The manner in which the parts are arranged in the flower-bud with respect to each other before opening is theaestivationorpraefloration. The latter terms are applied to the flower-bud in the same way as vernation is to the leaf-bud, and distinctive names have been given to the different arrangements exhibited, both by the leaves individually and in their relations to each other. As regards each leaf of the flower, it is either spread out, as the sepals in the bud of the lime-tree, or folded upon itself (conduplicate), as in the petals of some species ofLysimachia, or slightly folded inwards or outwards at the edges, as in the calyx of some species of clematis and of some herbaceous plants, or rolled up at the edges (involute or revolute), or folded transversely, becomingcrumpledorcorrugated, as in the poppy. When the parts of a whorl are placed in an exact circle, and are applied to each other by their edges only, without overlapping or being folded, thus resembling the valves of a seed-vessel, the aestivation isvalvate(fig. 42). The edges of each of the parts may be turned either inwards or outwards; in the former case the aestivation isinduplicate(fig. 43), in the latter casereduplicate(fig. 44). When the parts of a single whorl are placed in a circle, each of them exhibiting a torsion of its axis, so that by one of its sides it overlaps its neighbour, whilst its side is overlapped in like manner by that standing next to it, the aestivation istwistedorcontorted(fig. 45). This arrangement is characteristic of the flower-buds of Malvaceae and Apocynaceae, and it is also seen in Convolvulaceae and Caryophyllaceae. When the flower expands, the traces of twisting often disappear, but sometimes, as in Apocynaceae, they remain. Those forms of aestivation are such as occur in cyclic flowers, and they are included undercircularaestivation. But in spiral flowers we have a different arrangement; thus the leaves of the calyx ofCamellia japonicacover each other partially like tiles on a house. This aestivation isimbricate. At other times, as in the petals ofCamellia, the parts envelop each other completely, so as to becomeconvolute. This is also seen in a transverse section of the calyx ofMagnolia grandiflora, where each of the three leaves embraces that within it. When the parts of a whorl are five, as occurs in many dicotyledons, and the imbrication is such that there are two parts external, two internal, and a fifth which partially covers one of the internal parts by its margin, and is in its turn partially covered by one of the external parts, the aestivation isquincuncial(fig. 46). This quincunx is common in the corolla of Rosaceae. In fig. 47 a section is given of the bud ofAntirrhinum majus, showing the imbricate spiral arrangement. In this case it will be seen that the part marked 5 has, by a slight change in position, become overlapped by 1. This variety of imbricate aestivation has been termedcochlear. In flowers such as those of the pea (fig. 40), one of the parts, the vexillum, is often large and folded over the others, giving rise tovexillaryaestivation (fig. 48), or the carina may perform a similar office, and then the aestivation iscarinal, as in the Judas-tree (Cercis Siliquastrum). The parts of the several verticils often differ in their mode of aestivation. Thus, in Malvaceae the corolla is contorted and the calyx valvate, or reduplicate; in St John’s-wort the calyx is imbricate, and the corolla contorted. In Convolvulaceae, while the corolla is twisted, and has its parts arranged in a circle, the calyx is imbricate, and exhibits a spiral arrangement. InGuazumathe calyx is valvate, and the corolla induplicate. The circular aestivation is generally associated with a regular calyx and corolla, while the spiral aestivations are connected with irregular as well as with regular forms.
Thesepalsare sometimesfreeor separate from each other, at other times they are united to a greater or less extent; in the former case, the calyx ispolysepalous, in the lattergamosepalousormonosepalous. The divisions of theCalyx.calyx present usually the characters of leaves, and in some cases of monstrosity they are converted into leaf-like organs, as not infrequently happens in primulas. They are usually entire, but occasionally they are cut in various ways, as in the rose; they are rarely stalked. Sepals are generally of a more or less oval, elliptical or oblong form, with their apices either blunt oracute. In their direction they are erect or reflexed (with their apices downwards), spreading outwards (divergentorpatulous), or arched inwards (connivent). They are usually of a greenish colour (herbaceous); but sometimes they are coloured orpetaloid, as in the fuchsia, tropāeolum, globe-flower and pomegranate. Whatever be its colour, the external envelope of the flower is considered as the calyx. The vascular bundles sometimes form a prominent rib, which indicates the middle of the sepal; at other times they form several ribs. The venation is useful as pointing out the number of leaves which constitute a gamosepalous calyx. In a polysepalous calyx the number of the parts is indicated by Greek numerals prefixed; thus, a calyx which has three sepals istrisepalous; one with five sepals ispentasepalous. The sepals occasionally are of different forms and sizes. In Aconite one of them is shaped like a helmet (galeate). In a gamosepalous calyx the sepals are united in various ways, sometimes very slightly, and their number is marked by the divisions at the apex. These divisions either are simple projections in the form of acute or obtuse teeth (fig. 49); or they extend down the calyx as fissures about half-way, the calyx beingtrifid(three-cleft),quinquefid(five-cleft), &c., according to their number; or they reach to near the base in the form of partitions, the calyx beingtripartite,quadripartite,quinquepartite, &c. The union of the parts may be complete, and the calyx may be quite entire ortruncate, as in some Correas, the venation being the chief indication of the different parts. The cohesion is sometimes irregular, some parts uniting to a greater extent than others; thus a two-lipped orlabiatecalyx is formed. The upper lip is often composed of three parts, which are thus posterior or next the axis, while the lower has two, which are anterior. The part formed by the union of the sepals is called thetubeof the calyx; the portion where the sepals are free is thelimb.
Fig. 49.—Gamosepalous five-toothed calyx of Campion (Lychnis).
Fig. 50.—Obsolete calyx (c) of Madder (Rubia) adherent to the pistil, in the form of a rim.
Fig. 51.—Feathery pappus attached to the fruit of Groundsel (Senecio vulgaris).
Fig. 52.—Caducous calyx (c) of Poppy. There are two sepals which fall off before the petals expand.
Fig. 53.—Fruit of Physalis Alkekengi, consisting of the persistent calyx (s), surrounding the berry (fr), derived from the ovary. (After Duchartre.)
