FRUITS AND THE DISPERSAL OF FRUIT AND SEED

THE essential whorls of a flower are the andrcecium and the gyncecium. Without these a flowering plant would be faced with the alternative of extinction or immortality.

When corolla and calyx are present they play a subsidiary, but none the less very important, part in the work of pollination.

Fertilisation follows pollination, and this act gives such a stimulus to development that not only are ovule and ovary influenced, forming seed and fruit respectively, but neighbouring parts are also affected. The change in the receptacle of Strawberry and Apple, dependent upon fertilisation, is most pronounced. A fruit is not only the structure formed from the ovary after fertilisation. It is the whole result of a stimulus that influences ovule and ovary and, in many cases, the parts that surround the ovary as well.

Of fruits there is an infinite variety. They may be formed from one carpel or from many. They may contain one or a number of seeds. They may, or may not, open or dehisce to allow the seeds to escape. The fruit-wall or pericarp may be dry or juicy.

A good classification of fruits is not easy to frame. A preliminary three-fold division into Simple, Aggregate, and Composite makes a fairly satisfactory starting-point.

In some flowers there is only one carpel. This is true of the Leguminosx. Obviously, then, only one fruit can result from such a flower.

Simple Fruits.

It is equally obvious that only one fruit will result when a flower has two or more carpels if these are joined together.

In both cases the fruit is simple because it is produced from a single flower.

As there are a number of free carpels in the pistil of the Buttercup there must, of necessity, be a number of free fruits formed from one flower. Such a head of fruits is then spoken of as aggregate.

Composite fruits are not common. The Pine-apple, Fig and Mulberry are examples of this class. In each case much more than the ovary, more even than the ovary and receptacle, has taken part in the formation of the fruit.

The composite fruit is actually formed from a whole in- florescence. Each little area on the outside of a Pine-apple represents one flower. The whole axis of the inflorescence, and all the parts of the flowers it bears, become sweet and juicy and fuse together in the formation of this unusual type of fruit.

In the Mulberry each separate part of the fruit is not a single drupel, as in the Blackberry, but is made up of a central drupe, surrounded by four juicy perianth leaves .

Among the simple fruits some are described as dry because the pericarp of the mature fruit contains no juice. A nut is a typical dry fruit.

Others are juicy, either because the pericarp is juicy, as it is in a Plum – or because the seeds are embedded in a pulpy mass, as in an Orange – or because the receptacle has taken part in the formation of the fruit and has become juicy, as in the Pear.

Then again dry fruits may contain one seed or many. They may be formed from a varying number of carpels and dehisce in different ways.

Some dry fruits are formed from syncarpous ovaries, but, although the carpels were joined in the flower, the mature fruit divides into a number of parts, generally corresponding to the number of carpels from which it was formed. Such fruits are called schizocarps.

All these points are taken into consideration in framing the classification of fruits which is shown in tabular form below :

One-seeded fruits are all indehiscent because there is no need for the fruit coat to open and let the seed escape. When germination takes place the pressure of the escaping root causes a rupture of both testa and pericarp.

The differences between one-seeded fruits are largely differences in the nature of the pericarp.

An achene is a dry, one-seeded fruit in which the pericarp is tough or leathery.

In the Lady’s Mantle, one of the Rosacex, the fruit is an achene. In other cases achenes occur in collections, being formed from an apocarpous (free carpels) pistil.

Collections of achenes are found in the Buttercup, Strawberry, Rose, and many members of Ranunculacex and Rosacem .

A nut is a dry, one-seeded fruit in which the pericarp is hard and shell-like. The Hazel and Filbert are familiar examples .

Many nuts popularly so-called are not nuts at all. The Brazil nut and Horse Chestnut are seeds. The Walnut, Almond, and Coco-nut are drupes.

The caryopsis is the fruit of all cereals. It is dry and one-seeded. Its distinctive characteristic is the very close adhesion of the fruit coat (pericarp) with the seed coat (testa) .

