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Saturday, December 21st, 2024
the Third Week of Advent
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Fruit
1911 Encyclopedia Britannica
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(through the French from the Lat. fructus; frui, to enjoy), in its widest sense, any product of the soil that can be enjoyed by man or animals; the word is so used constantly in the Bible, and extended, as a Hebraism, to offspring or progeny of man and of animals, in such expressions as " the fruit of the body," " of the womb," " fruit of thy cattle " (Deut. xxviii. 4), &c., and generally to the product of any action or effort. Between this wide and frequently figurative use of the word and its application in the strict botanical sense treated below, there is a popular meaning, regarding the objects denoted by the word entirely from the standpoint of edibility, and differentiating them roughly from those other products of the soil, which, regarded similarly, are known as vegetables. In this sense " fruit " is applied to such seed-envelopes of plants as are edible, either raw or cooked, and are usually sweet, juicy or of a refreshing flavour. But applications of the word in this sense are apt to be loose and shifting according to the fashion of the time.
Fruit, in the botanical sense, is developed from the flower as the result of fertilization of the ovule. After fertilization various changes take place in the parts of the flower. Those more immediately concerned in the process, the anther and stigma, rapidly wither and decay, while the filaments and style often remain for some time; the floral envelopes become dry, the petals fall, and the sepals are either deciduous, or remain persistent in an altered form; the ovary becomes enlarged, forming the pericarp; and the ovules are developed as the seeds, containing the embryo-plant. The term fruit is strictly applied to the mature pistil or ovary, with the seeds in its interior; but it often includes other parts of the flower, such as the bracts and floral envelopes. Thus the fruit of the hazel and oak consists of the ovary enveloped by the bracts; that of the apple and pear, of the ovary and floral receptacle; and that of the pineapple, of the whole inflorescence. Such fruits are sometimes distinguished as pseudocarps. In popular language, the fruit includes all those parts which exhibit a striking change as the result of fertilization. In general, the fruit is not ripened unless fertilization has been effected; but cases occur as the result of cultivation in which the fruit swells and becomes to all appearance perfect, while no seeds are produced. Thus, there are seedless oranges, grapes and pineapples. When the ovules are unfertilized, it is common to find that the ovary withers and does not come to maturity; but in the case of bananas, plantains and breadfruit, the non-development of seeds seems to lead to a larger growth and a greater succulence of fruit.
The fruit, like the ovary, may be formed of a single carpel or of several. It may have one cell or cavity, being unilocular; or many, multilocular, &c. The number and nature of the divisions depend on the number of carpels and the extent to which their edges are folded inwards. The appearances presented by the ovary do not always remain permanent in the fruit. Great changes are observed to take place, not merely as regards the increased size of the ovary, its softening or hardening, but also in its internal structure, owing to the suppression, additional formation or enlargement of parts. Thus, in the ash (fig. I) an ovary with two cells, each containing an ovule attached to a central placenta, is changed into a unilocular fruit with one seed; one ovule becomes abortive, while the other, g, gradually enlarging until the septum is pushed to one side, unites with the walls of the cell, and the placenta appears to be parietal. In the oak and hazel, an ovary with three and two cells respectively, and two ovules in each, produces a one-celled fruit with one seed. In the coco-nut, a trilocular and triovular ovary produces a onecelled, one-seeded fruit. This abortion may depend on the pressure caused by the development of certain ovules, or it may proceed from non-fertilization of all the ovules and consequent non-enlargement of the carpels. Again, by the growth of the placenta, or the folding inwards of parts of the carpels, divisions occur in the fruit which did not exist in the ovary. In Cathartocarpus Fistula a one-celled ovary is changed into a fruit having each of its seeds in a separate cell, in consequence of spurious dissepiments being produced horizontal from the inner wall of the ovary. In flax (Linum) by the folding inwards of the back of the carpels a five-celled ovary becomes a ten-celled fruit. In Astragalus the folding inwards of the dorsal suture converts a one-celled ovary into a two-celled fruit; and in Oxytropis the folding of the ventral suture gives rise to a similar change. The development of cellular or pulpy matter, and the enlargement of parts not forming whorls of the flower, frequently alter the appearance of the fruit, and render it difficult to discover its formation. In the gooseberry (fig. 29), grape, guava, tomato and pomegranate, the seeds nestle in pulp formed by the placentas. In the orange the pulpy matter surrounding the seeds is formed by succulent cells, which are produced from the inner partitioned lining of the pericarp. In the strawberry the receptacle becomes succulent, and bears the mature carpels on its convex surface (fig. 2); in the rose there is a fleshy hollow receptacle which bears the carpels on its concave surface (fig. 3). In the juniper the scaly bracts grow up round the seeds and become succulent, and in the fig (fig. 4) the receptacle becomes succulent and encloses an inflorescence.