Occasionally, certain parts of the sepals undergo marked enlargement. In the violet the calycine segments are prolonged downwards beyond their insertions, and in the Indian cress (Tropaeolum) this prolongation is in the form of a spur (calcar), formed by three sepals; in Delphinium it is formed by one. In Pelargonium the spur from one of the sepals is adherent to the flower-stalk. InPotentillaand allied genera anepicalyxis formed by the development of stipules from the sepals, which form an apparent outer calyx, the parts of which alternate with the true sepals. In Malvaceae an epicalyx is formed by the bracteoles. Degenerations take place in the calyx, so that it becomes dry, scaly and glumaceous (like the glumes of grasses), as in the rushes (Juncaceae); hairy, as in Compositae; or a mere rim, as in some Umbelliferae and Acanthaceae, and in Madder (Rubia tinctorum, fig. 50), when it is calledobsoleteormarginate. In Compositae, Dipsacaceae and Valerianaceae the calyx is attached to the pistil, and its limb is developed in the form of hairs calledpappus(fig. 51). This pappus is either simple (pilose) or feathery (plumose). InValerianathe superior calyx is at first an obsolete rim, but as the fruit ripens it is shown to consist of hairs rolled inwards, which expand so as to waft the fruit. The calyx sometimes falls off before the flower expands, as in poppies, and iscaducous(fig. 52); or along with the corolla, as inRanunculus, and isdeciduous; or it remains after flowering (persistent) as in Labiatae, Scrophulariaceae, and Boraginaceae; or its base only is persistent, as inDatura Stramonium. InEschscholtziaandEucalyptusthe sepals remain united at the upper part, and become disarticulated at the base or middle, so as to come off in the form of a lid or funnel. Such a calyx isoperculateorcalyptrate. The existence or non-existence of an articulation determines the deciduous or persistent nature of the calyx.
The receptacle bearing the calyx is sometimes united to the pistil, and enlarges so as to form a part of the fruit, as in the apple, pear, &c. In these fruits the withered calyx is seen at the apex. Sometimes a persistent calyx increases much after flowering, and encloses the fruit without being incorporated with it, becomingaccrescent, as in various species ofPhysalis(fig. 53); at other times it remains in a withered ormarcescentform, as inErica; sometimes it becomesinflatedorvesicular, as in sea campion (Silene maritima).
The corolla is the more or less coloured attractive inner floral envelope; generally the most conspicuous whorl. It is present in the greater number of Dicotyledons. Petals differ more from ordinary leaves than sepals do, and areCorolla.much more nearly allied to the staminal whorl. In some cases, however, they are transformed into leaves, like the calyx, and occasionally leaf-buds are developed in their axil They are seldom green, although occasionally that colour is met with, as in some species ofCobaea,Hoya viridiflora,Gonolobus viridiflorusandPentatropis spiralis. As a rule they are highly coloured, the colouring matter being contained in the cell-sap, as in blue or red flowers, or in plastids (chromoplasts), as generally in yellow flowers, or in both forms, as in many orange-coloured or reddish flowers. The attractiveness of the petal is often due wholly or in part to surface markings; thus the cuticle of the petal of a pelargonium, when viewed with a ½ or ¼-in. object-glass, shows beautiful hexagons, the boundaries of which are ornamented with several inflected loops in the sides of the cells.
Petals are generally glabrous or smooth; but, in some instances, hairs are produced on their surface. Petaline hairs, though sparse and scattered, present occasionally the same arrangement as those which occur on the leaves; thus, in Bombaceae they are stellate. Coloured hairs are seen on the petals ofMenyanthes, and on the segments of the perianth ofIris. They serve various purposes in the economy of the flower, often closing the way to the honey-secreting part of the flower to small insects, whose visits would be useless for purposes of pollination. Although petals are usually very thin and delicate in their texture, they occasionally become thick and fleshy, as inStapeliaandRafflesia; or dry, as in heaths; or hard and stiff, as inXylopia. A petal often consists of two portions—the lower narrow, resembling the petiole of a leaf, and called theunguisorclaw; the upper broader, like the blade of a leaf, and called thelaminaorlimb. These parts are seen in the petals of the wallflower (fig. 54). The claw is often wanting, as in the crowfoot (fig. 55) and the poppy, and the petals are thensessile. According to the development of veins and the growth of cellular tissue, petals present varieties similar to those of leaves. Thus the margin is either entire or divided into lobes or teeth. These teeth sometimes form a regular fringe round the margin, and thepetal becomesfimbriated, as in the pink; orlaciniated, as inLychnis Flos-cuculi; orcrested, as inPolygala. Sometimes the petal becomes pinnatifid, as inSchizopetalum. The median vein is occasionally prolonged beyond the summit of the petals in the form of a long process, as inStrophanthus hispidus, where it extends for 7 in.; or the prolonged extremity is folded downwards or inflexed, as in Umbelliferae, so that the apex approaches the base. The limb of the petal may be flat or concave, or hollowed like a boat. In Hellebore the petals become folded in a tubular form, resembling a horn (fig. 56); in aconite (fig. 58) some of the petals resemble a hollow-curved horn, supported on a grooved stalk; while in columbine, violet (fig. 57), snapdragon andCentranthus, one or all of them are prolonged in the form of a spur, and arecalcarate. InValeriana,AntirrhinumandCorydalis, the spur is very short, and the corolla or petal is said to begibbous, orsaccate, at the base. These spurs, tubes and sacs serve as receptacles for the secretion or containing of nectar.
Fig. 54.—Unguiculate or clawed petal of Wallflower (Cheiranthus Cheiri).c, The claw or unguis;l, the blade or lamina.
Fig. 55.—Petal of Crowfoot (Ranunculus), without a claw, and thus resembling a sessile leaf. At the base of the petal a nectariferous scale is seen.
Fig. 56.—Tubular petal of Hellebore (Helleborus).
Fig. 57.—Pansy (Viola tricolor). Longitudinal section of flower;v, bracteole on the peduncle;l, sepals;ls, appendage of sepal;c, petals;cs, spur of the lower petals;fs, glandular appendage of the lower stamens;a, anthers. (After Sachs.)
(From Vines’ Students’Text-Book of Botany, by permission of Swan Sonnenschein & Co.)
Fig. 58.—Part of the flower of Aconite (Aconitum Napellus), showing two irregular horn-like petals (p) supported on grooved stalks (o). These serve as nectaries,s, the whorl of stamens inserted on the thalamus and surrounding the pistil.
A corolla isdipetalous,tripetalous,tetrapetalousorpentapetalousaccording as it has two, three, four or five separate petals. The general name ofpolypetalousis given to corollas having separate petals, whilemonopetalous,gamopetalousorsympetalousis applied to those in which the petals are united. This union generally takes place at the base, and extends more or less towards the apex; inPhyteumathe petals are united at their apices also. In some polypetalous corollas, as that of the vine, the petals are separate at the base and adhere by the apices. When the petals are equal as regards their development and size, the corolla isregular; when unequal, it isirregular. When a corolla is gamopetalous it usually happens that the lower portion forms a tube, while the upper parts are either free or partially united, so as to form a common limb, the point of union of the two portions being thethroat, which often exhibits a distinct constriction or dilatation. The number of parts forming such a corolla can be determined by the divisions, whether existing as teeth, crenations, fissures or partitions, or if, as rarely happens, the corolla is entire, by the venation. The union may be equal among the parts, or some may unite more than others.