The cypsela is the fruit of all members of the Compositx . It is dry and one-seeded, and its pericarp is frequently tough. It is, however, formed from an inferior ovary, whereas the typical achene results from an ovary that is superior. The striped cypsela of the Sunflower is always found in packets of parrot food.

In the samara the pericarp has undergone such a drastic change that it forms a wing . The samara is a dry, one-seeded fruit in which the pericarp is flattened and expanded to form a wing. Many-seeded fruits all agree in being dehiscent because there must be separation of the seeds. The method of opening in the ripe fruit varies, but in every case the seeds escape completely.

A capsule is a dry, many-seeded fruit formed from more than one carpel. It dehisces in various ways.

A common form of dehiscence is by means of valves . Longitudinal slits run the length of the capsule and correspond in number to the number of carpels that formed the fruit. The capsules of Willow and Poplar open by two valves – the Iris by three – the Willowherb by four.

Many capsules open at the apex by teeth . The number of teeth is either the same, or twice as many, as the number of carpels. The Primrose capsule opens by five teeth – the Campion by ten. Each of these flowers has a syncarpous ovary formed from five carpels.

In the Poppy a number of pores are caused by a breaking away and bending back of tissue in the ripe fruit . They occur just under the flat crown formed by the old stigmas. The number of pores, as well as the number of stigma rays, corresponds to the number of carpels in the flower. Large pores are found in the fruits of Snapdragon and Canterbury Bell .

A very fascinating mode of dehiscence is seen in the fruit of the Scarlet Pimpernel . The whole top of the capsule comes off most neatly as a little lid. As the old style still projects from the top of the fruit, the separating lid looks a little like an elfin cap, and this particular capsule is called a pyxidium.

In the basal half of the capsule the seeds are seen attached to a central column because the placentation is free-central.

A similar pyxidium is found in the Plantain, whose rosettes of foliage leaves cover the bare patches of many a waste bit of ground.

The pod, or legume, is the typical fruit of Leguminosx . It is dry, many-seeded, and formed from one carpel. It opens to let the seeds escape by splitting down both sides – that is, along the dorsal and ventral sutures – in other words, along the line that corresponds to the midrib of the leaf and that which corresponds to its united margins.

A follicle is also a dry, many-seeded fruit formed from one carpel . Unlike the pod, it splits down one side only, and that is the ventral. Follicles are seldom found singly because they occur in flowers with apocarpous pistils, as in the Columbine, Monkshood, and Christmas Rose.

A siliqua is long and narrow, and a silicula is short and broad . These are the characteristic fruits of Cruciferx. They are formed from two carpels. The fruit appears two-chambered, but this condition has been brought about by the growth of a false septum between the two carpels.

When the carpels lie flat so that they are almost in contact, the septum lies in the same plane as the carpels. This condition is found in Honesty . These mericarps are put into salads and may be used instead of capers for making sauce .

In Treacle Mustard the carpellary leaf is partly folded along the midrib, so that the fruit is approximately square in section.

More acute bending along the midrib produces the condition found in Shepherd’s Purse, where the septum is in the narrow plane of the fruit. The four mericarps of the White Dead Nettle result from a sharper angular bend, or keeling of the leaf at the midrib, gives the condition found in Penny-cress, where the septum is at its narrowest .

In the siliqua and silicula the seeds are borne on thickened marginal placentas which are connected by the septum. These placentas form a sort of framework, which is called the rep/um. In the Geranium there are five mericarps and in the Hollyhock and Mallow the number is indefinite .

The replum is seen well in Honesty as it is sold in the streets in autumn . The fruit coat is no longer there, but the septum persists and is the flat, silvery ornamental part. The framework round this is the replum. Impressions of the seeds are often seen on the septum, and often, too, seed-stalks are still attached to the replum and lie fiat upon the membrane.

Typical siliquas occur in Wallflower and Stock . A drupe is a juicy, one-seeded fruit in which the pericarp consists of three layers : epicarp, mesocarp, and endocarp.