The pericarp consists usually of three layers, the external, or epicarp (fig. 5, ep); the middle, or mesocarp, m; and the internal, FIG. 4.
FIG. I. - Samara or winged fruit of Ash (Fraxinus). I, Entire, with its wing a; 2, lower portion cut transversely, to show that it consists of two cells; one of which, 1, is abortive, and is reduced to a very small cavity, while the other is much enlarged and filled with a seed g. FIG. 2. - Fruit of the Strawberry (Fragaria vesca), consisting of an enlarged succulent receptacle, bearing on its surface the small dry seed-like fruits (achenes). (After Duchartre.) From Strasburger's Lehrbuch der Botanik, by permission of Gustav Fischer.
FIG. 3. - Fruit of the Rose cut vertically. s', Fleshy hollowed receptacle; s, persistent sepals; fr, ripe carpels; e, stamens, withered.
FIG. 4. - Peduncle of Fig (Ficus Carica), ending in a hollow receptacle enclosing numerous male and female flowers.
FIG. 5. - Fruit of Cherry (Prunus Cerasus) in longitudinal section. ep, Epicarp; m, mesocarp; en, endocarp.
From Strasburger's Lehrbuch der Botanik, by permission of Gustav Fischer.
or endocarp, en. These layers are well seen in such a fruit as the peach, plum or cherry, where they are separable one from the other; in them the epicarp forms what is commonly called the skin; the mesocarp, much developed, forms the flesh or pulp, and hence has sometimes been called sarcocarp; while the endocarp, hardened by the production of woody cells, forms the stone or putamen immediately covering the kernel or seed. The pulpy matter found in the interior of fruits, such as the gooseberry, grape and others, is formed from the placentas, and must not be confounded with the sarcocarp. In some fruits, as in the nut, the three layers become blended together and are indistinguishable. In bladder senna (Colutea arborescens) the pericarp retains its leaflike appearance, but in most cases it becomes altered both in consistence and in colour. Thus in the date the epicarp is the outer brownish skin, the pulpy matter is the mesocarp or sarcocarp, and the thin papery-like lining is the endocarp covering the hard seed. In the medlar the endocarp becomes of a stony hardness. In the melon the epicarp and endocarp are very thin, while the mesocarp forms the bulk of the fruit, differing in texture and taste in its external and internal parts. The rind of the orange consists of epicarp and mesocarp, while the endocarp forms partitions in the interior, filled with pulpy cells. The part of the pericarp attached to the peduncle is the base, and the point where the style or stigma existed is the apex. This latter is not always the apparent apex, as in the case of the ovary; it may be lateral or even basilar. The style sometimes remains in a hardened form, rendering the fruit apiculate; at other times it falls off, leaving only traces of its existence. The presence of the style or stigma serves to distinguish certain singleseeded pericarps from seeds.
FIG. 3.
FIG. 2. FIG. I.
FIG. 5.
When the fruit is mature and the seeds are ripe, the carpels usually give way either at the ventral or dorsal suture or at both, and so allow the seeds to escape. The fruit in this case is dehiscent. But some fruits are indehiscent, falling to the ground entire, and the seeds eventually reaching the soil by their decay. By dehiscence the pericarp becomes divided into different pieces, or valves, the fruit being univalvular, bivalvular or multivalvular, &c., according as there are one, two or many valves. The splitting extends the whole length of the fruit, or is partial, the valves forming teeth at the apex, as in the order Caryo phyllaceae (fig. 6). Sometimes the valves are detached only at certain points, and thus dehiscence takes place by pores at the apex, as in poppy (fig. 7), or at the base, as in Campanula. Indehiscent fruits are either dry, as the nut, or fleshy, as the cherry and apple.
They are formed of one or several FIG. 6. FIG. 7. carpels. In the former case they usually contain only a single seed, which may become so incorporated with the pericarp as to appear to be naked, as in the grain of wheat and generally in grasses. In such cases the presence of the remains of style or stigma determines their true nature.