Amongst regular polypetalous corollas may be noticed therosaceouscorolla (fig. 59), in which there are five spreading petals, having no claws, and arranged as in the rose, strawberry andPotentilla; thecaryophyllaceouscorolla, in which there are five petals with long, narrow, tapering claws, as in many of the pink tribe; thecruciform, having four petals, often unguiculate, placed opposite in the form of a cross, as seen in wallflower, and in other plants calledcruciferous. Of irregular polypetalous corollas the most marked is thepapilionaceous(fig. 40), in which there are five petals:—one superior (posterior), st, placed next to the axis, usually larger than the rest, called thevexillumorstandard; two lateral, a, thealaeor wings; two inferior (anterior), partially or completely covered by the alae, and often united slightly by their lower margins, so as to form a single keel-like piece,car, calledcarina, or keel, which embraces the essential organs. This form of corolla is characteristic of British leguminous plants.
Regular gamopetalous corollas are sometimescampanulateorbell-shaped, as in (Campanula) (fig. 60);infundibuliformorfunnel-shaped, when the tube is like an inverted cone, and the limb becomes more expanded at the apex, as in tobacco;hypocrateriformorsalver-shaped, when there is a straight tube surmounted by a flat spreading limb, as in primula (fig. 61);tubular, having a long cylindrical tube, appearing continuous with the limb, as inSpigeliaand comfrey;rotateorwheel-shaped, when the tube is very short, and the limb flat and spreading, as in forget-me-not,Myosotis(when the divisions of the rotate corolla are very acute, as inGalium, it is sometimes calledstellateorstar-like);urceolateorurn-shaped, when there is scarcely any limb, and the tube is narrow at both ends, and expanded in the middle, as in bell-heath (Erica cinerea). Some of these forms may become irregular in consequence of certain parts being more developed than others. Thus, inVeronica, the rotate corolla has one division much smaller than the rest, and in foxglove (Digitalis) there is a slightly irregular companulate corolla. Of irregular gamopetalous corollas there may be mentioned thelabiateorlipped(fig. 62), having two divisions of the limb in the form of lips (the upper one,u, composed usually of two united petals, and the lower,l, of three), separated by a gap. In such cases the tube varies in length, and the parts in their union follow the reverse order of what occurs in the calyx, where two sepals are united in the lower lip and three in the upper. When the upper lip of a labiate corolla is much arched, and the lips separated by a distinct gap, it is calledringent(fig. 62). The labiate corolla characterizes the natural order Labiatae. When the lower lip is pressed against the upper, so as to leave only a chink between them, the corolla is said to bepersonate, as in snapdragon, and some other Scrophulariaceae. In some corollas the two lips become hollowed out in a remarkable manner, as in calceolaria, assuming a slipper-like appearance, similar to what occurs in the labellum of some orchids, asCypripedium. When a tubular corolla is split in such a way as to form a strap-like process on one side with several tooth-like projections at its apex, it becomesligulateorstrap-shaped(fig. 63). This corolla occurs in many composite plants, as in the florets of dandelion, daisy and chicory. The number of divisions at the apex indicates the number of united petals, some of which, however, may beabortive. Occasionally some of the petals become more united than others, and then the corolla assumes abilabiateortwo-lippedform, as seen in the division of Compositae called Labiatiflorae.
Petals are sometimes suppressed, and sometimes the whole corolla is absent. InAmorphaandAfzeliathe corolla is reduced to a single petal, and in some other Leguminous plants it is entirely wanting. In the natural order Ranunculaceae, some genera, such asRanunculus, globe-flower and paeony, have both calyx and corolla, while others, such as clematis, anemone andCaltha, have only a coloured calyx. Flowers become double by the multiplication of the parts of the corolline whorl; this arises in general from a metamorphosis of the stamens.
Fig. 61.—Flower of cowslip (Primula veris) cut vertically.s, Sepals joined to form a gamosepalous calyx;c, corolla consisting of tube and spreading limb;a, stamens springing from the mouth of the tube;p, pistil.
Fig. 62.—Irregular gamopetalous labiate corolla of the Dead-nettle (Lamium album). The upper lipuis composed of two petals united, the lower lip (l) of three. Between the two lips there is a gap. The throat is the part where the tube and the labiate limb join. From the arching of the upper lip this corolla is called ringent.
Fig. 63.—Irregular gamopetalous ligulate flower of Ragwort (Senecio). It is a tubular floret, split down on one side, with the united petals forming a straplike projection. The lines on the flat portion indicate the divisions of the five petals. From the tubular portion below, the bifid style projects slightly.
Certain structures occur on the petals of some flowers, which received in former days the name ofnectaries. The term nectary was very vaguely applied by Linnaeus to any part of the flower which presented an unusual aspect, as the crown (corona) of narcissus, the fringes of the Passion-flower, &c. If the name is retained it ought properly to include only those parts which secrete a honey-like substance, as the glandular depression at the base of the perianth of the fritillary, or on the petal ofRanunculus(fig. 55), or on the stamens of Rutaceae. The honey secreted by flowers attracts insects, which, by conveying the pollen to the stigma, effect fertilization. The horn-like nectaries under the galeate sepal of aconite (fig. 58) are modified petals, so also are the tubular nectaries of hellebore (fig. 56). Other modifications of some part of the flower, especially of the corolla and stamens, are produced either by degeneration or outgrowth, or bychorisis, ordeduplication. Of this nature are the scales on the petals inLychnis,SileneandCynoglossum, which are formed in the same way as the ligules of grasses. In other cases, as in Samolus, the scales are alternate with the petals, and may represent altered stamens. InNarcissusthe appendages are united to form a crown, consisting of a membrane similar to that which unites the stamens inPancratium. It is sometimes difficult to say whether these structures are to be referred to the corolline or to the staminal row.
Petals are attached to the axis usually by a narrow base. When this attachment takes place by an articulation, the petals fall off either immediately after expansion (caducous) or after fertilization (deciduous). A corolla which is continuous with the axis and not articulated to it, as in campanula and heaths, may be persistent, and remain in a withered or marcescent state while the fruit is ripening. A gamopetalous corolla falls off in one piece; but sometimes the base of the corolla remains persistent, as inRhinanthusandOrobanche.
Thestamensand thepistilare sometimes spoken of as the essential organs of the flower, as the presence of both is required in order that perfect seed may be produced. As with few exceptions the stamen represents a leaf which has been specially developed to bear the pollen or microspores, it is spoken of in comparative morphology as a microsporophyll; similarly the carpels which make up the pistil are the megasporophylls (seeAngiosperms).Hermaphroditeorbisexualflowers are those in which both these organs are found;unisexualordiclinousare those in which only one of these organs appears,—those bearing stamens only, beingstaminiferousor “male”; those having the pistil only,pistilliferousor “female.” But even in plants with hermaphrodite flowers self-fertilization is often provided against by the structure of the parts or by the period of ripening of the organs. For instance, inPrimulaandLinumsome flowers have long stamens and a pistil with a short style, the others having short stamens and a pistil with a long style. The former occur in the so-called thrum-eyed primroses (fig. 61), the latter in the “pin-eyed.” Such plants are calleddimorphic. Other plants aretrimorphic, as species ofLythrum, and proper fertilization is only effected by combination of parts of equal length. In some plants the stamens are perfected before the pistil; these are calledproterandrous, as inRanunculus repens,Silene maritima,Zea Mays. In other plants, but more rarely, the pistil is perfected before the stamens, as inPotentilla argentea,Plantago major,Coix Lachryma, and they are termedproterogynous. Plants in which proterandry or proterogyny occurs are calleddichogamous. When in the same plant there are unisexual flowers, both male and female, the plant is said to bemonoecious, as in the hazel and castor-oil plant. When the male and female flowers of a species are found on separate plants, the termdioeciousis applied, as inMercurialisand hemp; and when a species has male, female and hermaphrodite flowers on the same or different plants, as inParietaria, it ispolygamous.
