Part 23

_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.

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.

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.

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_.

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.

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.