In Shepherd’s Purse, Candytuft, and Sweet Alyssum the fruit is a silicula .

In both types dehiscence takes place by separation of the valves from below upwards .

A schizocarp is a fruit formed from a syncarpous pistil. In the process of maturing it divides into separate parts, the Juicy Fruits.

Number of these being generally determined by . It occurs in many members of the Rosaceaa, as in the Plum, Cherry, Apricot, and Peach.

The number of carpels. Each part into which the schizocarp divides is called a mericarp. It contains one seed and closely resembles an achene. In the Almond, which also belongs to Rosacew, the drupaceous character is not so obvious because, in Almonds as we buy them, only the endocarp and the enclosed seed remain. This gives the general impression that the Almond is a nut. The epicarp and mesocarp are familiar to all who have Almond trees in the garden, for, even in England, some fruits are formed each year. The epicarp is green and rather plush-like. The mesocarp has by no means the juicy character of the Plum and Cherry.

There are two mericarps in the double samara of the Sycamore, and in all the fruits of Umbelliferx to which the Caraway belongs. Seed cakes are, therefore, mericarp cakes, strictly speaking . The Walnut and Coco-nut are other rather unrecognisable drupes. They do not belong to Rosacew.

In the Garden Nasturtium the schizocarp divides into three. In pickled Walnuts the drupe-like character of the fruit is obvious. As they are sold by the greengrocer they, like the Almond, consist of endocarp and seed only.

The Coco-nut, too, has its epicarp and mesocarp removed before it is offered for sale.

The epicarp is thick, tough, and shiny. It makes a waterproof cover for the fruit when it is being carried by the waves.

The mesocarp in this case is not at all juicy. It is, on the other hand, very fibrous and is used for making Coco-nut matting. The waste from this process is the fibre sold for growing bulbs. Its value for this purpose depends upon the fact that it holds so much air. This makes it quite a difficult matter to mix the dry fibre with water. This quality of the mesocarp helps greatly in buoying up the fruit when it falls from the palm tree to the sea.

The hard brown endocarp of the Coco-nut has three dints at one end. One of these is so soft that a hole can easily be made to let the milk escape. The dints are germ pores, but now only one embryo develops in the Coco-nut, so two of the germ pores are permanently closed.

The very small root lies in the very large seed close to the functional germ pore of the endocarp.

In a berry the whole pericarp except the outside skin becomes succulent.

In the Tomato, Orange, and Grape, the berry is formed from a superior ovary.

The Gooseberry, Currant, Vegetable Marrow, Cucumber, and Banana are all inferior .

In the Banana vegetative propagation has reached such a degree of perfection that in the cultivated species seeds do not develop. All that remains of them is a sort of brown dustiness, occasionally seen in the centre of the fruit.

A Date is a berry with one seed only. The reserve food in this case is stored up as cellulose, and that is why the date-stone, or seed, is so hard all through. The date stone is in no way comparable to the stone of a drupe.

In the seed of one species of palm the cellulose store is so great and so hard that it may be carved into little figures. It is sold as vegetable ivory.

The pome has already been described . The true fruit of the Apple and Pear is the core. The succulent tissue that encloses the core is the stem or receptacle and the external skin is the epidermis of the stem .

The fruit of the Hawthorn is difficult to place. It is variously described as a berry, a drupe, or a pome.

A flower can only produce a collection of fruits when its pistil is made up of free carpels. As the apocarpous pistil is characteristic of Ranunculacex and Rosacex, collective fruits are found in these families.

In Ranunculacex the fruits of Delphinium, Monkshood, and Columbine, of the Peony, Christmas Rose, and Kingcup (Marsh Marigold) are collections of follicles .

The fruits of all the Butter- cups, of Clematis, and Ane- mone are collections of achenes .

In Rosa-cex there are heads of achenes in the Rose, Avens (Geum), and Strawberry . The Raspberry, Blackberry, and Loganberry are not berries at all, but collections of very small drupes, called drupels .