The stamens arise from the thalamus or torus within the petals, with which they generally alternate, forming one or more whorls, which collectively constitute theandroecium. Their normal position is below the pistil, and whenStamens.they are so placed (fig. 64,a) upon the thalamus they arehypogynous. Sometimes they become adherent to the petals, or areepipetalous, and the insertion of both is looked upon as similar, so that they are still hypogynous, provided they are independent of the calyx and the pistil. In other cases they are perigynous or epigynous (fig. 65). Numerous intermediate forms occur, especially amongst Saxifragaceae, where the parts arehalf superiororhalf inferior. Where the stamens become adherent to the pistil so as to form a column, the flowers are said to begynandrous, as inAristolochia(fig. 66). These arrangements of parts are of great importance in classification. The stamens vary in number from one to many hundreds. In acyclic flowers there is often a gradual transition from petals to stamens, as in the white water-lily (fig. 31). When flowers become double by cultivation, the stamens are converted into petals, as in the paeony, camellia, rose, &c. When there is only one whorl the stamens are usually equal in number to the sepals or petals, and are arranged opposite to the former, and alternate with the latter. The flower is thenisostemonous. When the stamens are not equal in number to the sepals or petals, the flower isanisostemonous. When there is more than one whorl of stamens, then the parts of each successive whorl alternate with those of the whorl preceding it. The staminal row is more liable to multiplication of parts than the outer whorls. A flower with a single row of stamens ishaplostemonous. If the stamens are double the sepals or petals as regards number, the flower isdiplostemonous; if more than double,polystemonous. The additional rows ofstamens may be developed in the usual centripetal (acropetal) order, as in Rhamnaceae; or they may be interposed between the pre-existing ones or be placed outside them,i.e.develop centrifugally (basipetally), as in geranium and oxalis, when the flower is said to beobdiplostemonous. When the stamens are fewer than twenty they are said to bedefinite; when above twenty they areindefinite, and are represented by the symbol ∞. The number of stamens is indicated by the Greek numerals prefixed to the termandrous; thus a flower with one stamen ismonandrous, with two, three, four, five, six or many stamens, di-, tri-, tetr-, pent-, hex- or polyandrous, respectively.
The function of the stamen is the development and distribution of the pollen. The stamen usually consists of two parts, a contracted portion, often thread-like, termed thefilament(fig. 25f), and a broader portion, usually of two lobes, termed theanther(a), containing the powderypollen(p), and supported upon the end of the filament. That portion of the filament in contact with the anther-lobes is termed theconnective. If the anther is absent the stamen is abortive, and cannot perform its functions. The anther is developed before the filament, and when the latter is not produced, the anther is sessile, as in the mistletoe.
The filament is usually, as its name imports, filiform or thread-like, and cylindrical, or slightly tapering towards its summit. It is often, however, thickened, compressed and flattened in various ways, becomingpetaloidinCanna,Marania, water-lily (fig. 32);subulateor slightly broadened at the base and drawn out into a point like an awl, as inButomus umbellatus; or clavate, that is, narrow below and broad above, as inThalictrum. In some instances, as inTamarix gallica,Peganum Harmala, andCampanula, the base of the filament is much dilated, and ends suddenly in a narrow thread-like portion. In these cases the base may give off lateral stipulary processes, as inAlliumandAlyssum calycinum. The filament varies much in length and in firmness. The length sometimes bears a relation to that of the pistil, and to the position of the flower, whether erect or drooping. The filament is usually of sufficient solidity to support the anther in an erect position; but sometimes, as in grasses, and other wind-pollinated flowers, it is very delicate and hair-like, so that the anther is pendulous (fig. 105). The filament is generally continuous from one end to the other, but in some cases it is bent or jointed, becominggeniculate; at other times, as in the pellitory, it is spiral. It is colourless, or of different colours. Thus in fuchsia andPoinciana, it is red; inAdamiaandTradescantia virginica, blue; inOenotheraandRanunculus acris, yellow.
Hairs, scales, teeth or processes of different kinds are sometimes times developed on the filament. In spiderwort (Tradescantia virginica) the hairs are beautifully coloured, moniliform or necklace-like, and afford good objects for studying rotation of the protoplasm. Filaments are usually articulated to the thalamus or torus, and the stamens fall off after fertilization; but inCampanulaand some other plants they are continuous with the torus, and the stamens remain persistent, although in a withered state. Changes are produced in the whorl of stamens by cohesion of the filaments to a greater or less extent, while the anthers remain free; thus, all the filaments of the androecium may unite, forming a tube round the pistil, or a central bundle when the pistil is abortive, the stamens becomingmonadelphous, as occurs in plants of the Mallow tribe; or they may be arranged in two bundles, the stamens beingdiadelphous, as inPolygala,Fumariaand Pea; in this case the bundles may be equal or unequal. It frequently happens, especially in Papilionaceous flowers, that out of ten stamens nine are united by their filaments, while one (the posterior one) is free (fig. 68). When there are three or more bundles the stamens aretriadelphous, as inHypericum aegyptiacum, orpolyadelphous, as inRicinus communis(castor-oil). In some cases, as in papilionaceous flowers, the stamens cohere, having been originally separate, but in most cases each bundle is produced by the branching of a single stamen. When there are three stamens in a bundle we may conceive the lateral ones as of a stipulary nature. In Lauraceae there are perfect stamens, each having at the base of the filament two abortive stamens or staminodes, which may be analogous to stipules. Filaments sometimes are adherent to the pistil, forming a column (gynostemium), as inStylidium, Asclepiadaceae,Rafflesia, and Aristolochiaceae (fig. 66); the flowers are then termedgynandrous.
Fig. 68.—Stamens and pistil of Sweet Pea (Lathyrus). The stamens are diadelphous, nine of them being united by their filaments (f), while one of them (e) is free;st, stigma;c, calyx.
Fig. 69.—Portion of wall of anther of Wallflower (Cheiranthus).ce, Exothecium;cf, endothecium; highly magnified.
Fig. 70.—Quadrilocular or tetrathecal anther of the flowering Rush (Butomus umbellatus). The anther entire (a) with its filament; section of anther (b) showing the four loculi.
