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 3¹/₃ 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.

CHAPTER IV

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. 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. 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. Sea-water favourable for plankton life may contain several millions of individuals in every litre (about 1³/₄ pints); while as a fair estimate for the seas which surround our own islands “at least one” organism for every drop has been suggested.[6]

In the great abysses of the ocean, where vegetable life is absent, the strange creatures which live there in utter darkness prey upon others, and they again on others which belong to lesser depths, the ultimate source of life being again the minute surface organisms which, possessing chlorophyll, can make organic out of inorganic substances by the energy obtained from sunlight. Thus only is life made possible in

On the land, the dependence of animals on plants is in large measure direct, as the supply of vegetable food is abundant and widespread. The largest land animals are all vegetable feeders; so are the majority of our own native mammals, and in a great measure our birds; while most of the creatures upon which the flesh-eating animals prey are themselves vegetable feeders. The distribution of land animals over the globe is thus dependent in large measure on the distribution of plants. On account of the profusion and variety of plant life, and the fact that most vegetable feeders can thrive on various sorts of plants, few animals are restricted in their range by the presence or absence of any particular species or genus, but complete dependence of this sort is by no means unknown. The larvæ of some Butterflies, for instance, eat the leaves of one plant only; the Peacock (Vanessa io) and the Small Tortoiseshell (V. urticæ) are cases in point. The caterpillars of both these species feed exclusively on the Common Nettle (Urtica dioica). Should the efforts of farmers and gardeners succeed in exterminating this unwelcome plant, these two butterflies would disappear from the Earth. Sometimes absolute mutual dependence is found on both the animal and vegetable sides. The American Yucca filamentosa, often grown in our gardens, depends solely on the little moth Pronuba yuccasella for its pollination, just as the insect is absolutely dependent on the plant (see p. 80), and other species of Yucca have each its particular dependent moth, which feeds on no other plant, and whose flowers are pollinated by no other.

Apart from such special cases, the general dependence of animals upon plants is obvious, and is by no means confined to food-supply. Animals of all grades, from human beings to Caddis Worms, construct houses of vegetable materials; trees are the chosen home of large sections of our fauna, and the herbs of the field are the world for millions of tiny beings.

Turning to the other side of the picture, no such general dependence of the plant world upon the animal world is found, but the inter-relations of the two are many and varied, and in the absence of animals of one kind or another whole groups of plants would become extinct. The cases where plants derive their food-supply wholly from animals are indeed rare, save near the bottom of the vegetable scale, and most of such parasites are minute; one of the most noticeable in our own country is the fungus Cordyceps militaris, which may be found growing on the dead bodies of larvæ or pupæ which it has killed—a little scarlet, club-shaped plant, about an inch in height. But some of the most highly organized plants obtain portions of their food-supply from animal sources. Mention has already been made of the Sundews (Drosera), Butterworts (Pinguicula), and Bladderworts (Utricularia), which capture live insects, etc., by means of sensitive organs (as in the first two cases) or ingenious traps (as in the last), and subsequently digest them, and they will be dealt with later on (p. 186). Then there is the Venus’ Fly-trap (Dionæa) and the well-known Pitcher Plants (Nepenthes), which actively, as in the former case, or passively, as in the latter, catch insects and digest them, by means of leaves modified in very extraordinary ways. In all these instances the advantage lies entirely on the side of the plant, just as in the case of most of the plant-eating animals the advantage is wholly with the animal. But in a large number of instances—many of them of a most interesting nature—the inter-relations are such as to benefit both the actors, each obtaining from the other what is useful to it. One of the most conspicuous and widespread relationships of this kind is that prevailing between flowers and insects, the insect receiving food in the form of nectar, and at the same time carrying pollen from flower to flower, without which transfer no fertile seed would be formed. To this interchange of favours we shall return later (p. 81); meanwhile, it will be well to consider a few of the cases in which the relationship between plant and animal is continuous and more intimate, the two living in very close relations to each other: to such cases the term symbiosis or “living together” is applied by naturalists. The relations existing between certain trees and some species of ant are of high interest, and illustrate well this phase of life. The Candelabra Tree (Cecropia peltata) of the South American forests is liable to attack by leaf-cutting ants (Œcodoma), which climb trees and bite off thousands of leaves; these they cut up on the ground and carry to their nests, where they form a basis for the growth of certain small fungi which are a favourite food of the ants (compare the cultivation of mushrooms as practised by gardeners). The Candelabra Tree protects itself from these ravages by forming an alliance with another kind of ant (Azteca). Along the hollow stems are little pits through which the ants easily bore, and reach the convenient houses within, where they live and bring up their young. At the base of the leaf-stalks, where the greatest danger lies from the leaf-cutting ants, little tufts of hairs are situated, among which are small white masses of nutritious material much liked by the ants, and collected by them and stored within their houses. So that these desirable trees are swarming with Aztec ants, fierce little creatures—“it is one of the most bellicose ants that I know, and its sting is most irritating,” writes Kerner—which congregate especially at the leaf-stalks, the point of attack of the leaf-cutters. The advantages of these arrangements to both the trees and the Aztec ants are obvious.

A very remarkable instance of a different kind is supplied by the relations existing between the American species of Yucca and the small white-winged moths of the genus Pronuba. The following succinct account is given by Professor G. H. Carpenter:[7] “The female of these moths has not only the palps of the first maxillæ developed, but the region of the maxillæ (palpiger) whence they spring produced into a pair of long, flexible, hairy processes. By means of these she collects from the anthers pollen, which she deliberately carries to the stigma to ensure fertilization. With her piercing ovipositor—a most abnormal development among moths—she bores through the tissue of the pistil, and by means of the flexible egg-tube, protrusible beyond the ovipositor, lays her eggs close to the ovules of the Yucca. The caterpillar when hatched feeds on the growing seed of the plant, which would never develop were it not for the action of the Pronuba moth. This action is most wonderful, in that the moth herself gets no benefit from it. Her food canal is degenerate, and her jaws, useless for sucking, are devoted altogether to the gathering of the pollen; she does not feed in the perfect state. Doubtless her ancestors did so, and were first attracted to the Yucca in search of honey, though the act of pollination is now performed only for the sake of the offspring.”