When fertilisation has taken place in the flower, so that the ovule has become a seed and the ovary a fruit, the next consideration is the satisfactory distribution of the seeds. There must, of necessity, be a good deal of waste in all operations that take place on a very big scale, but if seeds simply dropped from fruits in a haphazard way, the waste would be so excessive as to be a very serious matter. Such a state of overcrowding would result that very few young plants would survive. They would struggle one against another for space, food, and air, and, in the case of woody perennials, the parent plant would be a most formidable competitor.

For this reason more or less adequate adaptations with a view to efficient seed-dispersal are found. These may be adaptations of the fruit or, in a many-seeded fruit, of the seeds themselves, so that separation of the seeds is effected.

One of the most important external agents in seed-dispersal is wind. Appendages in the forms of hairs and wings are found on many fruits and seeds and give a certain buoyancy that greatly aids the carrying power of the wind.

In fruits the wing may be a modification of the pen- carp. The formation of the wing depends upon the expansion and flattening of the pen-carp during development, except in the area that encloses the seed. Such a fruit is called a samara .

The Ash and Elm are single samaras. The Birch is a single samara, but each fruit has a divided wing. The three-pointed leaves, rather like scout badges, which are found among Birch fruits are bracts, in the axil of each of which three flowers grew .

The Sycamore fruit is a double samara and divides before dispersal into two mericarps . It is only necessary to fling a mericarp up to the ceiling to realise the help given by the wing in dispersal. The spin results from the fact that the wing is heavy on one side and light on the other.

In some fruits the wing is formed by persistent bracts which, unlike those of the Birch, remain attached to the fruit. Three joined bracts remain in connection with the nut of the Hornbeam and help in its dispersal . In the Lime the axis of the inflorescence is firmly attached to a long slender bract for nearly half its length . In this case the bract keeps rain from reaching the pollen when the flower is out ; when the flower is succeeded by the fruit it serves as a wing to help in dispersal.

When a fruit is provided with hairs . these may be in the form of a pappus, as in many of the Compositx, where the calyx is often represented by this ring of hairs.

In the Dandelion the pappus is supported by a slender column and acts like a parachute . Thistle fruits float very lightly on the air. Their attachment to the pappus is so slight that a roughness in the breeze, or any contact, is enough to separate them. The fruit then falls to the ground and, as thistledown, the pappus floats away. In all probability this accounts for the fact that Thistles so often grow under walls and hedges, which have acted as obstacles to the floating fruits.

In some cases fruits owe their lightness to numerous hairs which grow from persistent styles. Traveller’s Joy, or old Nlan’s Beard, is one of the best-known examples of this method of dispersal .

It is extremely important not to confound fruits and seeds. In the foregoing examples the special aid for dispersal is some adaptation of the fruit itself, the seed being carried within the fruit.

Similar adaptations occur in seeds. As might be expected, the expansion and flattening of the testa make a serviceable wing. Beautifully lobed wings are formed in this way in Bignonia seeds. Bignonias are tropical plants, but it is possible to get their seeds from many of our British parks and botanical gardens . From our own flower- beds we can collect the fruits of Gladiolus and see the winged seeds within the capsule .

The wing of the Pine,

Cedar, and allied seeds is formed quite differently . It is a thin, membraneous detachment from the inner part of the cone-scale, upon which the seed lies. An ovule of the Pine may miss fertilisation, but the wing still becomes detached from the cone, ready to buoy up a seed that will never be formed. Pine kernels, detached from the wings, take the place of Almonds on the cheaper Dundee cakes.

Delicate hairs that help in seed-dispersal are in some cases outgrowths of the testa. To see them to the best advantage a fruit of Greater Willowherb should be carefully opened . The most satisfactory way is to hold the short stalk of the capsule between the finger and thumb of one hand and, with the other, give a pinch and a slight twist to the apex of the inferior fruit, just by the scar of the fallen petals. The fruit then begins to split by four valvular openings and reveals a marvellous instance of packing. The column that stands erect in the centre of the open capsule is the placenta to which the seeds were attached.