Dehiscent fruits, when composed of single carpels, may open by the ventral suture only, as in the paeony, hellebore, Aquilegia (fig. 28) and Caltha; by the dorsal suture only, as in magnolias and some Proteaceae, or by both together, as in the pea (fig. 8) and bean; in these cases the dehiscence is sutural. When composed of several FIG. 12. FIG. 13.
FIG. 8. - Dry dehiscent fruit. The pod (legume) of the Pea; r, the dorsal suture; b, the ventral; c, calyx; s, seeds.
From Vines' Students' Text-Book of Botany, by permission of Swan Sonnenschein & Co.
FIG. 9. - (I) Fruit or capsule of Meadow Saffron (Colchicum autumnale), dehiscing along the septa (septicidally); (2) same cut across, showing the three chambers with the seeds attached along the middle line (axile placentation).
FIG. Io. - Diagram to illustrate the septicidal dehiscence in a pentalocular capsule. The loculaments l correspond to the number of the carpels, which separate by splitting through the septa, s. FIG. II. - The seed vessel (capsule) of the Flower-de-Luce (Iris), opening in a loculicidal manner. The three valves bear the septa in the centre, and the opening takes place through the back of the loculaments. Each valve is formed by the halves of contiguous carpels.
FIG. 12. - Diagram to illustrate loculicidal dehiscence. The loculaments 1, split at the back, and the valves separate, bearing the septa s on their centres.
FIG. 13. - Diagram to illustrate septifragal dehiscence, in which the dehiscence takes place through the back of the loculaments 1, and the valves separate from the septa s, which are left attached to the p lacentas in the centre.
united carpels, two types of dehiscence occur - a longitudinal and a. transverse. In the longitudinal the separation may take place by the dissepiments throughout their length, so that the fruit is resolved into its original carpels, and each valve represents a carpel, as in rhododendron, Colchicum, &c.; this dehiscence, in consequence of taking place through the septum, is called septicidal (figs. 9, to). The valves separate from their commissure, or central line of union, carrying the placentas with them, or they leave the latter in the centre, so as to form with the axis a column of a cylindrical, conical or prismatic shape. Dehiscence is loculicidal when the union between the edges of the carpels is persistent, and they dehisce by the dorsal suture, or through the back of the loculaments, as in the lily and iris (figs. II, 12). In these cases each valve consists of a half of each of two contiguous carpels. The placentas either remain united to the axis, or they separate from it, being attached to the septa on the valves. When the outer walls of the carpels break off from the septa, leaving them attached to the central column, the dehiscence is said to be septifragal (fig. 13), and where, as in Linum catharticum and Calluna, the splitting takes place first of all in a septicidal manner, the fruit is described as septicidally septifragal; while in other cases, as in thorn apple (Datura Stramonium), where the splitting is at first loculicidal, the dehiscence is loculicidally septifragal. In all those forms the separation of the valves takes. place either from above downwards or from below upwards. In.
FIG. 16.
FIG. 14. FIG. 15. FIG. 17. FIG. 18.
FIG. 14. - Siliqua or seed-vessel of Wallflower (Cheiranthus Cheiri),. opening by two valves, which separate from the base upwards,. leaving the seeds attached to the dissepiment which is supported by the replum.
From Strasburger's Lehrbuch der Botanik, by permission of Gustav Fischer.
FIG. 15. - Capsule of an Orchid (Xylobium). v, valve.
FIG. 16. - Seed-vessel of Anagallisarvensis, opening by circumscissile dehiscence.
From Strasburger's Lehrbuch der Botanik, by permission of Gustav Fischer.
FIG. 17. - Lomentum of Hedysarum which, when ripe, separates. transversely into single-seeded portions or mericarps.
FIG. 18. - Fruit of Geranium pratense, after splitting.
Saxifraga a splitting for a short distance of the ventral sutures of the carpels takes place, so that a large apical pore is formed. In the fruit of Cruciferae, as wallflower (fig. 14), the valves separate from the base of the fruit, leaving a central replum, or frame, which supports the false septum formed by a prolongation from the parietal placentas on opposite sides of the fruit, extending between the ventral sutures of the carpels. In Orchidaceae (fig. 15) the pericarp, when ripe, separates into three valves in a loculicidal manner, but the midribs of the carpels, to which the placentas are attached, often remain adherent to the axis both at the apex and base after the valves bearing the seeds have fallen. The other type of dehiscence is transverse, or circumscissile, when the upper part of the united carpels falls off in the form of a lid or operculum, as in Anagallis and in henbane (Hyoscyamus) (fig. 16).
