Part 3

The object of the fruit in the economy of the plant is the protection and nursing of the developing seed and the dispersion of the ripe seeds. Hence, generally, one-seeded fruits are indehiscent, while fruits containing more than one seed open to allow of the dispersal of the seeds over as wide an area as possible. The form, colour, structure and method of dehiscence of fruits and the form of the contained seeds are intimately associated with the means of dispersal, which fall into several categories. (1) By a mechanism residing in the fruit. Thus many fruits open suddenly when they are dry, and the seeds are ejected by the twisting or curving of the valves, or in some other way; e.g. in gorse, by the spiral curving of the valves; in _Impatiens_, by the twisting of the cocci; in squirting cucumber, by the pressure exerted on the pulpy contents by the walls of the pericarp. (2) By aid of various external agencies such as water. Fruits or seeds are sometimes sufficiently buoyant to float for a long time on sea- or fresh-water; e.g. coco-nut, by means of its thick, fibrous coat (mesocarp), is carried hundreds of miles in the sea, the tough, leathery outer coat (epicarp) preventing it from becoming water-soaked. Fruits and seeds of West Indian plants are thrown up on the coasts of north-west Europe, having been carried by the Gulf Stream, and will often germinate; many are rendered buoyant by air-containing cavities, and the embryo is protected from the seawater by the tough coat of fruit or seed. Water-lily seeds are surrounded with a spongy tissue when set free from the fruit, and float for some distance before dropping to the bottom. (3) The most general agent in the dispersal of seeds is the wind or currents of air--the fruit or seed being rendered buoyant by wing-developments as in fruits of ash (fig. 1) or maple (fig. 21), seeds of pines and firs, or many members of the order Bignoniaceae; or hair-developments as in fruits of clematis, where the style forms a feathery appendage, fruits of many Compositae (dandelion, thistle, &c.), which are crowned by a plumose pappus, or seeds of willow and poplar, or _Asclepias_ (fig. 36), which bear tufts of silky hairs; to this category belong bladder-like fruits, such as bladder-senna, which are easily rolled by the wind, or cases like the so-called rose of Jericho, a small cruciferous plant (_Anastatica hierocuntica_), where the plant dries up after developing its fruits and becomes detached from the ground; the branches curl inwards, and the whole plant is rolled over the dry ground by the wind. The wind also aids the dispersal of the seeds in the case of fruits which open by small teeth (many Caryophyllaceae [fig. 6]) or pores (poppy [fig. 7], _Campanula_, &c.); the seeds are in these cases small and numerous, and are jerked through the pores when the capsules, which are generally borne on long, dry stems or stalks, are shaken by the wind. (4) In other cases members of the animal world aid in seed-dispersal. Fruits often bear stiff hairs or small hooks, which cling to the coat of an animal or the feathers of a bird; such are fruits of cleavers (_Galium Aparine_), a common hedge-row plant, _Ranunculus arvensis_ (fig. 20), carrot, _Geum_, &c.; or the fruit or seed has an often bright-coloured, fleshy covering, which is sought by birds as food, as in stone-fruits such as plum, cherry (fig. 5), &c., where the seed is protected from injury in the mouth or stomach of the animal by the hard endocarp; or the hips of the rose (fig. 3), where the succulent scarlet "fruit" (the swollen receptacle) envelops a number of small dry true fruits (achenes), which cling by means of stiff hairs to the beak of the bird.

[Illustration:

FIG. 22.--Vertical section of a grain of wheat, showing embryo below at the base of the endosperm e; s, scutellum separating embryo from endosperm; f.l, foliage leaf; p.s, sheath of plumule; p.r, primary root; s.p.r, sheath of primary root.

FIG. 23.--Fruit of Comfrey (_Symphytum_) surrounded by persistent calyx, c. The style s appears to arise from the base of the carpels, enlarged.

FIG. 24.--Ovary of _Foeniculum officinale_ with pendulous ovules, in longitudinal section. (After Berg and Schmidt, magnified.)

From Strasburger's _Lehrbuch der Botanik_, by permission of Gustav Fischer.

FIG. 25.--Fruit of _Carum Carui_. A, Ovary of the flower; B, ripe fruit. The two carpels have separated so as to form two mericarps (m). Part of the septum constitutes the carpophore (a). p, Top of flower-stalk; d, disk on top of ovary; n, stigma.

From Vines' _Students' Text-Book of Botany_, by permission of Swan Sonnenschein & Co.]

