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 S5P5St5+5C5, where St5+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. |
| 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. |
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.
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. 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, 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 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.
| From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer. |
| 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).
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 Corolla. 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 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 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.