lakes. Rivers are more effectual, as seeds may be transported long distances in their currents and thrown up finally on their banks or over flooded areas. When we consider the sea, we realize that there is here a possibility of almost unlimited dispersal provided that the seeds are not injured by salt water, and that they can remain afloat. It is on the latter point that the whole efficacy of water dispersal turns. This was long ago recognized, and investigations have been made by many naturalists to determine the buoyancy of seeds of all kinds. The results show that, taking the seeds of the plants of any country as a whole, not more than about 10 per cent. are capable of floating for more than a short period, while most of them sink at once in either fresh or salt water. So one’s vision of seeds transported in myriads over hundreds of miles of sea is rudely dispelled; and the fact that many seeds can survive prolonged immersion in sea-water uninjured is of little account. The 10 per cent. of our own flora which produce buoyant seeds are mainly riverside and seaside plants; and no doubt their dispersal is to a great extent due to streams and tidal currents. But the majority of the hundreds of thousands of seeds which a river transports annually find their last resting-place in quiet backwaters or on the floor of the sea.

It is different, however, with the flora which fringes beaches in the Tropics. Here many of the plants possess large fruits of great buoyancy, which are still afloat and alive after months of tossing on the waves, and if cast up germinate readily. These bold wanderers are a familiar feature of Tropic plant life, and their successful voyaging accounts for the uniformity of the beach flora on innumerable islands. Even our own inhospitable shores sometimes receive these waifs of warmer seas, brought from the West Indies by the Gulf Stream and the prevailing south-west winds. Of these the most frequent are the large bean-like seeds of _Entada scandens_, a Leguminous plant, which are originally enclosed in gigantic pods several feet in length, and the more globular seeds of the Bonduc (_Guilandina bonducella_), another species of the same order. But the most famous of all floating fruits is the Double Cocoanut, or Coco-de-mer, a huge nut weighing 40 or 50 lb. and containing several seeds a foot and a half long. It is the product of a Palm (_Lodoicea Sechellarum_); cast up on the shores of India, it was known centuries before its place of origin in the Seychelles was discovered, and fantastic legends grew up regarding it.

3. _Wind._--Everything that we know about the wind suggests that it is a potent agent of seed-dispersal, far excelling, for instance, that of flowing water. “All the rivers flow into the sea,” that cemetery of seeds, and their courses are at best mere spider-lines on a map. But the wind, blowing where it listeth, is everywhere, always ready to snatch up in its arms any seed of sufficient lightness, and to bear it away from the parent; in fancy we can see tiny seeds borne by gales across mountains and oceans. But we have to leave imagination out of account, and examine prosaically the mechanical laws according to which such transport is of necessity conducted. Any body liberated in still air will fall vertically with a velocity which increases according to well-known laws until the increasing resistance of the air to its passage equals the effect due to gravity; it thenceforward continues to fall at a uniform velocity, that velocity depending upon the nature of the falling body. In all seeds which are sufficiently light to be at all suitable for wind dispersal, the resistance of the air almost at once counteracts acceleration due to gravity, so that the rate of fall may be taken as uniform from the beginning. 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. Thus, if a seed liberated 100 feet from the ground falls that distance in half a minute, and the wind is blowing at the rate of, say, 1,000 feet in half a minute (or nearly 23 miles per hour, a good breeze), the seed will be carried 1,000 feet before it reaches the ground. Its course will be represented by the diagonal AD of the accompanying figure, where AB represents the distance which the seed falls in the given time, and AC the distance according to the same scale travelled by the wind in the same period.

