Capitulum of contracted Scorpioid Cymes (Glomerulus), Sea-pink.
| Fig. 22.—Flower of Sedum rubens. s, Sepals; p, petals; a, stamens; c, carpels. Fig. 23.—Diagram of a completely symmetrical flower, consisting of four whorls, each of five parts, s, Sepals; p, petals; a, stamens; c, carpels. Fig. 24.—Monochlamydeous (apetalous) flower of Goosefoot (Chenopodium), consisting of a single perianth (calyx) of five parts, enclosing five stamens, which are opposite the divisions of the perianth, owing to the absence of the petals. Fig. 25.—Stamen, consisting of a filament (stalk) f and an anther a, containing the pollen p, which is discharged through slits in the two lobes of the anther. Fig. 26.—The pistil of Tobacco (Nicotiana Tabacum), consisting of the ovary o, containing ovules, the style s, and the capitate stigma g. The pistil is placed on the receptacle r, at the extremity of the peduncle. |
| Fig. 27.—Calyx and pistil of Fraxinella (Dictamnus Fraxinella). The pistil consists of several carpels, which are elevated on a stalk or gynophore prolonged from the receptacle. |
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 flower. 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 ☿. When only one of those organs is present the flower is unisexual or diclinous, and is either male (staminate), ♁; or female (pistillate), ♀. 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).
| Fig. 28. Fig. 29. Fig. 30. |
| Figs. 28, 29 and 30.—Diagrams illustrating hypogyny, perigyny and epigyny of the flower. a, Stamens; c, carpels; p, petals; s, sepals. |
| Fig. 31. Fig. 32. |
| From Strasburger’s Text-Book of Botany, by permission of Macmillan & Co., Ltd. |
| 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.) |
| 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. |
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, Symmetry of the flower. 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.
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.
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.