Chapter X: Part 10

The flower consists of the floral axis bearing the sporophylls (stamens and carpels), usually with certain protective envelopes. The axis is usually very much contracted, no internodes being developed, and the portion bearing the floral leaves, termed the _thalamus_ or _torus_, frequently expands into a conical, flattened or hollowed expansion; at other times, though rarely, the internodes are developed and it is elongated. Upon this torus the parts of the flower are arranged in a crowded manner, usually forming a series of verticils, the parts of which alternate; but they are sometimes arranged spirally especially if the floral axis be elongated. In a typical flower, as in fig. 22, we recognize four distinct whorls of leaves: an outer whorl, the _calyx_ of _sepals_; within it, another whorl, the parts alternating with those of the outer whorl, the _corolla_ of _petals_; next a whorl of parts alternating with the parts of the corolla, the _androecium_ of _stamens_; and in the centre the _gynoecium_ of _carpels_. Fig. 23 is a diagrammatic representation of the arrangement of the parts of such a flower; it is known as a _floral diagram_. The flower is supposed to be cut transversely, and the parts of each whorl are distinguished by a different symbol. Of these whorls the two internal, forming the sporophylls, constitute the _essential_ organs of reproduction; the two outer whorls are the protective coverings or floral _envelopes_. The sepals are generally of a greenish colour; their function is mainly protective, shielding the more delicate internal organs before the flower opens. The petals are usually showy, and normally alternate with the sepals. Sometimes, as usually in monocotyledons, the calyx and corolla are similar; in such cases the term _perianth_, or _perigone_, is applied. Thus, in the tulip, crocus, lily, hyacinth, we speak of the parts of the perianth, in place of calyx and corolla, although in these plants there is an outer whorl (calyx), of three parts, and an inner (corolla), of a similar number, alternating with them. When the parts of the calyx are in appearance like petals they are said to be _petaloid_, as in Liliaceae. In some cases the petals have the appearance of sepals, then they are _sepaloid_, as in Juncaceae. In plants, as _Nymphaea alba_, where a spiral arrangement of the floral leaves occurs, it is not easy to say where the calyx ends and the corolla begins, as these two whorls pass insensibly into each other. When both calyx and corolla are present, the plants are _dichlamydeous_; when one only is present, the flower is termed _monochlamydeous_ or _apetalous_, having no petals (fig. 24). Sometimes both are absent, when the flower is _achlamydeous_, or naked, as in willow. The outermost series of the essential organs, collectively termed the _androecium_, is composed of the microsporophylls known as the staminal leaves or _stamens_. In their most differentiated form each consists of a stalk, the _filament_ (fig. 25, f), supporting at its summit the anther (a), consisting of the pollen-sacs which contain the powdery _pollen_ (p), the microspores, which is ultimately discharged therefrom. The _gynoecium_ or _pistil_ is the central portion of the flower, terminating the floral axis. It consists of one or more _carpels_ (megasporophylls), either separate (fig. 22, c) or combined (fig. 24). The parts distinguished in the pistil are the _ovary_ (fig. 26, o), which is the lower portion enclosing the _ovules_ destined to become seeds, and the _stigma_ (g), a portion of loose cellular tissue, the receptive surface on which the pollen is deposited, which is either sessile on the apex of the ovary, as in the poppy, or is separated from it by a prolonged portion called the _style_ (s). The androecium and gynoecium are not present in all flowers. When both are present the flower is hermaphrodite; and in descriptive botany such a flower is indicated by the symbol [female/male]. When only one of those organs is present the flower is _unisexual_ or _diclinous_, and is either male (_staminate_), [male] or female (_pistillate_), [female]. A flower then normally consists of the four series of leaves--calyx, corolla, androecium and gynoecium--and when these are all present the flower is _complete_. These are usually densely crowded upon the thalamus, but in some instances, after apical growth has ceased in the axis, an elongation of portions of the receptacle by intercalary growth occurs, by which changes in the position of the parts may be brought about. Thus in _Lychnis_ an elongation of the axis betwixt the calyx and the corolla takes place, and in this way they are separated by an interval. Again, in the passion-flower (_Passiflora_) the stamens are separated from the corolla by an elongated portion of the axis, which has consequently been termed the _androphore_, and in _Passiflora_ also, fraxinella (fig. 27), Capparidaceae, and some other plants, the ovary is raised upon a distinct stalk termed the _gynophore_; it is thus separated from the stamens, and is said to be _stipitate_. Usually the successive whorls of the flower, disposed from below upwards or from without inwards upon the floral axis, are of the same number of parts, or are a multiple of the same number of parts, those of one whorl alternating with those of the whorls next it.

