Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/153

Rh PLACENTA.] BOTANY 143 divided into cavities, cells, or loculaments, and it may be bitocular, trilocular (fig. 257), quadrilocular, quinquelocular, or multilocular, according as it is formed by two, three, four, five, or many carpels, each carpel corresponding to a single cell or loculament. In these cases the marginal placentas meet in the axis, and unite so as to form a single central one (figs. 257, 258), and the ovules appear in the central angle of the loculi, as in Canna, Lily (fig. 257). The number of loculaments is equal to that of the dissepi ments. When the carpels in a syncarpous pistil do not fold inwards completely so as to meet in the centre, but only partially, so that the dissepiments appear as projections on the walls of the ovary, then the ovary is unilocular (fig. 2GO), and the placentas are parietal, as in Viola (fig. 261). In 260. Fig. 261. Fio. 260. Diagrammatic section of a quinquelocular ovary, in which the septa 3 proceed inwaids for a certain length, bearing the placentas and ovules o. In this case the ovary is unilocular, and the placentas are parietal. Dorsum, !. Fie. 2G1. Pistil of Viola tricolor, or Pansy. 1. Vertical section to show the ovules o, attached to the parletes! Two rows of ovules are seen, one in front, and the other in profile; p, a thickened line on the walls forming the placenta; c, calyx; d, ovary; s, hooded stigma terminating the short style. 2. Horizontal section of the same; ;&amp;gt;, placenta; o, ovules; s, suture. these instances the placentas may be formed at the margin of the united contiguous leaves, so as to appear single, or the margins may not be united, each developing a placenta. Frequently the margins of the carpels, which fold in to the centre, split there into two lamella?, each of which is curved outwards and projects into the loculament, dilating at the end into a placenta. This is well seen in Cucur- bitaceoe (fig. 262), Pyrola, &c. From this it will be seen that dissepiments are opposite to placentas formed by the union of the margins of two contiguous carpels, but alternate with those formed by the margins of the same carpel. The carpellary leaves may fold inwards very slightly, or they may be applied in a valvate manner, merely touching at their margins, the placentas then being parietal (fig. 259), and appearing as lines or thickenings FlO. 268. Diagrammatic section of a compound unilocuiar ovary in which there are no indications of partitions. The ovules o are attached to a free central placenta, which has no connection with the walls of the ovary. along the walls. Cases occur, however, in which the placentas are not connected with the walls of the ovary, and form what is called a free central placenta (fig. 263). Fig. 204. Fig. 265. This is seen in many of the Caryophyllaccoe and Primulaceai (figs. 264, 265). In Caryopliyllaccae, however, while the placenta is free in the centre, there are often traces found at the base of the ovary of the remains of septa, a:; if rupture had taken place, and, in rare instances, ovules are found on the margins of the carpels. But in Primulacea?, Myrsinaceie, and Santalaceoe, no vestiges of septa or marginal ovules can be perceived at any period of growth; the placenta is always free, and rises in the centre of the ovary, and the part [ uncovered by ovules gra dually extends into the style. Free central pla- centation, therefore, has been accounted for in two ways ; either by suppos ing that the placentas in the early state were formed on the margins of carpellary leaves, and that in the progress of ,, FIG. 264. Pistil of Cerastntm hirsuttim cr.t development these leaves vertically, o, unilocular or monotheciil Separated from them, ovary ^ free central placenta; ,7, ovules; leaving the placentas and FlG - 265. The same cut horizontally, and the , . ., . halves separated so as to show the interior OVUieS tree 111 the Centre ; O f the cavity of the ovary o, with the free or by supposing that the central P laecnta P- covered with ovules y. placentas are not marginal but axile formations, produced by an elongation of the axis, the ovules being lateral buds, and the carpels verticillate leaves, united together around the axis. The first of these views would apply well to Caryo- phyllaccie, the second to Primulaceas. The latter case has also been explained, on the marginal hypothesis, by con sidering the placentas as formed from the carpels by a pro cess of chorisis, and united together in the centre. Some consider the axile view of placentation as applic able to all cases, the axis in some 1 instances remaining free and independent, at other times sending prolongations along the margins of the car ellary leaves, and thus forming the marginal placentas. Occasionally, divisions take place in ovaries which are not formed by the edges of contiguous carpels. These are called spurious dissepiments. They are often horizontal, and are then called phragmata, as in Cathartocarpus Fistula, where they consist of transverse cellular prolongations from the walls of the ovary, only developed after fertilization, and therefore more properly noticed under fruit. At other times they are vertical, as in Datura, where the ovary, in place of being two-celled, be comes four-celled ; in Cruciferce, where the prolongation of the placentas forms arephim or partition ; in Astragalus and Thespesia, where the dorsal suture is folded inwards ; in Oxytropis, where the ventral suture is folded inwards ; and in Diplophractum, where the inner margin of the carpels is infiexed. In Cucurbitaceae divisions are formed in the ovary apparently by peculiar projections inwards from curved parietal placentas. In some cases horizontal dissepiments are supposed to be formed by the union of carpels situated at different heights, so that the base of one becomes united to the apex of another. In such cases the divisions are true dissepiments formed by carpellary leaves. The anomalous divisions in the ovary of the Pomegranate have been thus explained. The ovary is usually of a more or less spherical or curved form, sometimes smooth and uniform on its surface, at other times hairy and grooved. The grooves usually indicate the divisions between the caipels, and correspond to the dissepi ments. The dorsal suture may be marked by a slight pro jection, or by a superficial groove. When the ovary is situated on the centre of the receptacle, free from the other whorls, so that its base is above the insertion of the stamens, it is