Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/142

BOTANY [EEPEOEUCTIVE ORGANS. Plate VI II feathery (plumose). In Valeriana the superior calyx is at first an obsolete rim, but as the fruit ripens it is shown to consist of hairs rolled inwards, which expand so as to waft the fruit. The calyx sometimes falls off before the flower Plate I. expands, as in Poppies, and is caducous (fig. 199) ; or along with the corolla, as in Ranunculus, and is deciduous ; or it Plate XT. remains after flowering, as in Labiatue, Scrophulariacefe, and Boraginaceoe ; or its base only is persistent, as in Datura Stramonium. In Eschscholtzia and Eucalyptus the sepals remain united at the upper part, and become disarticulated at the base or middle, so as to come off in the form of a lid or funnel. Such a calyx is opercidate or calyptrate. The existence or non-existence of an articulation determines the deciduous or persistent nature of the calyx. In the case of Eschscholtzia the axis seems to be prolonged so as Plate II. to form a sort of tube, from which the calyx separates. In Eucalyptus the calyx consists of leaves, the laminae or petioles of which are articulated like those of the Orange, and the separation between the parts occurs at this articu lation. The receptacle bearing the calyx is sometimes united to the pistil, and enlarges so as to form a part of the fruit, as in the Apple, Pear, Pomegranate, Gooseberry, etc. In these fruits the withered calyx is seen at the apex. Sometimes a persistent calyx increases much after flowering, and encloses the fruit without being incorporated with it, becoming accrescent, as in various species of Physalis (fig. 200) ; at other times it remains in a withered or marcescent form, as in Erica ; sometimes it becomes inflated or vesicular, as in Sea Campion (Silene maritima). 2. Corolla. Corolla The corolla is the more or less coloured inner floral envelope, forming the whorl of leaves between the calyx and the stamens. It is generally the most conspicuous whorl. The gay colours and fragrant odours of flowers are resident in it. It is present in the greater number of Dicotyledons. Petals differ more from leaves than sepals do, and are much more nearly allied to the staminal whorl. In some cases, however, they are transformed into leaves, like the calyx, and occasionally leaf-buds are developed in their axil. They are seldom green, although occasionally that colour is met with, as in some Cobaeas, Hoya viridiflora, Gonolobus viridiflorus, and Pentatropis spiralis. As a rule they are highly coloured, the colouring matter being con tained in cells, and differing in its nature from the chlorophyll of the leaves. As regards their structure petals consist of cellular tissue, traversed by true spiral vessels and thin-walled tubes. In delicate flowers, as Convolvulus and Anagallis, these vessels are easily seen under the microscope. Petals do not usually present numerous layers of cells like the leaves, neither is the epidermis always distinct, although in some instances it may be detached, especially from the surface next the calyx. The cuticle of the petal of a Pelargonium, when viewed with a or ^-inch object glass, shows beautiful hexagons, the boundaries of which are ornamented with several inflected loops in the sides of the cells On the outer surface of petals, corresponding to the lower side of leaves, stomata are sometimes found. Petals are generally glabrous or smooth ; but, in some instances, hairs are produced on their surface. Petaline hairs, though sparse and scattered, present occasionally the same arrange ment as those which occur on the leaves ; thus, in Bombacese they are stellate. Coloured hairs arc seen on the petals of Menyanthes, and on the segments of the perianth of the Iris. Although petals are usually very thin and delicate in their texture, they occasionally become thick and fleshy, as in Stapelia and Rafflesia ; or dry, as in Heaths ; or hard and stiff, as in Xylopia. A petal often consists of two portions the lower narrow, resembling the petiole of a leaf, and called the unguis or claw ; the upper broader, like the blade of a leaf, and called the lamina or limb. These parts are seen in the petals of the Wallflower (fig. 201), where c is the claw and I the limb. The claw is often wanting, as in the Crowfoot (fig. 202) and the Poppy, and the petals are Plate I. then sessile. Petals having a claw are unguicidate. Accord ing to the development of veins and the growth of cellular tissue, petals present varieties similar to those already noticed in the case of leaves. Thus the margin is either entire or divided into lobes or teeth. These teeth sometimes form a regular fringe round the margin, and the petal becomes fimbriated, as in the Pink ; or laciniated, as in Lychnis Flos-cuculi ; or crested, as in Polygala. Sometimes the petal becomes pinnatifid, as in Schizopetalum. The median vein is occasionally prolonged beyond the summit Fig. 203. Fig. 204. FIG. 201. Unguiculate or stalked petal of Wallflower (Cheiranthus Cheiri). c, the claw or unguis ; /, the lilade or lamina. FIG. 202. Petal of Crowfoot (Kanunculus) , without a claw, and thus resembling a sessile leaf. At the base of the pefcil a nectariferous scale is seen. FIG. 203. Tubular petal of Hellebore (HeUeborus), formed by folding and adhesion, in the same manner as pitchers. FIG. 204. Horn-like hollow petal of Columbine (Aquihyia vulyaris), formed by folding and adhesion of the edges. FIG. 205. Part of the flower of Aconite (Aconitum Net) e his), showing two irre gular horn-like petals p, supported on grooved stalks o. These used to be called nectaries, s, the whorl of stamens inserted on the thalamus, and sur rounding the pistil. of the petals in the form of a long process, as in Strophanthus hispidus, where it extends for 7 inches ; and at other times it ends in a free point or cuspis, and the petal becomes cuspidate ; or the prolonged extremity is folded downw r ards or inflexed, as in Umbelliferse, so that the apex approaches the base. The limb of the petal may be flat or concave, or hollowed like a boat. In Hellebore the petals become folded in a tubular form, resembling a horn (fig. 203) ; in Aconite (fig. 205) some of the petals resemble a hollow-curved horn, supported on a grooved stalk ; while in Columbine (fig. 204-), Violet, Snapdragon, and Centranthus, one or all of them are prolonged in the form of a spur, and are calcarate. In Valeriana, Antirrhinum, and Corydalis, Plate VT1 the spur is very short, and the corolla or petal is said to be gibbous, or saccate, at the base. A corolla rarely consists of one petal, and when this occurs, as in Amorpha, it depends on the abortion or non- development of others. Such a corolla is unipetalous, a term quite distinct from monopetalous. A corolla is dipetalous, tripetalous, tetrapctalouz, or pentapetal&us, according as it has two, three, four, or five separate petals. The general name of polypctalous, dialypetalous, eleuthero- petalous, or apopetalous, is given to corollas having separate petals, while monopetalous, gamopetalous, or sympetalous is applied to those in which the petals are united. This union generally takes place at the base, and extends more or less towards the apex ; in Phyteuma the petals are united at their apices also. In some polypetalous corollas,

as that of the Vine, the petals are separate at the base., and