In the more primitive types of flowers the torus is more or less convex, and the series of organs follow in regular succession, culminating in the carpels, in the formation of which the growth of the axis is closed (fig. 28). This arrangement is known as hypogynous, the other series (calyx, corolla and stamens) being beneath (_hypo-_) the gynoecium. In other cases, the apex of the growing point ceases to develop, and the parts below form a cup around it, from the rim of which the outer members of the flower are developed around (_peri-_) the carpels, which are formed from the apex of the growing-point at the bottom of the cup. This arrangement is known as _perigynous_ (fig. 29). In many cases this is carried farther and a cavity is formed which is roofed over by the carpels, so that the outer members of the flower spring from the edge of the receptacle which is immediately above the ovary (epigynous), hence the term epigyny (fig. 30).

Symmetry of the flower.

When a flower consists of parts arranged in whorls it is said to be _cyclic_, and if all the whorls have an equal number of parts and are alternate it is _eucyclic_ (figs. 22, 23). In contrast to the cyclic flowers are those, as in Magnoliaceae, where the parts are in spirals (_acyclic_). Flowers which are cyclic at one portion and spiral at another, as in many Ranunculaceae, are termed _hemicyclic_. In spiral flowers the distinction into series is by no means easy, and usually there is a gradual passage from sepaloid through petaloid to staminal parts, as in the water-lily family, Nymphaeaceae (figs. 31, 32), although in some plants there is no such distinction, the parts being all petaloid, as in _Trollius_. Normally, the parts of successive whorls alternate; but in some cases we find the parts of one whorl opposite or _superposed_ to those of the next whorl. In some cases, as in the vine-family Ampelidaceae, this seems to be the ordinary mode of development, but the superposition of the stamens on the sepals in many plants, as in the pink family, Caryophyllaceae, is due to the suppression or abortion of the whorl of petals, and this idea is borne out by the development, in some plants of the order, of the suppressed whorl. As a rule, whenever we find the parts of one whorl superposed on those of another we may suspect some abnormality.

FIGS. 31 and 32.--White Water Lily. Fig. 31, flower; fig. 32, successive stages, a-f, in the transition from petals to stamens. (After Wossidlo.)]