Sometimes the axis is prolonged beyond the base of the carpels„ as in the mallow and castor-oil plant, the carpels being united to it throughout their length by their faces, and separating from it without opening. In the Umbelliferae the two carpels separate from the lower part of the axis, and remain attached by their apices to a prolongation of it, called a carpophore or podocarp, which splits into two (fig. 25) and suspends them; hence the fruit is termed a cremocarp, which divides into two mericarps. The general term schizocarp is applied to all dry fruits, which break up into two or more one-seeded indehiscent mericarps, as in Hedysarum (fig. 17). In the order Geraniaceae the styles remain attached to a central' column, and the mericarps separate from below upwards, before dehiscing by their ventral suture (fig. 18). Carpels which separate one from another in this manner are called cocci. They are well.
FIG. 9. FIG. IO.
FIG. 6. - Seed-vessel or capsule of Campion, opening by ten teeth at the apex. The calyx c is seen surrounding the seedvessel.
FIG. 7. - Capsule of Poppy, opening by pores p, under the radiating peltate stigma s. FIG. 8.
FIG. I I.
seen in the order Euphorbiaceae, where there are usually three such carpels, and the fruit is termed tricoccus. In many of them, as Hura crepitans, the cocci separate with great force and elasticity. In many leguminous plants, such as Ornithopus, Hedysarum (fig. 17), Entada, Coronilla and the gum-arabic plant (Acacia arabica), the fruit becomes a schizocarp by the formation of transverse partitions from the folding in of the sides of the pericarp, and distinct separations taking place at these partitions.
Fruits are formed by one flower, or are the product of several flowers combined. In the former case they are either apocarpous, of one mature carpel or of several separate free carpels; or syncarpous, of several carpels, more or less completely united. When the fruit is composed of the ovaries of several flowers united, it is usual to find the bracts and floral envelopes also joined with them, so as to form one mass; hence such fruits are known as multiple, confluent or anthocarpous. The term simple is applied to fruits which are formed by the ovary of a single flower, whether they are composed of one or several carpels, and whether these carpels are separate or combined.
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. (I) 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 seaor 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 watersoaked. 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. I) or maple (fig. 21), seeds of pines and firs, or many members of the order Bignoniaceae; or hairdevelopments as in fruits of clematis, where the style forms a feathery FIG. 19. - Dry one-seeded fruit of dock (Rumex) cut vertically. ,ov, Pericarp formed from ovary wall; s, seed; e, endosperm; pl, embryo with radicle pointing upwards and cotyledons downwards - enlarged.
FIG. 20. - Achene of Ranunculus arvensis in longitudinal section; e, endosperm; pl, embryo. (After Baillon, enlarged.) From Strasburger's Lehrbuch der Botanik, by permission of Gustav Fischer.
FIG. 21. - Fruit of Common Sycamore (Acer Pseudoplatanus), dividing into two mericarps in; s, pedicel; fl, wings (nat. size).
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 tee': (many Caryophyllaceae [fig. 6]) or pores (poppy [fig. 7], Campanu: &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 FIG. 24. FIG. 25.
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 Carton 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.
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.
FIG. 20.
FIG. 22.
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.
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.
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.
In the baobab there is a multiFIG. 31. - Transverse section locular syncarpous fruit, in which of the fruit of the Melon the seeds are immersed in pulp. (Cucumis Melo), showing the The pepo, another indehiscent placentas with the seeds attached syncarpous fruit, is illustrated by to them. The three carpels the fruit of the gourd, melon (fig. forming the pepo are separated 31) and other Cucurbitaceae. It by partitions. From the centre is formed of three carpels, surprocesses pass outwards, ending mounted by the calyx; the rind in the curved placenta.
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.
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.
FIG. 26. - Cupule of Quercus Aegilops. cp, cupule; gl, fruit. (After Duchartre.) 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.
FIG. 32. - Honesty (Lunaria biennis), showing the septum after the carpels have fallen away.
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.
FIG. 34. - Fruit of the pine-apple (Ananassa saliva), 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.
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.
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 truit is produced, not by the incorporation of two flowers, but by the abnormal development of a second carpel in the flower.