Theantherconsists of lobes containing the minute powdery pollen grains, which, when mature, are discharged by a fissure or opening of some sort. There is a double covering of the anther—the outer, orexothecium, resembles theThe anther.epidermis, and often presents stomata and projections of different kinds (fig. 69); the inner, orendothecium, is formed by a layer or layers of cellular tissue (fig. 69,cf), the cells of whichhave a spiral, annular, or reticulated thickening of the wall. The endothecium varies in thickness, generally becoming thinner towards the part where the anther opens, and there disappears entirely. The walls of the cells are frequently absorbed, so that when the anther attains maturity the fibres are alone left, and these by their elasticity assist in discharging the pollen. The anther is developed before the filament, and is always sessile in the first instance, and sometimes continues so. It appears at first as a simple cellular papilla of meristem, upon which an indication of two lobes soon appears. Upon these projections the rudiments of the pollen-sacs are then seen, usually four in number, two on each lobe. In each a differentiation takes place in the layers beneath the epidermis, by which an outer layer of small-celled tissue surrounds an inner portion of large cells. Those central cells are the mother-cells of the pollen, whilst the small-celled layer of tissue external to them becomes the endothecium, the exothecium being formed from the epidermal layer.
In the young state there are usually four pollen-sacs, two for each anther-lobe, and when these remain permanently complete it is aquadrilocularortetrathecalanther (fig. 70). Sometimes, however, only two cavities remain in the anther, by union of the sacs in each lobe, in which case the anther is said to bebilocularordithecal. Sometimes the anther has a single cavity, and becomesunilocular, ormonothecal, ordimidiate, either by the disappearance of the partition between the two lobes, or by the abortion of one of its lobes, as inStyphelia laetaandAlthaea officinalis(hollyhock). Occasionally there are numerous cavities in the anther, as inViscumandRafflesia. The form of the anther-lobes varies. They are generally of a more or less oval or elliptical form, or they may be globular, as inMercurialis annua; at other times linear or clavate: curved, flexuose, or sinuose, as in bryony and gourd. According to the amount of union of the lobes and the unequal development of different parts of their surface an infinite variety of forms is produced. That part of the anther to which the filament is attached is theback, the opposite being theface. The division between the lobes is marked on the face of the anther by a groove orfurrow, and there is usually on the face asuture, indicating the line of dehiscence. The suture is often towards one side in consequence of the valves being unequal. The stamens may cohere by their anthers, and becomesyngenesious, as in composite flowers, and in lobelia, jasione, &c.
The anther-lobes are united to theconnective, which is either continuous with the filament or articulated with it. When the filament is continuous with the connective, and is prolonged so that the anther-lobes appear to be unitedThe connective.to it throughout their whole length, and lie in apposition to it and on both sides of it, the anther is said to beadnateoradherent; when the filament ends at the base of the anther, then the latter isinnateorerect. In these cases the anther is to a greater or less degree fixed. When, however, the attachment is very narrow, and an articulation exists, the anthers are movable (versatile) and are easily turned by the wind, as inTritonia, grasses (fig. 105), &c., where the filament is attached only to the middle of the connective. The connective may unite the anther-lobes completely or only partially. It is sometimes very short and is reduced to a mere point, so that the lobes are separate or free. At other times it is prolonged upwards beyond the lobes, assuming various forms, as inAcalyphaand oleander; or it is extended backwards and downwards, as in violet (fig. 71), forming a nectar-secreting spur. InSalvia officinalisthe connective is attached to the filament in a horizontal manner, so as to separate the two anther-lobes (fig. 72), one only of which contains pollen, the other being imperfectly developed and sterile. The connective is joined to the filament by a movable joint forming a lever which plays an important part in the pollination-mechanism. InStachysthe connective is expanded laterally, so as to unite the bases of the anther-lobes and bring them into a horizontal line.
Fig. 71.—Two stamens of Pansy (Viola tricolor), with their two anther-lobes and the connectives (p) extending beyond them. One of the stamens has been deprived of its spur, the other shows its spurc.
Fig. 72.—Anther ofSalvia officinalis.lf, fertile lobe full of pollen;ls, barren lobe without pollen;e, connective;f, filament.
Fig. 73.—Stamen of Lady’s Mantle (Alchemilla), with the anther opening transversely.
Fig. 74.—Stamen of a species of Nightshade (Solanum), showing the divergence of the anther-lobes at the base, and the dehiscence by pores at the apex.
Fig. 75.—The stamen of the Barberry (Berberis vulgaris), showing one of the valves of the anther (v) curved upwards, bearing the pollen on its inner surface.
The opening ordehiscenceof the anthers to discharge their contents takes place either by clefts, by valves, or by pores. When the anther-lobes are erect, the cleft is lengthwise along the line of the suture—longitudinal dehiscence(fig. 25). At otherAntherdehiscence.times the slit is horizontal, from the connective to the side, as inAlchemilla arvensis(fig. 73) and inLemna; the dehiscence is thentransverse. When the anther-lobes are rendered horizontal by the enlargement of the connective, then what is really longitudinal dehiscence may appear to be transverse. The cleft does not always proceed the whole length of the anther-lobe at once, but often for a time it extends only partially. In other instances the opening is confined to the base or apex, each loculament opening by a single pore, as inPyrola,Tetratheca juncea, Rhododendron,VacciniumandSolanum(fig. 74), where there are two, andPoranthera, where there are four; whilst in the mistletoe the anther has numerous pores for the discharge of the pollen. Another mode of dehiscence is the valvular, as in the barberry (fig. 75), where each lobe opens by a valve on the outer side of the suture, separately rolling up from base to apex; in some of the laurel tribe there are two such valves for each lobe, or four in all. In some Guttiferae, asHebradendron cambogioides(the Ceylon gamboge plant), the anther opens by a lid separating from the apex (circumscissiledehiscence).
The anthers dehisce at different periods during the process of flowering; sometimes in the bud, but more commonly when the pistil is fully developed and the flower is expanded. They either dehisce simultaneously or in succession. In the latter case individual stamens may move in succession towards the pistil and discharge their contents, as inParnassia palustris, or the outer or the inner stamens may first dehisce, following thus a centripetal or centrifugal order. These variations are intimately connected with the arrangements for transference of pollen. The anthers are calledintrorsewhen they dehisce by the surface next to the centre of the flower; they areextrorsewhen they dehisce by the outer surface; when they dehisce by the sides, as inIrisand some grasses, they arelaterallydehiscent. Sometimes, from their versatile nature, anthers originally introrse become extrorse, as in the Passion-flower andOxalis.
The usual colour of anthers is yellow, but they present a great variety in this respect. They are red in the peach, dark purple in the poppy and tulip, orange inEschscholtzia, &c. The colour and appearance of the anthers often change after they have discharged their functions.