A flower is said to be _symmetrical_ when each of its whorls consists of an equal number of parts, or when the parts of any one whorl are multiples of that preceding it. Thus, a symmetrical flower may have five sepals, five petals, five stamens and five carpels, or the number of any of these parts may be ten, twenty or some multiple of five. Fig. 23 is a diagram of a symmetrical flower, with five parts in each whorl, alternating with each other. Fig. 33 is a diagram of a symmetrical flower of stone-crop, with five sepals, five alternating petals, ten stamens and five carpels. Here the number of parts in the staminal whorl is double that in the others, and in such a case the additional five parts form a second row alternating with the others. In the staminal whorl especially it is common to find additional rows. Fig. 34 shows a symmetrical flower, with five parts in the three outer rows, and ten divisions in the inner. In this case it is the gynoecium which has an additional number of parts. Fig. 35 shows a flower of heath, with four divisions of the calyx and corolla, eight stamens in two rows, and four divisions of the pistil. In fig. 36 there are three parts in each whorl; and in fig. 37 there are three divisions of the calyx, corolla and pistil, and six stamens in two rows. In all these cases the flower is symmetrical. In Monocotyledons it is usual for the staminal whorl to be double, it rarely having more than two rows, whilst amongst dicotyledons there are often very numerous rows of stamens. The floral envelopes are rarely multiplied. Flowers in which the number of parts in each whorl is the same, are _isomerous_ (of equal number); when the number in some of the whorls is different, the flower is _anisomerous_ (of unequal number). The pistillate whorl is very liable to changes. It frequently happens that when it is fully formed, the number of its parts is not in conformity with that of the other whorls. In such circumstances, however, a flower has been called symmetrical, provided the parts of the other whorls are normal,--the permanent state of the pistil not being taken into account in determining symmetry. Thus fig. 38 shows a pentamerous symmetrical flower, with dimerous pistil. Symmetry, then, in botanical language, has reference to a certain definite numerical relation of parts. A flower in which the parts are arranged in twos is called _dimerous_; when the parts of the whorls are three, four or five, the flower is _trimerous_, _tetramerous_ or _pentamerous_, respectively. The symmetry which is most commonly met with is trimerous and pentamerous--the former occurring generally among monocotyledons, the latter among dicotyledons. Dimerous and tetramerous symmetry occur also among dicotyledons.

FIG. 33.--Diagrammatic section of a symmetrical pentamerous flower of Stone-crop (_Sedum_), consisting of five sepals (s), five petals (p) alternating with the sepals, ten stamens (a) in two rows, and five carpels (c) containing ovules. The dark lines (d) on the outside of the carpels are glands.

FIG. 34.--Diagram of the flower of Flax (_Linum_), consisting of five sepals (s), five petals (p), five stamens (a), and five carpels (c), each of which is partially divided into two. The dots represent a whorl of stamens which has disappeared. It is pentamerous, complete, symmetrical and regular.

FIG. 35.--Diagram of the flower of Heath (_Erica_), a regular tetramerous flower.

FIG. 36.--Diagram of the trimerous symmetrical flower of Iris.

FIG. 37.--Diagram of the symmetrical trimerous flower of Fritillary (_Fritillaria_).

FIG. 38.--Diagram of the flower of Saxifrage (_Saxifraga tridactylites_). The calyx and corolla consist of five parts, the stamens are ten in two rows, while the pistil has only two parts developed.]

The various parts of the flower have a certain definite relation to the axis. Thus, in axillary tetramerous flowers (fig. 35), one sepal is next the axis, and is called _superior_ or _posterior_; another is next the bract, and is _inferior_ or _anterior_, and the other two are _lateral_; and certain terms are used to indicate that position. A plane passing through the anterior and posterior sepal and through the floral axis is termed the _median plane_ of the flower; a plane cutting it at right angles, and passing through the lateral sepals, is the _lateral plane_; whilst the planes which bisect the angles formed by the lateral and median planes are the _diagonal planes_, and in these flowers the petals which alternate with the sepals are cut by the diagonal planes.

FIG. 40.--Flower of Pea (_Pisum sativum_), showing a papilionaceous corolla, with one petal superior (st) called the standard (vexillum), two inferior (car) called the keel (carina), and two lateral (a) called wings (alae). The calyx is marked c.]

In a pentamerous flower one sepal may be superior, as in the calyx of Rosaceae and Labiatae; or it may be inferior, as in the calyx of Leguminosae (fig. 39)--the reverse, by the law of alternation, being the case with the petals. Thus, in the blossom of the pea (figs. 39, 40), the odd petal (vexillum) st is superior, while the odd sepal is inferior. In the order Scrophulariaceae one of the two carpels is posterior and the other anterior, whilst in Convolvulaceae the carpels are arranged laterally. Sometimes the twisting of a part makes a change in the position of other parts, as in Orchids, where the twisting of the ovary changes the position of the labellum.