Forms of fruit.

Simple fruits have either a _dry_ or _succulent_ pericarp. The _achene_ is a dry, one-seeded, indehiscent fruit, the pericarp of which is closely applied to the seed, but separable from it. It is solitary, forming a single fruit, as in the dock (fig. 19) and in the cashew, where it is supported on a fleshy peduncle; or _aggregate_, as in _Ranunculus_ (fig. 20), where several achenes are placed on a common elevated receptacle. In the strawberry the achenes (fig. 2) are aggregated on a convex succulent receptacle. In the rose they are supported on a concave receptacle (fig. 3), and in the fig the succulent receptacle completely encloses the achenes (fig. 4). In _Dorstenia_ the achenes are situated on a flat or slightly concave receptacle. Hence what in common language are called the seeds of the strawberry, rose and fig, are in reality ripe carpels. The styles occasionally remain attached to the achenes in the form of feathery appendages, as in _Clematis_. In Compositae, the fruit is an inferior achene (_cypsela_), to which the pappus (modified calyx) remains adherent. Such is also the nature of the fruit in Dipsacaceae (e.g. scabious). When the pericarp is thin, and appears like a bladder surrounding the seed, the achene is termed a _utricle_, as in Amarantaceae. When the pericarp is extended in the form of a winged appendage, a _samara_ or _samaroid achene_ is produced, as in the ash (fig. 1) and common sycamore (fig. 21). In these cases there are usually two achenes united, one of which, however, as in _Fraxinus_ (fig. 1), may be abortive. The wing surrounds the fruit longitudinally in the elm. When the pericarp becomes so incorporated with the seed as to be inseparable from it, as in grains of wheat (fig. 22), maize, oats and other grasses, then the name _caryopsis_ is given. The one-seeded portions (mericarps) of schizocarps often take the form of achenes, e.g. the mericarps of the mallows or of umbellifers (figs. 24, 25). In Labiatae and Boraginaceae (e.g. comfrey, fig. 23), where the bicarpellary ovary becomes our one-seeded portions in the fruit, the partial fruits are of the nature of achenes or nutlets according to the texture (leathery or hard) of the pericarp.

[Illustration: From Strasburger's _Lehrbuch der Botanik_, by permission of Gustav Fischer.

FIG. 26.--Cupule of _Quercus Aegilops_. cp, cupule; gl, fruit. (After Duchartre.)]

The _nut_ or _glans_ is a dry one-celled indehiscent fruit with a hardened pericarp, often surrounded by bracts at the base, and, when mature, containing only one seed. In the young state the ovary often contains two or more ovules, but only one comes to maturity. It is illustrated by the fruits of the hazel and chestnut, which are covered by leafy bracts, in the form of a _husk_, and by the acorn, in which the bracts and receptacle form a _cupula_ or _cup_ (fig. 26). The parts of the pericarp of the nut are united so as to appear one. In common language the term nut is very vaguely applied both to fruit and seeds.

The _drupe_ is a succulent usually one-seeded indehiscent fruit, with a pericarp easily distinguishable into epicarp, mesocarp and endocarp. This term is applied to such fruits as the cherry (fig. 5), peach, plum, apricot or mango. The endocarp is usually hard, forming the stone (putamen) of the fruit, which encloses the kernel or seed. The mesocarp is generally pulpy and succulent, so as to be truly a sarcocarp, as in the peach, but it is sometimes of a tough texture, as in the almond, and at other times is more or less fibrous, as in the coco-nut. In the almond there are often two ovules formed, only one of which comes to perfection. In the raspberry and bramble several small drupes or _drupels_ are aggregated so as to constitute an _etaerio_.

The _follicle_ is a dry unilocular many-seeded fruit, formed from one carpel and dehiscing by the ventral suture. It is rare to meet with a solitary follicle forming the fruit. There are usually several aggregated together, either in a whorl on a shortened receptacle, as in hellebore, aconite, larkspur, columbine (figs. 27, 28) or the order Crassulaceae, or in a spiral manner on an elongated receptacle, as in _Magnolia_ and _Banksia_. Occasionally, follicles dehisce by the dorsal suture, as in _Magnolia grandiflora_ and _Banksia_.

[Illustration:

FIG. 27.--Fruit of Columbine (_Aquilegia_), formed of five follicles.

FIG. 28.--Single follicle, showing dehiscence by the ventral suture.