The cotton of commerce is obtained from the long hairs on the seeds of the Cotton plant.

In other cases hairs grow at the base of seeds and are not developed until after fertilisation. They are, therefore, of the nature of an aril . The dispersal of Poplar and Willow seeds depends upon such an aril of hairs .

On the whole the development of winged and hairy appendages on fruit and seed is effectual in securing a fairly wide range of distribution. Where dispersal consists in nothing more than the shaking of the ripe fruit, the scattered seeds fall over a very limited area. It is, however, a sufficiently good method for annuals.

This method, known as censer mechanism, is well seen in the Poppy . The long, stiff fruit-stalk waves in the breeze – the seeds inside the capsule have become detached from their parietal placentas and are shaken out of the porzs at the top of the fruit. This method is by no means limited to annuals. It occurs in biennials and perennials also, as in Foxglove and Snapdragon.

There is an ingenious device in different Campanula (Canterbury Bell) capsules, which open by three to five pores. The fruit of some species is erect and then the pores are at the top, close under the persistent calyx. In others the fruit droops and the pores are at its base . In either case the pores are uppermost in space and, therefore, more force is required to expel the seeds and they are more wide-flung. There is, here, the apparent danger of rain entering the capsule and wetting the seeds, because of this position of the pores. The walls of the capsule, however, are very hygroscopic, that is, very sensitive to moisture, and the pores close at once in damp weather.

In some cases seeds are so light that no additional device is necessary to secure their dispersal. In Orchids they are like fine dust.

Dispersal by water is not nearly so common as one might suppose. The extremely buoyant fruits of Water Lilies, for instance, rest on the surface of ponds. But this is stagnant water, and the actual movement of the fruit depends upon wind.

Some plants that grow by the sides of streams drop their fruits or seeds into the water. This often happens in the case of Alders. Sometimes the seeds are carried by the current and lodge in a more distant part of the bank. But, speaking generally, in such cases the fruits and seeds get water-logged and sink. Coco-nuts are certainly carried by waves from one island to another, and this explains the palm-girt character of the islands. This is a highly specialised case.

Actual dispersal by water does take place when seeds of wall- and rock-plants are carried in trickles of rain and lodge in a crevice some little distance from the parent plant. Seeds of Snapdragon, growing on old walls and chalky cliffs, of Wallflowers on old walls, of Herb Robert, also on old walls, are all dispersed in this way.

For dispersal over a really wide area, animals, especially birds, are largely responsible. Web-footed water-birds carry seeds in the mud that clings to their feet.

Other birds, jackdaws, swallows, and wagtails, often get smeared with mud. Later, when they preen their feathers and clean up their feet, the dry mud, with any seeds it may contain, falls to the ground, probably at a distance from the original place of contact.

In his very careful investigations, Darwin removed 6/ ounces of mud from water-fowls and mixed it with sterilised soil. From this 6/ ounces, 537 plants germinated.

Our own gardens show how very frequently fruits must be dropped by birds in flight. Oak, Hawthorn, and Beech seedlings spring up in suburban gardens, often far from the trees that bore them.

Some fruits and seeds are expelled in the excretions of birds.

This implies that the fruits have a very hard pericarp or the seeds a very hard testa, so that these coats resist digestion in their passage along the bird’s alimentary tract. Numbers of hard seeds are dispersed in this way by thrushes and black- birds especially. The missel-thrush gets its name from its connection with Mistletoe. It eats the viscous fruits and expels the undigested seeds. Probably this accounts for the almost invariable position of Mistletoe, which hangs from the undersides of the branches of Apple and Poplar. The excre- tion of thrushes is extremely liquid – it trickles down the curve of the branch and lodges the seed in the crevice of the bark towards its lower surface.

It is just possible that in some few cases dispersal may depend upon neglected food-stores. The squirrel and jay make stores of fruits and seeds for use in the barren winter months. If these are forgotten, or if the animal dies, germination may take place. But, for the most part, these stores are too crowded to be regarded as an effective means of dispersal.

