Page:EB1911 - Volume 10.djvu/591

Rh and the summit of the style only. Various intermediate states exist, such as partial union of the ovaries, as in the rue, where they coalesce at their base; and partial union of the styles, as in Malvaceae. The union is usually most complete at the base; but in Labiatae the styles are united throughout their length, and in Apocynaceae and Asclepiadaceae the stigmas only. When the union is incomplete, the number of the parts of a compound pistil may be determined by the number of styles and stigmas; when complete, the external venation, the grooves on the surface, and the internal divisions of the ovary indicate the number.

The ovules are attached to the placenta, which consists of a mass of cellular tissue, through which the nourishing vessels pass to the ovule. The placenta is usually formed on the edges of the carpellary leaf (fig. 91)—marginal.

In many cases, however, the placentas are formations from the axis (axile), and are not connected with the carpellary leaves. In marginal placentation the part of the carpel bearing the placenta is the inner or ventral suture, corresponding to the margin of the folded carpellary leaf, while the outer or dorsal suture corresponds to the midrib of the carpellary leaf. As the placenta is formed on each margin of the carpel it is essentially double. This is seen in cases where the margins of the carpel do not unite, but remain separate, and consequently two placentas are formed in place of one. When the pistil is formed by one carpel the inner margins unite and form usually a common marginal placenta, which may extend along the whole margin of the ovary as far as the base of the style (fig. 91), or may be confined to the base or apex only. When the pistil consists of several separate carpels, or is apocarpous, there are generally separate placentas at each of their margins. In a syncarpous pistil, on the other hand, the carpels are so united that the edges of each of the contiguous ones, by their union, form a septum or dissepiment, and the number of these septa consequently indicates the number of carpels in the compound pistil (fig. 92). When the dissepiments extend to the centre or axis, the ovary is divided into cavities or cells, and it may be bilocular, triloculur (fig. 92), quadrilocular, quinquelocular, or multilocular, according as it is formed by two, three, four, five or many carpels, each carpel corresponding to a single cell. In these cases the marginal placentas meet in the axis, and unite so as to form a single central one (figs. 92, 93), and the ovules appear in the central angle of the loculi. When the carpels in a syncarpous pistil do not fold inwards so that the placentas appear as projections on the walls of the ovary, then the ovary is unilocular (fig. 95) and the placentas are parietal, as in Viola (fig. 96). In 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 lamellae, each of which is curved outwards and projects into the loculament, dilating at the end into a placenta. This is well seen in Cucurbitaceae (fig. 97), Pyrola, &c. 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. 94), and appearing as lines or thickenings 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. 98). This is seen in many of the Caryophyllaceae and Primulaceae (figs. 99, 100). In Caryophyllaceae, 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, as if rupture had taken place, and, in rare instances, ovules are found on the margins of the carpels. But in Primulaceae 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. Free central placentation, therefore, has been accounted for in two ways: either by supposing that the placentas in the early state were formed on the margins of