Stamens occasionally become sterile by the degeneration or non-development of the anthers, when they are known asstaminodia, or rudimentary stamens. InScrophulariathe fifth stamen appears in the form of a scale; and in many Pentstemons it is reduced to a filament with hairs or a shrivelled membrane at the apex. In other cases, as in double flowers, the stamens are converted into petals; this is also probably the case with suchplants asMesembryanthemum, where there is a multiplication of petals in several rows. Sometimes, as inCanna, one of the anther-lobes becomes abortive, and a petaloid appendage is produced. Stamens vary in length as regards the corolla. Some are enclosed within the tube of the flower, as inCinchona(included); others areexserted, or extend beyond the flower, as inLittorellaorPlantago. Sometimes the stamens in the early state of the flower project beyond the petals, and in the progress of growth become included, as inGeranium striatum. Stamens also vary in their relative lengths. When there is more than one row or whorl in a flower, those on the outside are sometimes longest, as in many Rosaceae; at other times those in the interior are longest, as inLuhea. When the stamens are in two rows, those opposite the petals are usually shorter than those which alternate with the petals. It sometimes happens that a single stamen is longer than all the rest. A definite relation, as regards number, sometimes exists between the long and the short stamens. Thus, in some flowers the stamens aredidynamous, having only four out of five stamens developed, and the two corresponding to the upper part of the flower longer than the two lateral ones. This occurs in Labiatae and Scrophulariaceae (fig. 76). Again, in other cases there are six stamens, whereof four long ones are arranged in pairs opposite to each other, and alternate with two isolated short ones (fig. 77), giving rise totetradynamousflowers, as in Cruciferae. Stamens, as regards their direction, may be erect, turned inwards, outwards, or to one side. In the last-mentioned case they are calleddeclinate, as in amaryllis, horse-chestnut and fraxinella.
The pollen-grains or microspores contained in the anther consist of small cells, which are developed in the large thick-walled mother-cells formed in the interior of the pollen-sacs (microsporangia) of the young anther. These mother-cells are either separated from one another and float in the granular fluid which fills up the cavity of the pollen-sac, or are not so isolated. A division takes place, by which four cells are formed in each, the exact mode of division differing in dicotyledons and monocotyledons. These cells are the pollen-grains. They increase in size and acquire a cell-wall, which becomes differentiated into an outer cuticular layer, orextine, and an inner layer, orintine. Then the walls of the mother-cells are absorbed, and the pollen-grains float freely in the fluid of the pollen-sacs, which gradually disappears, and the mature grains form a powdery mass within the anther. They then either remain united in fours, or multiples of four, as in some acacias,Periploca graecaandInga anomala, or separate into individual grains, which by degrees become mature pollen. Occasionally the membrane of the mother-cell is not completely absorbed, and traces of it are detected in a viscid matter surrounding the pollen-grains, as in Onagraceae. In orchidaceous plants the pollen-grains are united into masses, orpollinia(fig. 78), by means of viscid matter. In orchids each of the pollen-masses has a prolongation or stalk (caudicle) which adheres to a prolongation at the base of the anther (rostellum) by means of a viscid gland (retinaculum) which is either naked or covered. The termclinandriumis sometimes applied to the part of the column in orchids where the stamens are situated. In some orchids, asCypripedium, the pollen has its ordinary character of separate grains. The number of pollinia varies; thus, inOrchisthere are usually two, inCattleyafour, and inLaeliaeight. The two pollinia inOrchis Moriocontain each about 200 secondary smaller masses. These small masses, when bruised, divide into grains which are united in fours. In Asclepiadaceae the pollinia are usually united in pairs (fig. 79), belonging to two contiguous anther-lobes—each pollen-mass having a caudicular appendage, ending in a common gland, by means of which they are attached to a process of the stigma. The pollinia are also provided with an appendicular staminal covering (fig. 80). Theexineis a firm membrane, which defines the figure of the pollen-grain, and gives colour to it. It is either smooth, or covered with numerous projections (fig. 81), granules, points or crested reticulations. The colour is generally yellow, and the surface is often covered with a viscid or oily matter. The intine is uniform in different kinds of pollen, thin and transparent, and possesses great power of extension. In some aquatics, asZostera,Zannichellia,Naias, &c., only one covering exists.
Fig. 78.—Pollinia, or pollen-masses, with their retinacula (g) or viscid matter attaching them at the base. The pollen masses (p) are supported on stalks or caudicles (c). These masses are easily detached by the agency of insects. Much enlarged.
Fig. 79.—Pistil ofAsclepias(a) with pollen-masses (p) adhering to the stigma (s).b, pollen-masses, removed from the stigma, united by a gland-like body. Enlarged.
Fig. 80.—Stamen ofAsclepias, showing filament f, anther a, and appendages p. Enlarged.
Pollen-grains vary from1⁄300to1⁄700of an inch or less in diameter. Their forms are various. The most common form of grain is ellipsoidal, more or less narrow at the extremities, which are called itspoles, in contradistinction to a line equidistant from the extremities, which is its equator. Pollen-grains are also spherical; cylindrical and curved, as inTradescantia virginica; polyhedral in Dipsacaceae and Compositae; nearly triangular in section in Proteaceae and Onagraceae (fig. 82). The surface of the pollen-grain is either uniform and homogeneous, or it is marked by folds formed by thinnings of the membrane. There are also rounded portions of the membrane or pores visible in the pollen-grain; these vary in number from one to fifty, and through oneor more of them the pollen-tube is extended in germination of the spore. In Monocotyledons, as in grasses, there is often only one, while in Dicotyledons they number from three upwards; when numerous, the pores are either scattered irregularly, or in a regular order, frequently forming a circle round the equatorial surface. Sometimes at the place where they exist, the outer membrane, in place of being thin and transparent, is separated in the form of a lid, thus becomingoperculate, as in the passion-flower and gourd. Within the pollen-grain is the granular protoplasm with some oily particles, and occasionally starch. Before leaving the pollen-sac a division takes place in the pollen-grain into a vegetative cell or cells, from which the tube is developed, and a generative cell, which ultimately divides to form the male cells (seeAngiospermsandGymnosperms).
When the pollen-grains are ripe, the anther dehisces and the pollen is shed. In order that fertilization may be effected the pollen must be conveyed to the stigma of the pistil. This process, termedpollination(seePollination),Pollination.is promoted in various ways,—the whole form and structure of the flower having relation to the process. In some plants, asKalmiaand Pellitory (fig. 83), the mere elasticity of the filaments is sufficient to effect this; in other plants pollination is effected by the wind, as in most of our forest trees, grasses, &c., and in such cases enormous quantities of pollen are produced. These plants areanemophilous. But the common agents for pollination are insects. To allure and attract them to visit the flower the odoriferous secretions and gay colours are developed, and the position and complicated structure of the parts of the flower are adapted to the perfect performance of the process. It is comparatively rare in hermaphrodite flowers for self-fertilization to occur, and the various forms of dichogamy, dimorphism and trimorphism are fitted to prevent this.