When the different members of each whorl are like in size and shape, the flower is said to be _regular_; while differences in the size and shape of the parts of a whorl make the flower _irregular_, as in the papilionaceous flower, represented in fig. 39. When a flower can be divided by a single plane into two exactly similar parts; then it is said to be _zygomorphic_. Such flowers as Papilionaceae, Labiatae, are examples. In contrast with this are _polysymmetrical_ or actinomorphic flowers, which have a radial symmetry and can be divided by several planes into several exactly similar portions; such are all regular, symmetrical flowers. When the parts of any whorl are not equal to or some multiple of the others, then the flower is _asymmetrical_. This want of symmetry may be brought about in various ways. Alteration in the symmetrical arrangement as well as in the completeness and regularity of flowers has been traced to _suppression_ or the _non-development_ of parts, _degeneration_ or imperfect formation, _cohesion_ or union of parts of the same whorl, _adhesion_ or union of the parts of different whorls, _multiplication_ of parts, and _deduplication_ (sometimes called _chorisis_) or splitting of parts.

By _suppression_ or non-appearance of a part at the place where it ought to appear if the structure was normal, the symmetry or completeness of the flower is disturbed. This suppression when confined to the parts of certain verticils makes the flower asymmetrical. Thus, in many Caryophyllaceae, as _Polycarpon_ and _Holosteum_, while the calyx and corolla are pentamerous, there are only three or four stamens and three carpels; in _Impatiens Noli-me-tangere_ the calyx is composed of three parts, while the other verticils have five; in labiate flowers there are five parts of the calyx and corolla, and only four stamens; and in _Tropaeolum pentaphyllum_ there are five sepals, two petals, eight stamens and three carpels. In all these cases the want of symmetry is traced to the suppression of certain parts. In the last-mentioned plant the normal number is five, hence it is said that there are three petals suppressed, as shown by the position of the two remaining ones; there are two rows of stamens, in each of which one is wanting; and there are two carpels suppressed. In many instances the parts which are afterwards suppressed can be seen in the early stages of growth, and occasionally some vestiges of them remain in the fully developed flower. By the suppression of the verticil of the stamens, or of the carpels, flowers become _unisexual_ or _diclinous_, and by the suppression of one or both of the floral envelopes, monochlamydeous and achlamydeous flowers are produced. The suppression of parts of the flower may be carried so far that at last a flower consists of only one part of one whorl. In the Euphorbiaceae we have an excellent example of the gradual suppression of parts, where from an apetalous, trimerous, staminal flower we pass to one where one of the stamens is suppressed, and then to forms where two of them are wanting. We next have flowers in which the calyx is suppressed, and its place occupied by one, two or three bracts (so that the flower is, properly speaking, achlamydeous), and only one or two stamens are produced. And finally, we find flowers consisting of a single stamen with a bract. There is thus traced a _degradation_, as it is called, from a flower with three stamens and three divisions of the calyx, to one with a single bract and a single stamen.

_Degeneration_, or the transformation of parts, often gives rise either to an apparent want of symmetry or to irregularity in form. In unisexual flowers it is not uncommon to find vestiges of the undeveloped stamens in the form of filiform bodies or scales. In double flowers transformations of the stamens and pistils take place, so that they appear as petals. In _Canna_, what are called petals are in reality metamorphosed stamens. In the capitula of Compositae we sometimes find the florets converted into green leaves. The limb of the calyx may appear as a rim, as in some Umbelliferae; or as pappus, in Compositae and _Valeriana_. In _Scrophularia_ the fifth stamen appears as a scale-like body; in other Scrophulariaceae, as in _Pentstemon_, it assumes the form of a filament, with hairs at its apex in place of an anther.