FIG. 29.--Transverse section of berry of Gooseberry, showing the seeds attached to the parietal placentas and immersed in pulp, which is formed partly from the endocarp, partly from the seed-coat.

FIG. 30.--Section of the fruit of the Apple (_Pyrus Malus_), or pome, consisting of a fleshy covering formed by the floral receptacle and the true fruit or core with five cavities with seeds.]

The _legume_ or _pod_ is a dry monocarpellary unilocular many-seeded fruit, formed from one carpel, dehiscing both by the ventral and the dorsal suture. It characterizes leguminous plants, as the bean and pea (fig. 8). In the bladder-senna it forms an inflated legume. In some Leguminosae, as _Arachis_, _Cathartocarpus Fistula_ and the tamarind, the fruit must be considered a legume, although it does not dehisce. The first of these plants produces its fruit underground, and is called earth-nut; the second has a partitioned legume and is schizocarpic; and both the second and third have pulpy matter surrounding the seeds. Some legumes are schizocarpic by the formation of constrictions externally. Such a form is the _lomentum_ or _lomentaceous legume_ of _Hedysarum_ (fig. 17), _Coronilla_, _Ornithopus_, _Entada_ and of some Acacias. In _Medicago_ the legume ~~ is twisted like a snail, and in _Caesalpinia coriaria_, or Divi-divi, it is vermiform or curved like a worm. Sometimes the number of seeds is reduced, as in _Erythrina monosperma_ and _Geoffroya superba_, which are one-seeded, and in _Pterocarpus_ and _Dalbergia_, which are two-seeded.

[Illustration: FIG. 31.--Transverse section of the fruit of the Melon (_Cucumis Melo_), showing the placentas with the seeds attached to them. The three carpels forming the pepo are separated by partitions. From the centre processes pass outwards, ending in the curved placenta.

The _berry_ (_bacca_) is a term applied generally to all fruits with seeds immersed in pulp, and includes fruits of very various origin. In _Actaea_ (baneberry) or _Berberis_ (barberry) it is derived from a single free carpel; generally, however, it is the product of a syncarpous ovary, which is superior, as in grape or potato, or inferior, as in gooseberry (fig. 29) or currant. In the pomegranate there is a peculiar baccate many-celled inferior fruit, having a tough rind, enclosing two rows of carpels placed one above the other. The seeds are immersed in pulp, and are attached irregularly to the wall, base and centre of the loculi. In the baobab there is a multilocular syncarpous fruit, in which the seeds are immersed in pulp.

The _pepo_, another indehiscent syncarpous fruit, is illustrated by the fruit of the gourd, melon (fig. 31) and other Cucurbitaceae. It is formed of three carpels, surmounted by the calyx; the rind is thick and fleshy, and there are three or more seed-bearing parietal placentas, either surrounding a central cavity or prolonged inwards into it. The fruit of the papaw resembles the pepo, but the calyx is not superior.

The _hesperidium_ is the name given to such indehiscent fleshy syncarpous fruits as the orange, lemon and shaddock, in which the epicarp and mesocarp form a separable rind, and the endocarp sends prolongations inwards, forming triangular divisions, to the inner angle of which the seeds are attached, pulpy cells being developed around them from the wall. Both pepo and hesperidium may be considered as modifications of the berry.

The _pome_ (fig. 30), seen in the apple, pear, quince, medlar and hawthorn, is a fleshy indehiscent syncarpous fruit, in the formation of which the receptacle takes part. The outer succulent part is the swollen receptacle, the horny core being the true fruit developed from the usually five carpels and enclosing the seeds. In the medlar the core (or true pericarp) is of a stony hardness, while the outer succulent covering is open at the summit. The pome somewhat resembles the fruit of the rose (fig. 3), where the succulent receptacle surrounds a number of separate achenes.

The name _capsule_ is applied generally to all dry syncarpous fruits, which dehisce by valves. It may thus be unilocular or multilocular, one- or many-seeded. The true valvular capsule is observed in _Colchicum_ (fig. 9), lily and iris (fig. 11). The _porose capsule_ is seen in the poppy (fig. 7), _Antirrhinum_ and _Campanula_. In _Campanula_ the pores occur at the base of the capsule, which becomes inverted when ripe. When the capsule opens by a lid, or by circumscissile dehiscence, it is called a _pyxidium_, as in pimpernel (_Anagallis arvensis_) (fig. 16), henbane and monkey-pot (_Lecythis_). The capsule assumes a screw-like form in _Helicteres_, and a star-like form in star-anise (_Illicium anisatum_). In certain instances the cells of the capsule separate from each other, and open with elasticity to scatter the seeds. This kind of capsule is met with in the sandbox tree (_Hura crepitans_) and other Euphorbiaceae, where the cocci, containing each a single seed, burst asunder with force; and in Geraniaceae, where the cocci, each containing, when mature, usually one seed, separate from the carpophore, become curved upwards by their adherent styles, and open by the ventral suture (fig. 18).

