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).
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.
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| Fig. 28. | Fig. 29. | Fig. 30. |
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Figs. 28, 29 and 30.—Diagrams illustrating hypogyny, perigyny and epigyny of the flower. a, Stamens; c, carpels; p, petals; s, sepals. | ||
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| Fig. 32.Fig. 31. |
| From Strasburger’s Text-Book of Botany, by permission of Macmillan & Co., Ltd. |
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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.) |
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| Fig. 33. | Fig. 34. |
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| Fig. 35. | Fig. 36. |
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| Fig. 37. | Fig. 38. |
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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. | |
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