I. 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 the endosperm is formed preparatory to fertilization. The fertilized egg enlarges and becomes multicellular, forming theembryo. 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 FIG. 35. FIG. 36.
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.
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. I) and Acer (fig. 21).
Sometimes there is an additional covering to the seed, formed after fertilization, to which the name arillus fias been given (fig. 38). This is seen in the passion-flower, where the covering arises from the placenta or extremity of the funicle at the base of the ovule and passes upwards towards the apex, leaving the micropyle uncovered. In the nutmeg and spindle tree this additional coat is formed from above downwards, constituting in the former case a laciniated scarlet covering called mace. In such instances it has been called an arillode (fig. 39). This arillode, after growing downwards, may be reflected upwards so as to cover the micropyle. The fleshy scarlet covering formed around the naked seed in the yew is by some considered of the nature of an aril. On the testa, at various, points, there are produced at times other cellular bodies, to which the name of strophioles, or caruncles, has been given, the seeds being strophiolate or carunculate. These tumours may occur near the base of the seed, as in Polygala, or at the apex, as in Castor-oil. plant (Ricinus); or they may occur in the course of the raphe, as in blood-root (Sanguinaria) and Asarabacca. The funicles of the ovules frequently attain a great length in the seed, and in some magnolias, when the fruit dehisces, they appear as long scarlet cords suspending the seeds outside. The hilum or umbilicus of the seed is usually FIG. 33.
well marked, as a scar of varying size; in the calabar bean and in some species of Mucuna and Dolichos it extends along a large portion of the edge of the seed; it frequently exhibits marked colours, being black in the bean, white in many species of Phaseolus, &c. The micropyle (fig. 35, m) of the seed may be recognizable by the naked eye, as in the pea and bean tribe, Iris, &c., or it may be very minute or microscopic. It indicates the true apex of the seed, and is important as marking the point to which the root of the embryo is directed. At the micropyle in the bean is observed a small process of integument, which, when the young plant sprouts, is pushed up like a lid; it is called the embryotega. The chalaza (fig. 38, ch) is often of a different colour from the rest of the seed. In the orange (fig. 40) it is of a reddish-brown colour, and is easily recognized at one end of the seed when the integuments are carefully removed. In anatropal seeds the raphe forms a distinct ridge along one side of the seed (fig. 41).
The position of the seed as regards the pericarp resembles that of the ovule in the ovary, and the same terms are applied - erect, ascending, pendulous, suspended, curved, &c. These terms have no reference to the mode in which the fruit is attached to the axis. Thus the seed may be erect while the fruit itself is pendent, in the ordinary meaning of that term. The part of the seed next the axis or the ventral suture is its face, the opposite side being the back. Seeds exhibit great varieties of form. They may be flattened laterally (compressed), or from above downwards (depressed). They may be round, oval, triangular, polygonal, rolled up like a snail, as in Physostemon, or coiled up like a snake, as in Ophiocaryon paradoxum. FIG. 37. FIG. 38. FIG. 39. FIG. 40. FIG. 41.
FIG. 37. - Seed of Pine (Pinus), with a membranous appendage w to the testa, called a wing.
FIG. 38. - Young anatropal seed of the white Water-lily (Nymphaea alba), cut vertically. It is attached to the placenta by the funicle f, cellular prolongations from which form an aril a a. The vessels of the cord are prolonged to the base of the nucellus n by means of the raphe r. The base of the nucellus is indicated by the chalaza ch, while the apex is at the micropyle m. The covering of the seed is marked i. n is the nucellus or perisperm, enclosing the embryo-sac es, is which the endosperm is formed. The embryo e, with its suspensor, is contained in the sac, the radicle pointing to the micropyle m.
FIG. 39. - Arillode a, or false aril, of the Spindle-tree (Euonymus), arising from the micropyle f. FIG. 40. - Anatropal seed of the Orange (Citrus Aurantium) opened to show the chalaza c, which forms a brown spot at one end.
FIG. 41. - Entire anatropal seed of the Orange (Citrus Aurantium), with its rugose or wrinkled testa, and the raphe r ramifying in the thickness of the testa on one side.