_Cohesion_, or the union of parts of the same whorl, and _adhesion_, or the growing together of parts of different whorls, are causes of change both as regards form and symmetry. Thus in _Cucurbita_ the stamens are originally five in number, but subsequently some cohere, so that three stamens only are seen in the mature flower. Adhesion is well seen in the _gynostemium_ of orchids, where the stamens and stigmas adhere. In Capparidaceae the calyx and petals occupy their usual position, but the axis is prolonged in the form of a gynophore, to which the stamens are united.

_Multiplication_, or an increase of the number of parts, gives rise to changes. We have already alluded to the interposition of new members in a whorl. This takes place chiefly in the staminal whorl, but usually the additional parts produced form a symmetrical whorl with the others. In some instances, however, this is not the case. Thus in the horse-chestnut there is an interposition of two stamens, and thus seven stamens are formed in the flower, which is asymmetrical.

Parts of the flower are often increased by a process of _deduplication_, or _chorisis_, i.e. the splitting of a part so that two or more parts are formed out of what was originally one. Thus in Cruciferous plants the staminal whorl consists of four long stamens and two short ones (_tetradynamous_). The symmetry in the flower is evidently dimerous, and the abnormality in the androecium, where the four long stamens are opposite the posterior sepals, takes place by a splitting, at a very early stage of development, of a single outgrowth into two. Many cases of what was considered chorisis are in reality due to the development of stipules from the staminal leaf. Thus in _Dicentra_ and _Corydalis_ there are six stamens in two bundles; the central one of each bundle alone is perfect, the lateral ones have each only half an anther, and are really stipules formed from the staminal leaf. Branching of stamens also produces apparent want of symmetry; thus, in the so-called polyadelphous stamens of Hypericaceae there are really only five stamens which give off numerous branches, but the basal portion remaining short, the branches have the appearance of separate stamens, and the flower thus seems asymmetrical.

_Cultivation_ has a great effect in causing changes in the various parts of plants. Many alterations in form, size, number and adhesion of parts are due to the art of the horticulturist. The changes in the colour and forms of flowers thus produced are endless. In the dahlia the florets are rendered quilled, and are made to assume many glowing colours. In pelargonium the flowers have been rendered larger and more showy; and such is also the case with the _Ranunculus_, the auricula and the carnation. Some flowers, with spurred petals in their usual state, as columbine, are changed so that the spurs disappear; and others, as _Linaria_, in which one petal only is usually spurred, are altered so as to have all the petals spurred, and to present what are called _pelorian_ varieties.

FIG. 41.--Tetramerous monochlamydeous male flower of the Nettle (_Urtica_).

FIG. 42.--Diagram to illustrate valvular or valvate aestivation, in which the parts are placed in a circle, without overlapping or folding.

FIG. 43.--Diagram to illustrate induplicative or induplicate aestivation, in which the parts of the verticil are slightly turned inwards at the edges.]

As a convenient method of expressing the arrangement of the parts of the flower, _floral formulae_ have been devised. Several modes of expression are employed. The following is a very simple mode which has been proposed:--The several whorls are represented by the letters S (sepals), P (petals), St (stamens), C (carpels), and a figure marked after each indicates the number of parts in that whorl. Thus the formula S5P5St5C5 means that the flower is perfect, and has pentamerous symmetry, the whorls being isomerous. Such a flower as that of Sedum (fig. 33) would be represented by the formula S5P5St_(5+5)C5, where St_(5+5) indicates that the staminal whorl consists of two rows of five parts each. A flower such as the male flower of the nettle (fig. 41) would be expressed S4P0St4C0. When no other mark is appended the whorls are supposed to be alternate; but if it is desired to mark the position of the whorls special symbols are employed. Thus, to express the superposition of one whorl upon another, a line is drawn between them, e.g. the symbol S5P5 | St5C5 is the formula of the flower of Primulaceae.

FIG. 44.--Diagram to illustrate reduplicative or reduplicate aestivation, in which the parts of the whorl are slightly turned outwards at the edges.