But most seeds sufficiently light to be capable of extended flights are liberated only a few feet from the ground; they are dependent on upward eddies to raise them if they are to achieve more than a very short migration. That such eddies, both upward and downward, occur on a windy day we all know from experience; and it is they that make or mar the fortune of most wind-borne seeds. Only some local or accidental excess of upward over downward eddies will assist a seed on its journey; and as every upward eddy must be compensated somewhere by a downward eddy, the longer the journey is, the more such eddies tend to neutralize each other. Over the sea--that most formidable barrier to plant migration--eddies do not prevail as they do over rough ground, so that, unless by a series of lucky eddies a seed is whirled up to a considerable elevation before it leaves the shore, the chances of its successful passage across a stretch of water are remote. Discussing the possibility of seeds of Portuguese plants reaching the Azores, lying 800 miles to the westward, H. B. Guppy[4] shows, from observations on the rate of fall of seeds made by several workers, that with a 50 miles per hour horizontal wind the light-plumed seed of the Common Groundsel (_Senecio vulgaris_), for instance, would require to be liberated at a height of 9 miles above the ground if it is to reach the islands: or to express it differently, if liberated at ground-level, the seed would need to be raised 9 miles by upward eddies during its journey, even if corresponding downward eddies were absent--which they certainly never are. It is clear that if even light seeds are to achieve anything more than short journeys, they must depend on exceptional disturbances of the air, such as whirlwinds and tornadoes.

It is now time to examine the devices by which many seeds achieve a more or less wide dispersal by means of the wind. Seeds possessing these adaptations may be divided into three classes: (i.) Powder seeds, (ii.) winged seeds, (iii.) plumed seeds.

By powder seeds are meant seeds of very small dimensions. Reduction in size, if carried far enough, greatly facilitates dispersal by wind. This is because the resistance offered by the air is relatively greater for a smaller body than for a larger one, so that rate of fall decreases as the size of the falling body diminishes--we all know how even a heavy material, if reduced to powder, will fall more slowly than when forming a single mass. Most of the spores of the “Flowerless Plants”--Ferns, Mosses, Fungi, etc.--are exceedingly minute, and have as a result a very slow rate of fall, and a consequent power of long-distance dispersal by wind. For instance, the microscopic spore of the puff-ball _Lycoperdon_ falls so slowly that, if we take again Guppy’s Azores example, it could traverse the 800 miles in a 50 miles an hour gale if it commenced its flight only 86 feet above the ground. Such spores are, in fact, so buoyant that they form a normal constituent of the air--as we know, for instance, by the rapidity with which they will discover and germinate upon a piece of cheese, forming bluemould--and with little doubt they are capable of reaching under favourable circumstances the most distant of oceanic islands. But in the Flowering Plants with which we are mainly concerned reduction in size is not carried far enough to confer any great amount of buoyancy. The minute seeds of the Poppies (_Papaver_), for instance, fall about 10 feet in a second. Applying again Guppy’s Azorean case, we find that though these would cover the distance in sixteen hours, they would fall in that time about 100 miles, unless raised during the journey to that extent by the excess of upward eddies as compared with downward ones--a quite impracticable proposition. In the Orchids alone do we find among the powder-seeded Flowering Plants a really effective buoyancy; this is due to the fact that great reduction in size is accompanied by very loosely disposed tissue enclosing the seed in a kind of net, and by the resistance to the air thus offered, greatly reducing the rate of fall. The seed of the Marsh Helleborine (_Epipactis longifolia_) falls only about 1/15 as fast as that of the Poppies, and would thus, under the same conditions, be carried fifteen times as far.

To pass on. Some seeds, many of them of considerable size as compared with those which we have just considered, have coverings which are furnished with a membranous wing (Fig. 13, _d_), sometimes extending all round the seed, as in the Elm (_Ulmus_), more often placed at one side, as in the Sycamore (_Acer_). The effect of such wings is to reduce the rate of fall, imparting to the seed an irregular zigzag motion, as in the former case, or a spinning motion as in the latter. A Sycamore seed with the wing removed will fall four or five times as fast as with the wing present. But while a well-developed wing forms a more efficient dispersal device than mere reduction in size as found in Seed Plants, the rate of fall of wing seeds as a whole shows that these appendages do not fit them for anything but short voyages.