The endosperm formed in the embryo-sac of angiosperms after fertilization, and found previous to it in gymnosperms, consists of cells containing nitrogenous and starchy or fatty matter, destined for the nutriment of the embryo. It occupied the whole cavity of the embryo-sac, or is formed only at certain portions of it, at the apex, as in Rhinanthus, at the base, as in Vaccinium, or in the middle, as in Veronica. As the endosperm increases in size along with the embryo-sac and the embryo, the substance of the original nucellus of the ovule is gradually absorbed. Sometimes, however, as in M usaceae, Cannaceae, Zingiberaceae, no endosperm is formed; the cells of the original nucellus, becoming filled with food-materials for the embryo, are not absorbed, but remain surrounding the embryo-sac with the embryo, and constitute the perisperm. Again, in other plants, as Nymphaeaceae (fig. 38) and Piperaceae, both endosperm and perisperm are present. It was from observations on cases such as these that old authors, imagining a resemblance betwixt the plant-ovule and the animal ovum, applied the name albumen to the outer nutrient mass or perisperm, and designated the endosperm as vitellus. The term albumen is very generally used as including all the nutrient matter stored up in the seed, but it would be advisable to discard the name as implying a definite chemical substance. There is a large class of plants in which although at first after fertilization a mass of endosperm is formed, yet, as the embryo increases in size, the nutrient matter from the endospermic cells passes out from them, and is absorbed by the cells of the embryo plant. In the mature seed, in such cases, there is no separate mass of tissue containing nutrient food-material apart from the embryo itself. Such a seed is said to be exalbuminous, as in Compositae, Cruciferae and most Leguminosae (e.g. pea, fig. 35).
When either endosperm or perisperm or both are present the seed is said to be albuminous. The albumen varies much in its nature and consistence, and furnishes important characters. It may be farinaceous or mealy, consisting chiefly of cells filled with starch, as in cereal grains, where it is abundant; fleshy or cartilaginous, consisting of thicker cells which are still soft, as in the coco-nut, and which sometimes contain oil, as in the oily albumen of Croton, Ricinus and poppy; horny, when the cell-walls are slightly thickened and capable of distension, as in date and coffee; the cell-walls sometimes become greatly thickened, filling up the testa as a hard mass, as in vegetable ivory (Phytelephas). The albumen may be uniform throughout, or it may present a mottled appear- y f ance, as in the nutmeg, the seeds of Anonaceae and some Palms, where it is called ruminated. This mottled appearance is due to a protrusion of a dark lamella of the integument between folded protuberances of albumen. A cavity is sometimes left in the centre which is usually filled with fluid, as in the coco-nut. The relative size of the embryo rand of the endosperm varies much. In Monocotyledons the embryo is usually small, and the endosperm large, and the same is true in the case of coffee and many other plants amongst Dicotyledons. The opposite is the case in other plants, as in the Labiatae, Plumbaginaceae, &c.
The embryo consists of an axis bearing the cotyledons (fig. 42, c), or the first leaves of the plant. To that part of this axis immediately beneath the cotyledons the terms hypocotyl, caulicle or tigellum (t) have been applied, and continuous backwards with it is the young root or radicle (r), the descending axis, their point of union being the collar or neck. The terminal growing bud of the axis is called the plumule or gemmule (g), and represents the ascending axis. The radicular extremity points towards the micropyle, while the cotyledonary extremity is pointed towards the base of the ovule or the chalaza. Hence, by ascertaining the position of the micropyle and chalaza, the two extremities of the embryo can in general be discovered. It is in many cases difficult to recognize the parts in an embryo; thus in Cuscuta, the embryo appears as an elongated axis without divisions; and in Caryocar the mass of the embryo is made up by the radicular extremity and hypocotyl, in a groove of which the cotyledonary extremity lies embedded (fig. 52). In some monocotyledonous embryos, as in Orchidaceae, the embryo is a cellular mass showing no parts. In parasitic plants also which form no chlorophyll, as Orobanche, Monotropa, &c., the embryo remains without differentiation, consisting merely of a mass of cells until the ripening of the seed. When the embryo is surrounded by the endosperm on all sides except its radicular extremity it is internal (see figs. 19, 20); when lying outside the endosperm, and only coming into contact with it at certain points, it is external, as in grasses (e.g. wheat, fig. 22). When the embryo follows the direction of the axis of the seed, it is axile or axial (fig. 43); when it is not in the direction of the axis, it becomes abaxile or abaxial. In campylotropal seeds the embryo is curved, and in place of being embedded in endosperm, is frequently external to it, following the concavity of the seed (fig. 44), and becoming peripherical, with the chalaza situated in the curvature of the embryo, as in Caryophyllaceae.