FIG. 45.--Diagram to illustrate contorted or twisted aestivation, in which the parts of the whorl are overlapped by each other in turn, and are twisted on their axis.

FIG. 46.--Diagram to illustrate the quincuncial aestivation, in which the parts of the flower are arranged in a spiral cycle, so that 1 and 2 are wholly external, 4 and 5 are internal, and 3 is partly external and partly overlapped by 1.]

The manner in which the parts are arranged in the flower-bud with respect to each other before opening is the _aestivation_ or _praefloration_. 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 of _Lysimachia_, 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, becoming _crumpled_ or _corrugated_, 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 is _valvate_ (fig. 42). The edges of each of the parts may be turned either inwards or outwards; in the former case the aestivation is _induplicate_ (fig. 43), in the latter case _reduplicate_ (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 is _twisted_ or _contorted_ (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 under _circular_ aestivation. But in spiral flowers we have a different arrangement; thus the leaves of the calyx of _Camellia japonica_ cover each other partially like tiles on a house. This aestivation is _imbricate_. At other times, as in the petals of _Camellia_, the parts envelop each other completely, so as to become _convolute_. This is also seen in a transverse section of the calyx of _Magnolia 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 is _quincuncial_ (fig. 46). This quincunx is common in the corolla of Rosaceae. In fig. 47 a section is given of the bud of _Antirrhinum 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 termed _cochlear_. 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 to _vexillary_ aestivation (fig. 48), or the carina may perform a similar office, and then the aestivation is _carinal_, 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. In _Guazuma_ the 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.

FIG. 47.--Diagram to illustrate imbricated aestivation, in which the parts are arranged in a spiral cycle, following the order indicated by the figures 1, 2, 3, 4, 5.

FIG. 48.--Diagram of a papilionaceous flower, showing vexillary aestivation.

1 and 2, The alae or wings.

3, A part of the carina or keel.

4, The vexillum or standard, which, in place of being internal, as
marked by the dotted line, becomes external.

5, The remaining part of the keel.

The order of the cycle is indicated by the figures.]

Calyx.

The _sepals_ are sometimes _free_ or separate from each other, at other times they are united to a greater or less extent; in the former case, the calyx is _polysepalous_, in the latter _gamosepalous_ or _monosepalous_. The divisions of the 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 or acute. In their direction they are erect or reflexed (with their apices downwards), spreading outwards (_divergent_ or _patulous_), or arched inwards (_connivent_). They are usually of a greenish colour (_herbaceous_); but sometimes they are coloured or _petaloid_, as in the fuchsia, tropaeolum, 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 is _trisepalous_; one with five sepals is _pentasepalous_. 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 being _trifid_ (three-cleft), _quinquefid_ (five-cleft), &c., according to their number; or they reach to near the base in the form of partitions, the calyx being _tripartite_, _quadripartite_, _quinquepartite_, &c. The union of the parts may be complete, and the calyx may be quite entire or _truncate_, 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 or _labiate_ calyx 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 the _tube_ of the calyx; the portion where the sepals are free is the _limb_.

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. In _Potentilla_ and allied genera an _epicalyx_ is 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 called _obsolete_ or _marginate_. In Compositae, Dipsacaceae and Valerianaceae the calyx is attached to the pistil, and its limb is developed in the form of hairs called _pappus_ (fig. 51). This pappus is either simple (_pilose_) or feathery (_plumose_). In _Valeriana_ the 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 is _caducous_ (fig. 52); or along with the corolla, as in _Ranunculus_, and is _deciduous_; or it remains after flowering (_persistent_) as in Labiatae, Scrophulariaceae, and Boraginaceae; or its base only is persistent, as in _Datura Stramonium_. In _Eschscholtzia_ and _Eucalyptus_ the 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 is _operculate_ or _calyptrate_. 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, becoming _accrescent_, as in various species of _Physalis_ (fig. 53); at other times it remains in a withered or _marcescent_ form, as in _Erica_; sometimes it becomes _inflated_ or _vesicular_, as in sea campion (_Silene maritima_).