The _siliqua_ is a dry syncarpous bilocular many-seeded fruit, formed from two carpels, with a false septum, dehiscing by two valves from below upwards, the valves separating from the placentas and leaving them united by the septum (fig. 32). The seeds are attached on both sides of the septum, either in one row or in two. When the fruit is long and narrow it is a _siliqua_ (fig. 14); when broad and short, _silicula_ (fig. 33). It occurs in cruciferous plants, as wallflower, cabbage and cress. In _Glaucium_ and _Eschscholtzia_ (Papaveraceae) the dissepiment is of a spongy nature. It may become transversely constricted (_lomentaceous_), as in radish (_Raphanus_) and sea-kale, and it may be reduced, as in woad (_Isatis_), to a one-seeded condition.

It sometimes happens that the ovaries of two flowers unite so as to form a double fruit (_syncarp_). This may be seen in many species of honeysuckle. But the fruits which are now to be considered consist usually of the floral envelopes, as well as the ovaries of several flowers united into one, and are called _multiple_ or _confluent_. The term _anthocarpous_ has also been applied as indicating that the floral envelopes as well as the carpels are concerned in the formation of the fruit.

The _sorosis_ is a succulent multiple fruit formed by the confluence of a spike of flowers, as in the fruit of the pine-apple (fig. 34), the bread-fruit and jack-fruit. Similarly the fruit of the mulberry represents a catkin-like inflorescence.

The _syconus_ is an anthocarpous fruit, in which the receptacle completely encloses numerous flowers and becomes succulent. The fig (fig. 4) is of this nature, and what are called its seeds are the achenes of the numerous flowers scattered over the succulent hollowed receptacle. In _Dorstenia_ the axis is less deeply hollowed, and of a harder texture, the fruit exhibiting often very anomalous forms.

The _strobilus_, or _cone_, is a seed-bearing spike, more or less elongated, covered with scales, each of which may be regarded as representing a separate flower, and has often two seeds at its base; the seeds are naked, no ovary being present. This fruit is seen in the cones of firs, spruces, larches and cedars, which have received the name of Coniferae, or cone-bearers, on this account. Cone-like fruit is also seen in most Cycadaceae. The scales of the strobilus are sometimes thick and closely united, so as to form a more or less angular and rounded mass, as in the cypress; while in the juniper they become fleshy, and are so incorporated as to form a globular fruit like a berry. The dry fruit of the cypress and the succulent fruit of the juniper have received the name of _galbulus_. In the hop the fruit is called also a strobilus, but in it the scales are thin and membranous, and the seeds are not naked but are contained in pericarps.

[Illustration:

FIG. 32.--Honesty (_Lunaria biennis_), showing the septum after the carpels have fallen away.

From Strasburger's _Lehrbuch der Botanik_, by permission of Gustav Fischer.

FIG. 33.--Silicula or pouch of shepherd's purse (_Capsella_), opening by two folded valves, which separate from above downwards. The partition is narrow, hence the silicula is angustiseptal.

From Strasburger's _Lehrbuch der Botanik_, by permission of Gustav Fischer.

FIG. 34.--Fruit of the pine-apple (_Ananassa sativa_), developed from a spike of numerous flowers with bracts, united so as to form a collective or anthocarpous fruit. The crown of the pine-apple, c, consists of a series of empty bracts prolonged beyond the fruit.]

The same causes which produce alterations in the other parts of the flower give rise to anomalous appearances in the fruit. The carpels, in place of bearing seeds, are sometimes changed into leaves, with lobes at their margins. Leaves are sometimes produced from the upper part of the fruit. In the genus _Citrus_, to which the orange and lemon belong, it is very common to meet with a separation of the carpels, so as to produce what are called horned oranges and fingered citrons. In this case a syncarpous fruit has a tendency to become apocarpous. In the orange we occasionally find a supernumerary row of carpels produced, giving rise to the appearance of small and imperfect oranges enclosed within the original one; the navel orange is of this nature. It sometimes happens that, by the union of flowers, double fruits are produced. Occasionally a double fruit is produced, not by the incorporation of two flowers, but by the abnormal development of a second carpel in the flower.