It has been already stated that the radicle of the embryo is directed to the micropyle, and the cotyledons to the chalaza. In some cases, by the growth of the integuments, the former is turned round so as not to correspond with the apex of the nucellus, and then the embryo has the radicle directed to one side, and is called excentric, as is seen in Primulaceae, Plantaginaceae and many palms, especially the date. The position of the embryo in different kinds of seeds varies. In an orthotropal seed the embryo is inverted or anttropal, the radicle pointing to the apex of the seed, or to the part opposite the hilum. Again, in an anatropal seed the embryo is erect or homotropal (fig. 43), the radicle being directed to the base of the seed. In curved or campylotropal seeds the embryo is folded so that its radicular and cotyledonary extremities are approximated, and it becomes amphitropal (fig. 44). In this instance the seed may be exalbuminous, and the embryo may be folded on itself; or albuminous, the embryo surrounding more or less completely the endosperm and being peripherical. According to the mode in which the seed is attached to the pericarp, the radicle may be directed upwards or downwards, or laterally, as regards the ovary. In an orthotropal seed attached to the base of the pericarp it is superior, as also in a suspended anatropal seed. In other anatropal seeds the radicle is inferior. When the seed is horizontal as regards the pericarp, the radicle is either centrifugal, when it points to the outer wall of the ovary; or centripetal, when it points to the axis or inner wall of the ovary. These characters are of value for purposes of classification, as they are often constant in large groups of genera.
FIG. 42. - The dicotyledonous embryo of the Pea laid open. c, c, The two fleshy cotyledons, or seed-lobes, which remain under ground when the plant sprouts; r, the radicular extremity of the axis whence the root arises; t, the axis (hypocotyl) bearing the young stalk and leaves g (plumule), which lie in a depression of the cotyledons f. Plants in which there are two cotyledons produced in the embryo are dicotyledonous. The two cotyledons thus formed are opposite to each other (figs. 42 and 45), but are not always of the same size. Thus, in Abronia and other members of the order Nyctaginaceae, one of them is smaller than the other (often very small), and in Carapa guianensis there appears to be only one, in consequence of the intimate union which takes place between the two. The union between the cotyledonary leaves may continue after the young plant begins to germinate. Such embryos have been called pseudomonocotyledonous. The texture of the cotyledons varies. They may be thick, as in the pea (fig. 42), exhibiting no traces of venation, with their flat internal surfaces in contact, and their backs more or less convex; or they may be in the form of thin and delicate laminae, flattened on both sides, and having distinct venation, as in Ricinus, Jatropha, Euonymus, &c. The cotyledons usually form the greater part of the mature embryo, and this is remarkably well seen in such exalbuminous seeds as the bean and pea.
Cotyledons are usually entire and sessile. But they occasionally become lobed, as in the walnut and the lime; or petiolate, as in Geranium molle; or auriculate, as in the ash. Like leaves in the FIG. 43. - Seed of Pansy (Viola tricolor) cut vertically. The embryo p1 is axial, in the midst of fleshy endosperm al. The seed is anatropal, and the embryo is homotropal; the cotyledons co point to the base of the nucellus or chalaza ch, while the radicle, or the other extremity of the embryo, points to the micropyle, close to the hilum h. The hilum or base of the seed, and the chalaza or base of the nucellus are united by means of the raphe r. FIG. 44. - Seed of the Red Campion (Lychnis), cut vertically, showing the peripherical embryo, with its two cotyledons and its radicle. The embryo is curved round the albumen, so that its cotyledons and radicle both come near the hilum (amphitropal). FIG. 45. - Mature dicotyledonous embryo of the Almond, with one of the cotyledons removed. r, Radicle; t, young stem or caulicle; c, one of the cotyledons left; i, line of insertion of the cotyledon which has been removed; g, plumule.
FIG. 46. - Exalbuminous seed of Wallflower (Cheiranthus) cut vertically. The radicle r is folded on the edges of the cotyledons c which are accumbent.