Corolla.

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 are 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 of _Cobaea_, _Hoya viridiflora_, _Gonolobus viridiflorus_ and _Pentatropis 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 1/2 or 1/4-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 of _Menyanthes_, and on the segments of the perianth of _Iris_. 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 in _Stapelia_ and _Rafflesia_; or dry, as in heaths; or hard and stiff, as in _Xylopia_. A petal often consists of two portions--the lower narrow, resembling the petiole of a leaf, and called the _unguis_ or _claw_; the upper broader, like the blade of a leaf, and called the _lamina_ or _limb_. 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 then _sessile_. 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 the petal becomes _fimbriated_, as in the pink; or _laciniated_, as in _Lychnis Flos-cuculi_; or _crested_, as in _Polygala_. Sometimes the petal becomes pinnatifid, as in _Schizopetalum_. The median vein is occasionally prolonged beyond the summit of the petals in the form of a long process, as in _Strophanthus 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 and _Centranthus_, one or all of them are prolonged in the form of a spur, and are _calcarate_. In _Valeriana_, _Antirrhinum_ and _Corydalis_, the spur is very short, and the corolla or petal is said to be _gibbous_, or _saccate_, 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 is _dipetalous_, _tripetalous_, _tetrapetalous_ or _pentapetalous_ according as it has two, three, four or five separate petals. The general name of _polypetalous_ is given to corollas having separate petals, while _monopetalous_, _gamopetalous_ or _sympetalous_ is 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; in _Phyteuma_ the 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 is _regular_; when unequal, it is _irregular_. 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 the _throat_, 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 the _rosaceous_ corolla (fig. 59), in which there are five spreading petals, having no claws, and arranged as in the rose, strawberry and _Potentilla_; the _caryophyllaceous_ corolla, in which there are five petals with long, narrow, tapering claws, as in many of the pink tribe; the _cruciform_, having four petals, often unguiculate, placed opposite in the form of a cross, as seen in wallflower, and in other plants called _cruciferous_. Of irregular polypetalous corollas the most marked is the _papilionaceous_ (fig. 40), in which there are five petals:--one superior (posterior), st, placed next to the axis, usually larger than the rest, called the _vexillum_ or _standard_; two lateral, a, the _alae_ or 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_, called _carina_, or keel, which embraces the essential organs. This form of corolla is characteristic of British leguminous plants.

FIG. 60.--Flower of _Campanula medium_; d, bract; v, bracteoles.]

Regular gamopetalous corollas are sometimes _campanulate_ or _bell-shaped_, as in (_Campanula_) (fig. 60); _infundibuliform_ or _funnel-shaped_, when the tube is like an inverted cone, and the limb becomes more expanded at the apex, as in tobacco; _hypocrateriform_ or _salver-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 in _Spigelia_ and comfrey; _rotate_ or _wheel-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 in _Galium_, it is sometimes called _stellate_ or _star-like_); _urceolate_ or _urn-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, in _Veronica_, 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 the _labiate_ or _lipped_ (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 called _ringent_ (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 be _personate_, 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, as _Cypripedium_. 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 becomes _ligulate_ or _strap-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 be abortive. Occasionally some of the petals become more united than others, and then the corolla assumes a _bilabiate_ or _two-lipped_ form, as seen in the division of Compositae called Labiatiflorae.

Petals are sometimes suppressed, and sometimes the whole corolla is absent. In _Amorpha_ and _Afzelia_ the 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 as _Ranunculus_, globe-flower and paeony, have both calyx and corolla, while others, such as clematis, anemone and _Caltha_, 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 lip u is 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 of _nectaries_. 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 of _Ranunculus_ (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 by _chorisis_, or _deduplication_. Of this nature are the scales on the petals in _Lychnis_, _Silene_ and _Cynoglossum_, 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. In _Narcissus_ the appendages are united to form a crown, consisting of a membrane similar to that which unites the stamens in _Pancratium_. 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 in _Rhinanthus_ and _Orobanche_.