Under the termdiskis included every structure intervening between the stamens and the pistil. It was to such structures that the name ofnectarywas applied by old authors. It presents great varieties of form, such as a ring, scales,Disk.glands, hairs, petaloid appendages, &c., and in the progress of growth it often contains saccharine matter, thus becoming truly nectariferous. The disk is frequently formed by degeneration or transformation of the staminal row. It may consist of processes rising from the torus, alternating with the stamens, and thus representing an abortive whorl; or its parts may be opposite to the stamens. In some flowers, asJatropha Curcas, in which the stamens are not developed, their place is occupied by glandular bodies forming the disk. In Gesneraceae and Cruciferae the disk consists of tooth-like scales at the base of the stamens. The parts composing the disk sometimes unite and form a glandular ring, as in the orange; or they form a dark-red lamina covering the pistil, as inPaeonia Moutan(fig. 84); or a waxy lining of the hollow receptacle, as in the rose; or a swelling at the top of the ovary, as in Umbelliferae, in which the disk is said to be epigynous. The enlarged torus covering the ovary inNymphaea(Castalia) andNelumbiummay be regarded as a form of disk.
The pistil orgynoeciumoccupies the centre or apex of the flower, and is surrounded by the stamens and floral envelopes when these are present. It constitutes the innermost whorl, which after flowering is changed into the fruitThe pistil.and contains the seeds. It consists essentially of two parts, a basal portion forming a chamber, theovary, containing the ovules attached to a part called theplacenta, and an upper receptive portion, thestigma, which is either seated on the ovary (sessile), as in the tulip and poppy, or is elevated on a stalk called thestyle, interposed between the ovary and stigma. The pistil consists of one or more modified leaves, thecarpels(ormegasporophylls). When a pistil consists of a single carpel it issimpleor monocarpellary (fig. 85). When it is composed of several carpels, more or less united, it iscompoundorpolycarpellary(fig. 86). In the first-mentioned case the terms carpel and pistil are synonymous. Each carpel has its own ovary, style (when present), and stigma, and may be regarded as formed by a folded leaf, the upper surface of which is turned inwards towards the axis, and the lower outwards, while from its margins are developed one or moreovules. This comparison is borne out by an examination of the flower of the double-flowering cherry. In it no fruit is produced, and the pistil consists merely of sessile leaves, the limb of each being green and folded, with a narrow prolongation upwards, as if from the midrib, and ending in a thickened portion. InCycasthe carpels are ordinary leaves, with ovules upon their margin.
Fig.85.—Pistil of Broom (Cytisus) consisting of ovaryo, styles, and stigmat. It is formed by a single carpel.
Fig.86.—Vertical section of the flower of Black Hellebore (Helleborus niger). The pistil is apocarpous, consisting of several distinct carpels, each with ovary, style and stigma. The stamens are indefinite, and are inserted below the pistil (hypogynous).
Fig.87.—Fruit of the Strawberry (Fragaria vesca), consisting of an enlarged succulent receptacle, bearing on its surface the small dry seed-like fruits (achenes).
Fig.88.—Fruit ofRosa alba, consisting of the fleshy hollowed axis s´, the persistent sepals s, and the carpelsfr. The stamens (c) have withered. (After Duchartre.)
Fig.89.—Pistil ofRanunculus.x, Receptacle with the points of insertion of the stamensa, most of which have been removed.
Fig.90.—Syncarpous Pistil of Flax (Linum), consisting of five carpels, united by their ovaries, while their styles and stigmas are separate.
A pistil is usually formed by more than one carpel. The carpels may be arranged either at the same or nearly the same height in a verticil, or at different heights in a spiral cycle. When they remain separate and distinct, thus showing at once the composition of the pistil, as inCaltha, Ranunculus, hellebore (fig. 86), andSpiraea, the termapocarpousis applied. Thus, in Sedum (fig. 22) the pistil consists of five verticillate carpelso, alternating with the stamense. In magnolia andRanunculus(fig. 89) the separate carpels are numerous and are arranged in a spiral cycle upon an elongated axis or receptacle. In the raspberry the carpels are on a conical receptacle; in the strawberry, on a swollen succulent one (fig. 87); and in the rose (fig. 88), on a hollow one. When the carpels are united, as in the pear, arbutus and chickweed, the pistil becomessyncarpous. The number of carpels in a pistil is indicated by the Greek numeral. A flower with a simple pistil is monogynous; with two carpels, digynous; with three carpels, trigynous, &c.
The union in a syncarpous pistil is not always complete; it may take place by the ovaries alone, while the styles and stigmas remain free (fig. 90), and in this case, when the ovaries form apparently a single body, the organ receives the name ofcompoundovary; or the union may take place by the ovaries and styles while the stigmas are disunited; or by the stigmasand the summit of the style only. Various intermediate states exist, such as partial union of the ovaries, as in the rue, where they coalesce at their base; and partial union of the styles, as in Malvaceae. The union is usually most complete at the base; but in Labiatae the styles are united throughout their length, and in Apocynaceae and Asclepiadaceae the stigmas only. When the union is incomplete, the number of the parts of a compound pistil may be determined by the number of styles and stigmas; when complete, the external venation, the grooves on the surface, and the internal divisions of the ovary indicate the number.
Fig. 92.—Trilocular ovary of the Lily (Lilium), cut transversely.s, Septum;o, ovules, which form a double row in the inner angle of each chamber. Enlarged.
Fig. 93.—Diagrammatic section of a quinquelocular ovary, composed of five carpels, the edges of which are folded inwards, and meet in the centre forming the septa,s. The ovules (o) are attached to a central placenta, formed by the union of the five ventral sutures. Dorsal suture,l.
Fig. 94.—Diagrammatic section of a five-carpellary ovary, in which the edges of the carpels, bearing the placentas and ovuleso, are not folded inwards. The placentas are parietal, and the ovules appear sessile on the walls of the ovary. The ovary is unilocular.
Fig.95.—Diagrammatic section of a five-carpellary ovary, in which the septa (s) proceed inwards for a certain length, bearing the placentas and ovules (o). In this case the ovary is unilocular, and the placentas are parietal. Dorsal suture,l.
Fig.96.—Pistil of Pansy (Viola tricolor), enlarged. 1, Vertical; 2, horizontal section;c, calyx;d, wall of ovary;o, ovules;p, placenta;s, stigma.
Fig.97.—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 go to circumference, ending in curved placentas bearing the ovules.
Fig.98.—Diagrammatic section of a compound unilocular ovary, in which there are no indications of partitions. The ovules (o) are attached to a free central placenta, which has no connexion with the walls of the ovary.
