Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/138

128 BOTANY [itEPllODUCTIVE ORGANS. usual for the staniinal 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 iu which the number of parts Fkr. 184. Fig. 185. FIG. 182. Diagram of the flower of Heath (Erica), having four sepals, four divisions of the corolla, eight stamens in two rows, and four divisions of the pistil. The flower is tetramerous, complete, symmetrical, and regular. FIG. 183. Diagram of the trimerous symmetrical flower of Iris. There are three alternating divisions of each whorl. Below is a single bract. FIG. 184. Diagram of the symmetrical trimerous flower of Fritillary, having three divisions of the two outer whorls, and of the pistil in the centre, and six stamens in two rows. FIG. 185. 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 flower is, nevertheless, called symmetrical, although the outer whorls are pcntamerous, and the central one dimerous. 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 pistilline 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 cir cumstances, 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. 185 shows a pentamerous symmetrical flower, with dimerous pistils. Symmetry, then, in botanical language, has refer ence to a certain definite numerical relation of parts. A flower in which the parts are arranged in twos, is called dimerous, the symmetry being binary and the arrangement marked thus %/. This may be considered either as ana logous to opposite decussating leaves, or as composed of distichous alternate leaves belonging to the one-half series. When the parts of the floral whorls are three, the flower is trimerous, and the symmetry being ternary or trigonal is marked ff. This may be looked upon as composed of verticillate leaves, or of tristichous alternate leaves with the angular divergence L When floral whorls have parts in a series of four, the flower is tetramerous, its symmetry being quaternary, and marked ^/. A pentamerous flower, marked /, has quinary or pentagonal symmetry. The symmetry which is most commonly met with in the vege table kingdom is trimerous and pentamerous the former occurring generally among Monocotyledons, the latter among Dicotyledons. Dimerous or tetramerous symmetry occurs also among Dicotyledonous plants, and the numbers 2 and 4 prevail in the reproductive organs of Acotyledons. The various parts of the flower have a certain definite relation to the axis. Thus, in axillary tetramerous flowers (fig. 182), one sepal is next the axis, and is called superior or jiosterior ; 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 thsse flowers the petals which alternate with the sepals are cut by the diagonal planes. In a pentamerous flower one sepal may be superior, as in the calyx of Rosacese and Labiatre ; or it may be inferior, as in the calyx of Leguminosre (fig. ISO), the reverse, by the law of alternation, being the case with the petals. Thus, in the blossom of the Pea (fig. 187), the odd petal (vexil- st FIG. 186. Diagram of flower of Sweet-pea (Lathyrus), showing five parts of the calyx, of which two are superior, one inferior, and two lateral; five parts of the corolla, of which one is superior, two inferior, and two lateral; ten stamens in two rows; one carpel, in consequence of four being undeveloped. FIG. 187. Flower of Pea (Pisum satimtm), 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 (aloe). The calyx is marked c. lum) st is superior, while the odd sepal is inferior. In the Figwort order one of the two carpels is posterior and the other anterior, whilst in Convolvulacere 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. 187. When a flower can be divided by a single median plane into two exactly similar parts, then it is said to be zyyomorphic. Such flowers as Papilionacese, Labiatse, are examples. In contrast with this are poly symmetrical flowers, which 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 unsymmetrical. This want of symmetry may be brought about in various ways. A consideration of the various unsymmetrical, irregular, and incomplete forms of the flower, and the processes by which they have been brought about, constitutes Teratology. 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, multipli cation 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 unsymmetrical. Thus, in the flowers of Staphylea there are five parts of the calyx, five petals, five stamens, and only two carpels ; in many Caryophyllaceaa, as Polycarpon and Holosteum, while the calyx and corolla arc pentamerous, there are only three or four stamens and

three carpels ; in Impatiens noli-mc-tangere the calyx is