We may then pass on to consider the plumed seeds, which possess by far the most efficient as well as the most beautiful devices for aiding dispersal found among wind-borne seeds. These plumed seeds belong to many different groups of plants, and the tufts of delicate hairs which give them their buoyancy arise in different ways. Among the _Compositæ_, the Order which furnishes the most familiar of our plumed seeds, the plume is formed by modification of the upper part of the calyx, which in so many common plants is small, green, and leaf-like; the lower part of the calyx in the _Compositæ_ is tough, persistent, and close-fitting, forming an additional protection for the seed. The plume springs either from the top of the seed, as in the Thistle, or is borne on a slender stalk, as in the Dandelion. It consists of a ring or radiating mass of hairs of beautiful delicacy, often bearing short

branches; these hairs are tightly packed together when the fruit is young or during damp weather, but on a dry day when it is ripe they spread out, and the seed, breaking away from its attachment, is floated off by the wind. In many species the plume or _pappus_ is only lightly attached to the seed, so that if on a voyage an obstacle is encountered the seed drops off, while the now useless parachute drifts away. But though the plume seeds of the _Compositæ_ are the largest and most beautiful among our common plants, they are not the most efficient for dispersal. The fluffy seeds of the Willowherbs (_Epilobium_) and of the Willows (_Salix_), for instance, fall at a slower rate than those of almost any _Compositæ_, while by far the most buoyant seed in the British flora is that of the Reed-mace (_Typha_). In this case the seed itself is minute, and is situated on a very slender stalk, from near the base of which springs a tuft of delicate hairs. This seed takes thirty-four seconds to fall twelve feet. Using once more the Azorean example, it could cross the 800 miles of sea if it had an initial elevation of 3-1/3 miles, or was raised to that amount during the sixteen hours occupied by its passage.

Summing up, then, we find that the plume seeds are the most efficient of all seeds for extended flights by the agency of the wind. If the efficiency of the seeds of the Reed-mace, the most buoyant among British plants, be taken as 100, the efficiency of the Willowherbs is between 60 and 70, of Willows 45 to 70, the best of the Thistles 35 to 40, Dandelion 25. Even the best of the winged seeds are much less efficient, Elm and Scotch Fir being about 20, Sycamore and Ash 9 or 10. Of powder seeds, the efficiency of several Orchids tested ranges from 35 to 65, and Broomrapes (_Orobanche_) from 20 to 25. Most of the powder seeds are far below these, the efficiency of seeds of _Papaver dubium_, for example, being only 4·5 on the same scale. This last figure is representative of the many small-seeded plants in the British flora such as are found among the _Crucifercæ_, _Caryophyllaceæ_, _Scrophulariaceæ_, etc. The relative efficiency of such comparatively large seeds as those of many of our Leguminous plants would be about 1 on the same scale.

