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.
In a pentamerous flower one sepal may be superior, as in the calyx of Rosaceae and Labiatae; or it may be inferior, as in the calyx of Leguminosae (fig. 39)—the reverse, by the law of alternation, being the case with the petals. Thus, in the blossom of the pea (figs. 39, 40), the odd petal (vexillum) st is superior, while the odd sepal is inferior. In the order Scrophulariaceae one of the two carpels is posterior and the other anterior, whilst in Convolvulaceae 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.
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Fig. 39.—Diagram of flower of Sweet-pea (Lathyrus), showing five sepals (s), two superior, one inferior, and two lateral; five petals (p), one superior, two inferior, and two lateral; ten stamens in two rows (a); and one carpel (c). |
Fig. 40.—Flower of Pea (Pisum sativum), 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 (alae). The calyx is marked c. |
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. 39. When a flower can be divided by a single plane into two exactly similar parts; then it is said to be zygomorphic. Such flowers as Papilionaceae, Labiatae, are examples. In contrast with this are polysymmetrical or actinomorphic flowers, which have a radial symmetry and 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 asymmetrical. This want of symmetry may be brought about in various ways. 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, multiplication 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 asymmetrical. Thus, in many Caryophyllaceae, as Polycarpon and Holosteum, while the calyx and corolla are pentamerous, there are only three or four stamens and three carpels; in Impatiens Noli-me-tangere the calyx is composed of three parts, while the other verticils have five; in labiate flowers there are five parts of the calyx and corolla, and only four stamens; and in Tropaeolum pentaphyllum there are five sepals, two petals, eight stamens and three carpels. In all these cases the want of symmetry is traced to the suppression of certain parts. In the last-mentioned plant the normal number is five, hence it is said that there are three petals suppressed, as shown by the position of the two remaining ones; there are two rows of stamens, in each of which one is wanting; and there are two carpels suppressed. In many instances the parts which are afterwards suppressed can be seen in the early stages of growth, and occasionally some vestiges of them remain in the fully developed flower. By the suppression of the verticil of the stamens, or of the carpels, flowers become unisexual or diclinous, and by the suppression of one or both of the floral envelopes, monochlamydeous and achlamydeous flowers are produced. The suppression of parts of the flower may be carried so far that at last a flower consists of only one part of one whorl. In the Euphorbiaceae we have an excellent example of the gradual suppression of parts, where from an apetalous, trimerous, staminal flower we pass to one where one of the stamens is suppressed, and then to forms where two of them are wanting. We next have flowers in which the calyx is suppressed, and its place occupied by one, two or three bracts (so that the flower is, properly speaking, achlamydeous), and only one or two stamens are produced. And finally, we find flowers consisting of a single stamen with a bract. There is thus traced a degradation, as it is called, from a flower with three stamens and three divisions of the calyx, to one with a single bract and a single stamen.
Degeneration, or the transformation of parts, often gives rise either to an apparent want of symmetry or to irregularity in form. In unisexual flowers it is not uncommon to find vestiges of the undeveloped stamens in the form of filiform bodies or scales. In double flowers transformations of the stamens and pistils take place, so that they appear as petals. In Canna, what are called petals are in reality metamorphosed stamens. In the capitula of Compositae we sometimes find the florets converted into green leaves. The limb of the calyx may appear as a rim, as in some Umbelliferae; or as pappus, in Compositae and Valeriana. In Scrophularia the fifth stamen appears as a scale-like body; in other Scrophulariaceae, as in Pentstemon, it assumes the form of a filament, with hairs at its apex in place of an anther.
Cohesion, or the union of parts of the same whorl, and adhesion, or the growing together of parts of different whorls, are causes of change both as regards form and symmetry. Thus in Cucurbita the stamens are originally five in number, but subsequently some cohere, so that three stamens only are seen in the mature flower. Adhesion is well seen in the gynostemium of orchids, where the stamens and stigmas adhere. In Capparidaceae the calyx and petals occupy their usual position, but the axis is prolonged in the form of a gynophore, to which the stamens are united.
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