The _stamens_ and the _pistil_ are 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 (see ANGIOSPERMS). _Hermaphrodite_ or _bisexual_ flowers are those in which both these organs are found; _unisexual_ or _diclinous_ are those in which only one of these organs appears,--those bearing stamens only, being _staminiferous_ or "male"; those having the pistil only, _pistilliferous_ or "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, in _Primula_ and _Linum_ some 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 called _dimorphic_. Other plants are _trimorphic_, as species of _Lythrum_, 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 called _proterandrous_, as in _Ranunculus repens_, _Silene maritima_, _Zea Mays_. In other plants, but more rarely, the pistil is perfected before the stamens, as in _Potentilla argentea_, _Plantago major_, _Coix Lachryma_, and they are termed _proterogynous_. Plants in which proterandry or proterogyny occurs are called _dichogamous_. When in the same plant there are unisexual flowers, both male and female, the plant is said to be _monoecious_, as in the hazel and castor-oil plant. When the male and female flowers of a species are found on separate plants, the term _dioecious_ is applied, as in _Mercurialis_ and hemp; and when a species has male, female and hermaphrodite flowers on the same or different plants, as in _Parietaria_, it is _polygamous_.

FIG. 64.--Flower of _Paeonia peregrina_, in longitudinal section. k, Sepal; c, petal; a, stamens; g, pistil. (1/2 nat. size.)]

Stamens.

The stamens arise from the thalamus or torus within the petals, with which they generally alternate, forming one or more whorls, which collectively constitute the _androecium_. Their normal position is below the pistil, and when they are so placed (fig. 64, a) upon the thalamus they are hypogynous. Sometimes they become adherent to the petals, or are _epipetalous_, 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 are _half superior_ or _half inferior_. Where the stamens become adherent to the pistil so as to form a column, the flowers are said to be _gynandrous_, as in _Aristolochia_ (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 then _isostemonous_. When the stamens are not equal in number to the sepals or petals, the flower is _anisostemonous_. 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 is _haplostemonous_. If the stamens are double the sepals or petals as regards number, the flower is _diplostemonous_; if more than double, _polystemonous_. The additional rows of stamens 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 be _obdiplostemonous_. When the stamens are fewer than twenty they are said to be _definite_; when above twenty they are _indefinite_, and are represented by the symbol [infinity]. The number of stamens is indicated by the Greek numerals prefixed to the term _androus_; thus a flower with one stamen is _monandrous_, with two, three, four, five, six or many stamens, di-, tri-, tetr-, pent-, hex- or polyandrous, respectively.

FIG. 66.--Flowers of _Aristolochia Clematitis_ cut through longitudinally. I. Young flower in which the stigma (N) is receptive and the stamens (S) have not yet opened; II. Older flower with the stamens (S) opened, the stigma withered, and the hairs on the corolla dried up.]

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 the _filament_ (fig. 25 f), and a broader portion, usually of two lobes, termed the _anther_ (a), containing the powdery _pollen_ (p), and supported upon the end of the filament. That portion of the filament in contact with the anther-lobes is termed the _connective_. 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, becoming _petaloid_ in _Canna_, _Marania_, water-lily (fig. 32); _subulate_ or slightly broadened at the base and drawn out into a point like an awl, as in _Butomus umbellatus_; or clavate, that is, narrow below and broad above, as in _Thalictrum_. In some instances, as in _Tamarix gallica_, _Peganum Harmala_, and _Campanula_, 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 in _Allium_ and _Alyssum 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, becoming _geniculate_; at other times, as in the pellitory, it is spiral. It is colourless, or of different colours. Thus in fuchsia and _Poinciana_, it is red; in _Adamia_ and _Tradescantia virginica_, blue; in _Oenothera_ and _Ranunculus acris_, yellow.