4. _Dispersal by Animals._--The coverings of many seeds are provided with hooks or barbs, and others with stiff hairs, which render them liable to become entangled in the hair or fur of passing animals. Examples will occur at once to the reader, as this character occurs in the case of many familiar plants, such as Burdock (_Arctium_), Enchanter’s Nightshade (_Circæa_), Avens (_Geum_), and so on. Without doubt these hooked fruits often secure a wide local dispersal by the aid of cattle, sheep, rabbits, and so on: the state of one’s trousers or stockings after walking the autumn woods is often very suggestive in this regard. Again, herbivorous quadrupeds eat seeds in quantities, many of which are capable of germination after passing through the animal’s body. 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. Seeds are carried about by birds in two ways--by becoming attached to their feathers or feet, or by being eaten and subsequently ejected. The first case belongs to the class of phenomena which we have just been considering, save that the smooth plumage of birds, and their frequent preening of their feathers, tends to keep their coats free from extraneous material. But at least in wet weather minute seeds must often cling to feathers and to feet, and mud which may contain seeds may easily be present on a bird’s toes during flight. More important is the question of _endozoic_ dispersal--where seeds are transported in the alimentary canal of birds. Some families, like the Finches and Tits, which eat great numbers of seeds, are inimical instead of helpful to dispersal, because the seeds which they devour are crushed and afterwards digested. But in many cases the seeds are swallowed whole, and are usually in no way injured by their passage through a bird’s body. Frequently, indeed, the seeds have not to run the gauntlet of the digestive juices of the alimentary canal, being disgorged from the stomach along with other hard material prior to digestion. Birds which live on berries or other juicy fruits are the most important in seed-dispersal. As Barrows says: “The seed-eaters are not the seed-planters; on the contrary, the insectivorous birds more often sow seeds than the true seed-eaters.” “Seeds which _simply contain_ nourishment are eaten and destroyed, while seeds which _are contained in nourishment_ are eaten and survive.”[5] It is for this reason that, if we look under a tree on which Blackbirds or Thrushes perch, we shall often find young plants of Bramble (_Rubus_), Ivy (_Hedera_), Holly (_Ilex_), or Yew (_Taxus_). There can be no doubt that birds eat and subsequently eject vast numbers of seeds still capable of germination; many observations and calculations might be quoted. But when we come to apply the facts to the problem of long-distance dispersal, or the passage across serious barriers, we find that important limiting factors must be taken into account. The digestion of birds is remarkably rapid, food being ejected from a half to three hours after being eaten, so that a bird eating seeds and at once flying off in a straight line at, say, 50 miles per hour could not convey seeds more than 150 miles. Secondly, many observations show that on migration birds generally travel with empty stomachs and clean plumage and feet. 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. Thus, the heaviest of British seeds, the acorn, is carried about by Rooks, just as the hazelnut is scattered by Squirrels, or a head of Burdock fruits by a passing sheep.

Having thus arrived at some idea of the high efficiency for dispersal of many kinds of seeds, it is with some little surprise that we observe--as we may on any country walk--that the plants which arise from these are in general no more abundant or more widely distributed than others which possess seeds devoid of any apparent advantages in this respect--seeds which cannot fly nor float, nor cling to a passing creature, and which are not eaten to any extent by birds so far as observation goes. The truth is, we have to remember, as emphasized in a previous chapter, that the world is already densely populated by plants, all of which survive by reason of their being specially fitted for their several habitats. They have fought in the great struggle for existence, and have established their right to the places which they occupy; they will not readily give way to any newcomer whose seeds happen to be imported into their strongholds. Of course exceptions can be quoted, where plants accidentally or intentionally introduced by man into new areas have not only maintained a foothold, but have spread remarkably. Note the case of the Sweetbrier (_Rosa eglanteria_) in New Zealand, of the Mexican _Bryophyllum calycinum_ in many Tropical countries, of the American Monkey-flower (_Mimulus Langsdorfii_) in our own islands; but these are admittedly exceptional. It is nearer the truth to say that the troubles of an immigrant only begin where dispersal ends; and that the chance of seeds carrying out a successful migration is much greater than the chances of their giving rise to a new colony when that migration is successfully accomplished. Every head of the Reed-mace liberates about a quarter of a million seeds of marvellous lightness; yet the Reed-mace does not increase in the country, nor is it a particularly abundant plant even in its chosen habitats. The Foxgloves (_Digitalis purpurea_) in a wood shed, each plant, say a hundred thousand seeds; yet on an average only one of these attains maturity, otherwise the species would become more abundant in the area. This enormous destruction of seed is largely due to competition. The reception which a plant receives in its new home is the thing that matters, and that may usually be summed up in the phrase “House full.”

Nevertheless, the present flora of Great Britain is in the long run the result of migration from surrounding areas; so that ease of dispersal has undoubtedly played its part in the building up of our vegetation.

Conditions under which rapid dispersal has obviously an advantage occur when by some exceptional circumstances the natural vegetation is destroyed within an area, as by a flood or landslide. Such conditions are produced artificially each season over much of our own country by the operations of agriculture. Their results will be considered in a subsequent chapter.