Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/137

Rh SYMMETRY OF THE FLOWER.] BOTANY 127 only one of those organs is present the flower is unisexual or diclinous, and is cither male (staminate), ;$, or female (pistillate), ?. A ilower then normally consists of the four whorls of leaves, calyx, corolla, androceium, and gynoe- cium, and when these are all present th^ Ilower is com plete. These several whorls of the flower 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 Passiflora the stamens are separated from the corolla by an elongated por tion of the axis, which has consequently been termed the andropkore; and in Passiflora, Fraxinella (fig. 177), Cap- paridacete, and some other plants, the ovary is raised upon a distinct stalk termed the gynophore ; it is thus sepa rated from the stamens, and is said to be stipitate. The apical growth of the floral axis in the flower soon ceases, and therefore the parts are arranged in whorls; but at times, as in the Ranun- Fig. 177. culaceas, Magnoliacese, &c., the growth c.-iiyx and pistil of Frax- is of sufficient duration to permit a spiral arrangement of parts. Usually the successive whorb of the Ilower dis posed 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. 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 encyclic. In con trast to the cyclic flowers are those, as in Magnoliacese, where the parts are in spirals. When in such a flower the transition from one foliar structure to another, i.e., from calyx to corolla, from corolla to stamens, &c., does not coincide with a definite number of turns of the spiral it is said to be acyclic ; if it does so correspond it is termed hemicyclic ; the latter term also includes flowers which are cyclic at one portion and spiral at another, as in many Ranunculaceax In spiral flowers the distinction into whorls is by no means easy, and usually there is a gradual passage from sepaloid through petaloid to staminal parts, as in Nymphosacece (figs. 178, 179), although in some plants inella (Dictamnut Frax- ine!la~). The pistil con sists of several carpels, which are elevated on a stalk prolonged from the receptacle. The stalk is called gyno phore or thecaphcre. Fig. 178. Fig. 179. FIG. 178.--Flower of ffymphcca albft, White Water-lily, c, e, e, &amp;lt;-, the four foliola of the calyx or sepals; p, p,p, p, petals; e, stamens; s, pistil. Fio. 179. Parts of the flower separated to show the transition from the green sepals of the calyx c, and the white petals of the corolla p, to the stamens c. The latter present changes from their perfect state, 5, through intermediate forms, 4, 3, 2, and 1, which gradually resemble the petals. there is no such distinction, the parts being all petaloid, as in Trollius. We may again note here that some regard the cone in Conifers and Cycaclaceaa as a single flower, and in that case the whorls are much separated and arranged spirally along the floral axis. Normally, the parts of suc cessive whorls alternate; but in some cases we find the parts of one whorl opposite or svperjjosed to those of tho next whorl. In some cases, as in the Ampelidetc, this seems to be the ordinary mode of development, but in Caryophyllacece tho superposition of the stamens on tho sepals in many plants seems to be 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 tho suppressed whorl. In Primulacese, again, where there is a superposition of stamens and petals the abnormality is duo apparently to another cause. The petals are developed after the stamens, and are to be regarded as appendages from them, of the same nature as the appendages to tho stamens in Asclepiadacea3, so that morphologically in Pri- mulaceaa, according to this view, there are no petals. As a rule, whenever we find the parts of one whorl superposed to those of another we may suspect some abnormality. Frequently, when parts are superposed they become adherent to one another; thus the stamens become adherent to the petals or are epipetalous, or to the sepals and are episepalous. A flower is said to be symmetrical when each of its whorls consists of an equal number of parts, or when tho parts of any one whorl are multiples of that preceding it. Thus, a symmetrical flower may^iave 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. 173 is a diagram of a symmetrical flower, with five parts in each whorl, alternating with each other. In fig. 180 there is a section 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 com mon to find additional rows. In fig. 181 there is a Fig. 180. 181. Fio. 180. Diagrammatic section of a symmetrical pcntamerous flower of Stono- crop (Sedum), consisting of five sepals externally, five petals alternating with the sepals, ten stamens In two rows, and five carpels containing seeds. Tho dark lines on tho outside of the carpels are glands. FIG. 181. Diagram of the flower of Flax (Linum,, consisting of five sepals, five petals, five stamens, and five carpels, each of -vMiich is partially divided into two. It is pcntamerous, complete, symmetrical, and regular. 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. 182 shows a flower of Heath, with four divisions of tho calyx and corolla, eight stamens in two rows, and four divisions of the pistil. In fig. 183 there are three parts in each whorl; and in figs. 184 there are three divi sions of the calyx, corolla, and pistil, and six stamens in two rows. In all these cases the flower is symmetrical. Where, as in the Stone-crop, an extra row of parts is deve loped in any whorl they may be cither formed in regular succession within the first row, or they may be interposed to them, i.e., formed between them, or even external to and beneath them. We have examples of the former in many Caryophyllaccous plants, the latter being well seen in Crassulacec-c, Geraniaceoc, &c. In Monocotyledons it is