Another proverbially provident animal in the matter of food stores is the ant. Ants are particularly attracted by seeds which have, at the base, a little partial aril called a caruncle. The seeds of Violet, Periwinkle, and Greater Celandine all have such a caruncle. The ants labour along, collecting their store, seed by seed. At the end they eat nothing more than the caruncle, so that the seed itself is unharmed and germinates.

Apart from this, the value of the caruncle is not very evident. It is supposed, in some cases, to absorb water in a sponge-like way and pass it to the micropyle, which lies near it. This may be so in the Castor Oil where the testa is very resistant .

An aril is an additional covering to a seed. It develops after fertilisation has taken place. Very obvious arils are the bright red, juicy coverings that occur on the Yew, threading it with colour . The Spindle seed, too, has a bright orange aril contrasting strongly, when the fruit dehisces, with the pink pericarp. In these cases, without doubt, the aril is attractive to birds, which expel with their excretion the undigested seeds.

A more highly organised plan for dispersal by animals is found in those numerous fruits that are provided with adhesive appendages in one form or another. Kerner estimates that about io per cent. Of all Flowering Plants possess fruits or seeds which are dispersed by means of clawed or barbed processes.

On the elongated receptacle of Geum (Avens) the achenes have long, hooked styles. Originally the styles were straight. Later they form a loop. Later still the loop breaks in such a way that a hooked end remains .

In Goosegrass stiffly de-curving hairs, like those that help the stem to climb, occur all over the two halves of the schizo-carp . Similar hairs develop on the mericarps of • Enchanter’s Nightshade. After walking through the undergrowth of a wood these small burrs may be found in dozens, clinging to one’s stockings.

The burrs of the Burdock are quite different morphologically, for hooks are not developed on the fruits. Burdock belongs to Compositx, and each bract of the involucre curves sharply back at the tip . Thus the numerous small black cypselas are surrounded by a chaplet of hooks. The hairs of the imperfect pappus are extraordinarily irritant. It is unwise to let them touch the face or neck, for they cause a burning that is quite disagreeable.

The burrs in Agrimony are different again. Agrimony is a member of Rosacex and shows such extreme perigyny that it becomes, as in the Apple, epigyny. In this case the true fruit is only one-seeded, so the fruit is a sort of inferior nut. The burr results from the stiff hooked bristles that grow from the receptacle .

In the Bur-marigold, which is a Composite, two or more stiff spikes, representing the calyx, project from the top of the fruit. These are barbed and cling tenaciously to any object with which they come in contact .

All these barbed fruits may get fixed in the coats of passing animals, to be set free in some more distant area.

Many cases of seed-dispersal depend upon some mechan- ism in the fruit.

An extreme case is that of the Squirting Cucumber. When this separates from its stalk a hole remains at the point of junction.. The expansion of a particular layer of tissue within the fruit causes so much pressure that the seeds, and all the mucilage surrounding them, are shot out of the hole with force.

Among our own plants similar phenomena, but of a milder nature, are met with. Violent expulsion of the seeds of the Wood Sorrel is brought about by a layer , of cells on the inner side of the aril of each seed – this swells and thus imposes a great strain on the outer layers. The result is a sudden slit in the outer region of the aril, a swift rolling back of the slit edges, and the jerking of the seed from the fruit.

In the Balsam (Touch-me-not), the Bitter Cress, and Violet the expulsion is due to a rupturing of the fruit coat, as a result of inequality in stresses and strains .

In Geraniums the fruit is a schizocarp. The separation of the men-carps follows the lines of union of the five parts of the style. The rupture is sudden and begins from below, so that each released strip curves outwards and rolls up. As the rolling is a sudden process the seeds are flung out with a jerk, as a stone is flung from a catapult .

In many Leguminosw, seeds are expelled by a twisting of the two halves of the pod at the time of dehiscence .

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