_Arrangement of Fruits._

A. True fruits--developed from the ovary alone. 1. Pericarp not fleshy or fibrous. i. Indehiscent--not opening to allow the escape of the seeds--generally one-seeded. Achene; caryopsis; cypsela; nut; schizocarp. ii. Dehiscent--the pericarp splits to allow the escape of the seeds--generally many-seeded. Follicle; legume; siliqua; capsule. 2. Pericarp generally differentiated into distinct layers, one of which is succulent or fibrous. Drupe; berry. B. Pseudocarps--the development extends beyond the ovary. Pome; syconus; sorosis.

_The Seed._--The _seed_ is formed from the ovule as the result of fertilization. It is contained in a seed-vessel formed from the ovary in the plants called _angiospermous_; while in _gymnospermous_ plants, such as Coniferae and Cycadaceae, it is naked, or, in other words, has no true pericarp. It sometimes happens in Angiosperms, that the seed-vessel is ruptured at an early period of growth, so that the seeds become more or less exposed during their development; this occurs in mignonette, where the capsule opens at the apex, and in _Cuphea_, where the placenta bursts through the ovary and floral envelopes, and appears as an erect process bearing the young seeds. After fertilization the ovule is greatly changed, in connexion with the formation of the embryo. In the embryo-sac of most Angiosperms (q.v.) there is a development of cellular tissue, the endosperm, more or less filling the embryo-sac. In Gymnosperms (q.v.) the endosperm is formed preparatory to fertilization. The fertilized egg enlarges and becomes multicellular, forming the embryo. The embryo-sac enlarges greatly, displacing gradually the surrounding nucellus, which eventually forms merely a thin layer around the sac, or completely disappears. The remainder of the nucellus and the integuments of the ovules form the seed-coats. In some cases (fig. 35) a delicate inner coat or _tegmen_ can be distinguished from a tougher outer coat or _testa_; often, however, the layers are not thus separable. The consistency of the seed-coat, its thickness, the character of its surface, &c., vary widely, the variations being often closely associated with the environment or with the means of seed-dispersal. An account of the development of the seed from the ovule will be found in the article ANGIOSPERMS. When the pericarp is dehiscent the seed-covering is of a strong and often rough character; but when the pericarp is indehiscent and encloses the seed for a long period, the outer seed-coat is thin and soft. The cells of the testa are often coloured, and have projections and appendages of various kinds. Thus in _Abrus precatorius_ and _Adenanthera pavonina_ it is of a bright red colour; in French beans it is beautifully mottled; in the almond it is veined; in the tulip and primrose it is rough; in the snapdragon it is marked with depressions; in cotton and _Asclepias_ (fig. 36) it has hairs attached to it; and in mahogany, _Bignonia_, and the pines and firs it is expanded in the form of wing-like appendages (fig. 37). In _Collomia_, _Acanthodium_, _Cobaea scandens_ and other seeds, it contains spiral cells, from which, when moistened with water, the fibres uncoil in a beautiful manner; and in flax (_Linum_) and others the cells are converted into mucilage. These structural peculiarities of the testa in different plants have relation to the scattering of the seed and its germination upon a suitable nidus. But in some plants the pericarps assume structures which subserve the same purpose; this especially occurs in small pericarps enclosing single seeds, as achenes, caryopsides, &c. Thus in Compositae and valerian, the pappose limb of the calyx forms a parachute to the pericarp; in Labiatae and some Compositae spiral cells are formed in the epicarp; and the epicarp is prolonged as a wing in _Fraxinus_ (fig. 1) and _Acer_ (fig. 21).

[Illustration:

FIG. 35.--Seed of Pea (_Pisum_) with one cotyledon removed. c, Remaining cotyledon; ch, chalaza-point at which the nourishing vessels enter; e, tegmen or inner coat; f, funicle or stalk; g, plumule of embryo; m, micropyle; pl, placenta; r, radicle of embryo; t, tigellum or stalk between root and plumule; te, testa.

FIG. 36.--Seed of _Asclepias_, with a cluster of hairs arising from the edges of the micropyle.]