FIG. 47. - Transverse section of the seed of the Wallflower (Cheiranthus), showing the radicle r folded on the edges of the accumbent cotyledons c. FIG. 48. - Transverse section of the seed of the Dame's Violet (Hesperis). The radicle r is folded on the back of the cotyledons c, which are said to be incumbent.
bud, cotyledons may be either applied directly to each other, or may be folded in various ways. In geranium the cotyledons are twisted and doubled; in convolvulus they are corrugated; and in the potato and in Bunias, they are spiral, - the same terms being applied as to the foliage leaves. The radicle and cotyledons are either straight or variously curved. Thus, in some cruciferous plants, as the wallflower, the cotyledons are applied by their faces, and the radicle (figs. 46, 47) is folded on their edges, so as to be lateral; the cotyledons are here accumbent. In others, as Hesperis, the cotyledons (fig. 48) are applied to each other by their faces, and the radicle, r, is folded on their back, so as to be dorsal, and the cotyledons are incumbent. Again, the cotyledons are conduplicate when the radicle is dorsal, and enclosed between their folds. In other divisions the radicle is folded in a spiral manner, and the cotyledons follow the same course.
In many gymnosperms more than two cotyledons are present, and they are arranged in a whorl. This occurs in Coniferae, especially in the pine, fir (fig. 49), spruce and larch, in which six, nine, twelve and even fifteen have been observed. They are linear, and resemble in their form and mode of development the clustered or fasciculated leaves of the larch. Plants having numerous cotyledons are termed polycotyledonous. In species of Streptocarpus the cotyledons are permanent, and act the part of leaves. One of them is frequently largely developed, while the other is small or abortive.
In those plants in which there is only a single cotyledon in the embryo, hence called monocotyledonous, the embryo usually has a cylindrical form more or less rounded at the extremities, or elongated and fusiform, often oblique. The axis is usually very short compared with the cotyledon, which in general encloses the plumule by its lower portion, and exhibits on one side a small slit which indicates the union of the edges of the vaginal or sheathing portion of the leaf (fig. 50). In grasses, by the enlargement of the embryo in a particular direction, the endosperm is pushed on one side, and thus the embryo comes to lie outside at the base of the endosperm (figs. 22, 51). The lamina of the cotyledon is not developed. Upon the side of the embryo next the endosperm and enveloping it is a large shield-shaped body, termed the scutellum. This is an outgrowth from the base of the cotyledon, enveloping more or less the cotyledon r FIG. 49. FIG. 50. FIG. 51. FIG. 52.
FIG. 49. - Polycotyledonous embryo of the Pine (Pinus) beginning to sprout. t, Hypocotyl; r, radicle. The cotyledons c are numerous. Within the cotyledons the primordial leaves are seen, constituting the plumule or first bud of the plant.
FIG. 50. - Embryo of a species of Arrow-grass (Triglochin), showing a uniform conical mass, with a slit s near the lower part. The cotyledon c envelops the young bud, which protrudes at the slit during germination. The radicle is developed from the lower part of the axis r. FIG. 51. - Grain of Wheat (Triticum) germinating, showing (b) the cotyledon and (c) the rootlets surrounded by their sheaths (coleorrhizae). FIG. 52. - Embryo of Caryocar. t, Thick hypocotyl, forming nearly the whole mass, becoming narrowed and curved at its extremity, and applied to the groove s. In the figure this narrowed portion is slightly separated from the groove; c, two rudimentary cotyledons.
and plumule, in some cases, as in maize, completely investing it; in other cases, as in rice, merely sending small prolongations over its anterior face at the apex. By others this scutellum is considered as the true cotyledon, and the sheathing structure covering the plumule is regarded as a ligule or axillary stipule (see GRASSES). In many aquatic monocotyledons (e.g. Potamogeton, Ruppia and others) there is a much-developed hypocotyl, which forms the greater part of the embryo and acts as a store of nutriment in germination; these are known as macropodous embryos. A similar case is that of Caryocar among Dicotyledons, where the swollen hypocotyl occupies most of the embryo (fig. 52). In some grasses, as oats and rice, a projection of cellular tissue is seen upon the side of the embryo opposite to the scutellum, that is, on the anterior side. This has been termed the epiblast. It is very large in rice. This by some was considered the rudimentary second cotyledon; but is now generally regarded as an outgrowth of the sheath of the true cotyledon. (A. B. R.)
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Bibliography Information
Chisholm, Hugh, General Editor. Entry for 'Fruit'. 1911 Encyclopedia Britanica. https://www.studylight.org/encyclopedias/eng/bri/f/fruit.html. 1910.