Among certain lower animals and plants symbiotic connection is often most intimate. For instance, in the body-wall of certain Sea Anemones and Holothurians there are small green cells which were long believed to be part of the animal, and which puzzled naturalists because they contained chlorophyll, that remarkable green substance characteristic of plants, which gives to them the power of forming food out of its raw inorganic materials. These cells are now known to be minute seaweeds (Algæ), which spend their lives in the animal tissues to the benefit of both organisms. The plant, by virtue of its chlorophyll, absorbs carbon dioxide, decomposes it, and gives out oxygen, which is eagerly seized on by the animal. The animal in its turn liberates carbon dioxide, which is required by the plant. Similar relations exist between Algæ and some of the lowly Radiolarians and Foraminifera; in these cases, the animals being very minute, the plant partner plays a more conspicuous rôle. It is noteworthy that these Algæ are quite capable of living and multiplying separately, free from the body of the animals, and the animals also are capable of pursuing an independent existence.

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; and carpels, which contain ovules, each enclosing an embryo sac or megaspore, in which is an ovum or female cell.

Each stamen consists usually of a slender stalk, the filament, bearing an oblong head, the anther, which contains four chambers, or pollen sacs, filled with pollen grains; these, when mature, escape into the air by the rupturing of the walls of the chambers.

Each carpel contains in its lower part an ovary, while its upper part presents to the air a surface charged with nutrient substance, the stigma, which is often raised on a slender stalk, the style.

To secure the production of seed, the first necessary step is pollination, or the transfer of pollen from the stamen to the stigma. When this is effected—the means will be considered immediately—and a pollen grain alights on the surface of the stigma, which is usually sticky or hairy to aid its retention there, the pollen grain commences growth, and sends out a slender tube (the pollen tube), which pursues its way through the substance of the stigma, down the style, into the ovary, and from its tip a male cell passes out and fuses with the ovum. In most flowers the pollen tube is not called on to make any great effort of growth, the distance between stigma and ovary being very small; but occasionally, as in Crocus and Lily, this may amount to half a foot. The result of this act of fertilization is that the ovum and ovule grow, the former forming eventually the embryo, or young plant, the latter the seed in which the embryo is enclosed. In order that fertile seed may be produced it is often necessary, and usually desirable, that the pollen which reaches the stigma should not belong to the same flower, but to a different flower of the same species; cross-pollination being the rule among seed plants, self-pollination the exception. To secure the former, and to avoid the latter, many highly interesting devices are found, materially affecting the structure and development of flowers.

The essential parts of a flower, then, consist of stamens and carpels. Flowers consisting of no other parts but either or both of these are not common, but we may compare, for example, the rarely produced flowers of the Duckweeds (Lemna), in which a tiny group of two stamens and a carpel represents one flower, or, according to some views, a group of three flowers. More commonly the flower is much more composite, consisting mostly of four sets of organs, arranged in whorls or rings, or more rarely in close spirals. In the centre is a group of carpels; outside them—in other words, slightly lower on the stem—a ring, or two rings, of stamens, few or many; then a ring of petals, forming the corolla, usually coloured, leaf-like, and conspicuous; and outside of them a ring of sepals, forming the calyx, generally green and leaf-like. The main function of the calyx is protective; it encloses the essential organs and guards them till they are mature, when the flower opens and stamen and stigma play their parts. The calyx is usually tough, and often covered with hairs, or with a sticky substance, to keep the flower safe and ward off the attacks of insects or other small devourers. If we turn to the corolla we find a singular variety of size, form, and colour. To account for this, it is necessary to consider the means by which pollen is distributed. There are two chief ways in which pollen is conveyed from flower to flower—by means of the wind, and by means of flying insects. If we examine wind-pollinated flowers, such as Hazel (Corylus), Scotch Fir (Pinus), or Reed-mace (Typha), we note the small size of the flowers and the great abundance of pollen. Compare these with insect-fertilized flowers, such as Buttercup (Ranunculus), Flax (Linum), Snapdragon (Antirrhinum), or one of the Orchids. In these the flowers are much larger owing to the increased size of the petals, which are of brilliant colour and of various shape. Pollen is mostly much reduced in quantity, since insects flying direct from flower to flower afford a far more economical mode of distribution than is offered by the wind. The pollen grains, moreover, are sticky and covered with tiny spines or knobs, to render them more liable to adhere to the body or head of an insect; the pollen grains of wind-fertilized flowers being, on the other hand, smooth, dry, and dust-like. Again, these insect-pollinated flowers usually possess little glands which secrete nectar, the sugary syrup which by digestion in a bee’s body becomes honey. Here, then, is the inter-relation established: the insect helps the plant by carrying its pollen from flower to flower, and in its turn is helped by the provision of delicious food. And what about the showy petals, and the fragrance that so often marks these entomophilous flowers? They are advertisements, designed to catch the attention of the necessary insects as they fly about. Not only does the corolla by its bright colour attract insects, but markings of various shapes and tints upon the petals are generally held to be honey-guides—sign-posts directing the insects to the nectar and to the pollen. These are especially conspicuous in many of the irregular flowers to which reference will be made shortly, in which the insects are encouraged to approach the flowers in a particular way. An example