The ovules are attached to theplacenta, which consists of a mass of cellular tissue, through which the nourishing vessels pass to the ovule. The placenta is usually formed on the edges of the carpellary leaf (fig. 91)—marginal.The placenta.In many cases, however, the placentas are formations from the axis (axile), and are not connected with the carpellary leaves. In marginal placentation the part of the carpel bearing the placenta is theinnerorventral suture, corresponding to the margin of the folded carpellary leaf, while theouterordorsal suturecorresponds to the midrib of the carpellary leaf. As the placenta is formed on each margin of the carpel it is essentially double. This is seen in cases where the margins of the carpel do not unite, but remain separate, and consequently two placentas are formed in place of one. When the pistil is formed by one carpel the inner margins unite and form usually a common marginal placenta, which may extend along the whole margin of the ovary as far as the base of the style (fig. 91), or may be confined to the base or apex only. When the pistil consists of several separate carpels, or is apocarpous, there are generally separate placentas at each of their margins. In a syncarpous pistil, on the other hand, the carpels are so united that the edges of each of the contiguous ones, by their union, form aseptumordissepiment, and the number of these septa consequently indicates the number of carpels in the compound pistil (fig. 92). When the dissepiments extend to the centre or axis, the ovary is divided into cavities orcells, and it may bebilocular,triloculur(fig. 92),quadrilocular,quinquelocular, ormultilocular, according as it is formed by two, three, four, five or many carpels, each carpel corresponding to a single cell. In these cases the marginal placentas meet in the axis, and unite so as to form a singlecentralone (figs. 92, 93), and the ovules appear in the central angle of the loculi. When the carpels in a syncarpous pistil do not fold inwards so that the placentas appear as projections on the walls of the ovary, then the ovary isunilocular(fig. 95) and the placentas areparietal, as inViola(fig. 96). In these instances the placentas may be formed at the margin of the united contiguous leaves, so as to appear single, or the margins may not be united, each developing a placenta. Frequently the margins of the carpels, which fold in to the centre, split there into two lamellae, each of which is curved outwards and projects into the loculament, dilating at the end into a placenta. This is well seen in Cucurbitaceae (fig. 97),Pyrola, &c. The carpellary leaves may fold inwards very slightly, or they may be applied in a valvate manner, merely touching at their margins, the placentas then being parietal (fig. 94), and appearing as lines or thickenings along the walls. Cases occur, however, in which the placentas are not connected with the walls of the ovary, and form what is called afree central placenta(fig. 98). This is seen in many of the Caryophyllaceae and Primulaceae (figs. 99, 100). In Caryophyllaceae, however, while the placenta is free in the centre, there are often traces found at the base of the ovary of the remains of septa, as if rupture had taken place, and, in rare instances, ovules are found on the margins of the carpels. But in Primulaceae no vestiges of septa or marginal ovules can be perceived at any period of growth; the placenta is always free, and rises in the centre of the ovary. Free central placentation, therefore, has been accounted for in two ways: either by supposing that the placentas in the early state were formed on the margins ofcarpellary leaves, and that in the progress of development these leaves separated from them, leaving the placentas and ovules free in the centre; or by supposing that the placentas are notmarginalbutaxileformations, produced by an elongation of the axis, and the carpels verticillate leaves, united together around the axis. The first of these views applies to Caryophyllaceae, the second to Primulaceae.
Fig.99.—Pistil ofCerastium hirsutumcut vertically.o, Ovary;p, free central placenta;g, ovules;s, styles.
Fig.100.—The same cut horizontally, and the halves separated so as to show the interior of the cavity of the ovaryo, with the free central placentap, covered with ovulesg.
Fig. 101.—Carpel of Lady’s-mantle (Alchemilla) with lateral styles;o, ovary,st, stigma. Enlarged.
Fig. 102.—Pistil of Primrose (Primula) composed of five carpels which are completely united;o, ovary;s, style;st, stigma. Enlarged.
Fig. 103.—Gynoecium of the Flower-de-Luce (Iris), consisting of an inferior ovary (o) and a style which divides into three petaloid segments (s), each bearing a stigma (st).
Fig. 104.—Capsule of Poppy, opening by pores (p), under the radiating peltate stigma (s).
Occasionally, divisions take place in ovaries which are not formed by the edges of contiguous carpels. These are calledspurious dissepiments. They are often horizontal, as inCathartocarpus Fistula, where they consist of transverse cellular prolongations from the walls of the ovary, only developed after fertilization, and therefore more properly noticed under fruit. At other times they are vertical, as inDatura, where the ovary, in place of being two-celled, becomes four-celled; in Cruciferae, where the prolongation of the placentas forms a vertical partition; inAstragalusandThespesia, where the dorsal suture is folded inwards; and inOxytropis, where the ventral suture is folded inwards.
The ovary is usually of a more or less spherical or curved form, sometimes smooth and uniform on its surface, at other times hairy and grooved. The grooves usually indicate the divisions between the carpels and correspond to the dissepiments. The dorsal suture may be marked by a slight projection or by a superficial groove. When the ovary is situated on the centre of the receptacle, free from the other whorls, so that its base is above the insertion of the stamens, it is termedsuperior, as inLychnis,Primula(fig. 61) and Peony (fig. 64) (see also fig. 28). When the margin of the receptacle is prolonged upwards, carrying with it the floral envelopes and staminal leaves, the basal portion of the ovary being formed by the receptacle, and the carpellary leaves alone closing in the apex, the ovary isinferior, as in pomegranate, aralia (fig. 65), gooseberry and fuchsia (see fig. 30). In some plants, as many Saxifragaceae, there are intermediate forms, in which the termhalf-inferioris applied to the ovary, whilst the floral whorls arehalf-superior.
Thestyleproceeds from the summit of the carpel (fig. 102), and is traversed by a narrow canal, in which there are some loose projecting cells, a continuation of the placenta, constituting what is called conducting tissue, which ends in the stigma. This is particularly abundant whenThe style.the pistil is ready for fertilization. In some cases, owing to more rapid growth of the dorsal side of the ovary, the style becomeslateral(fig. 101); this may so increase that the style appears to arise from near the base, as in the strawberry, or from the base, as inChrysobalanus Icaco, when it is calledbasilar. In all these cases the style still indicates the organic apex of the ovary, although it may not be the apparent apex. When in a compound pistil the style of each carpel is thus displaced, it appears as if the ovary were depressed in the centre, and the style rising from the depression in the midst of the carpels seems to come from the torus. Such a style isgynobasic, and is well seen in Boraginaceae. The form of the style is usually cylindrical, more or less filiform and simple; sometimes it is grooved on one side, at other times it is flat, thick, angular, compressed and even petaloid, as inIris(fig. 103) andCanna. In Goodeniaceae it ends in a cuplike expansion, enclosing the stigma. It sometimes bears hairs, which aid in the application of the pollen to the stigma, and are calledcollecting hairs, as inCampanula, and also inAsterand other Compositae. These hairs, during the upward growth of the style, come into contact with the already ripened pollen, and carry it up along with them, ready to be applied by insects to the mature stigma of other flowers. InViciaandLobeliathe hairs frequently form a tuft below the stigma. The styles of a syncarpous pistil are either separate or united; when separate, they alternate with the septa; when united completely, the style is said to besimple(fig. 102). The style of a single carpel, or of each carpel of a compound pistil, may also be divided. Each division of the tricarpellary ovary ofJatropha Curcashas abifurcateor forked style, and the ovary ofEmblica officinalishas three styles, each of which is twice forked. The length of the style is determined by the relation which should subsist between the position of the stigma and that of the anthers, so as to allow the proper application of the pollen. The style is deciduous or persists after fertilization.