The placenta is discoidal and deciduate.
The ovary contains one or more ovules borne on a placenta, which is generally some part of the ovary-wall.
Sometimes a long funicle arises from a basal placenta, reaches the summit of the ovary, and there bending over suspends the ovule, as in Armeria (sea-pink); at other times the hilum appears to be in the middle, and the ovule becomes horizontal.
- Campylotropous ovule of wall-flower (Cheiranthus), showing the funicle f, which attaches the ovule to the placenta; p, the outer, s, the inner coat, n, the nucellus, ch, the chalaza.
91), the ovules, o, are attached to the extended marginal placenta, one above the other, forming usually two parallel rows corresponding to each margin of the carpel.
Finally, there is the hypothesis that marsupials are the descendants of placentals, in which case, as was suggested by its discoverer, the placenta of the bandicoots would be a true vestigial structure.
P, placenta; o, ovules; s, suture, or median line of carpel.
Modification of placenta from simple diffused to cotyledonary form.
Placenta with many cotyledons.
It consists essentially of two parts, a basal portion forming a chamber, the ovary, containing the ovules attached to a part called the placenta, and an upper receptive portion, the stigma, which is either seated on the ovary (sessile), as in the tulip and poppy, or is elevated on a stalk called the style, interposed between the ovary and stigma.
The ovules are attached to the placenta, which consists of a mass of cellular tissue, through which the nourishing vessels pass to the ovule.
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.
When the pistil is formed by one carpel the inner margins unite and form usually a common marginal placenta, which may extend FIG.
The ovules (o) are attached to a central placenta, formed by the union of the five ventral sutures.
I, Vertical; 2, horizontal section; c, calyx; d, wall of ovary; o, ovules; p, placenta; s, stigma.
The ovules (o) are attached to a free central placenta, which has no connexion with the walls of the ovary.
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.
O, Ovary; p, free central placenta; g, ovules; s, styles.
- The same cut horizontally, and the halves separated so as to show the interior of the cavity of the ovary o, with the free central placenta p, covered with ovules g.
102), and is traversed by a narrow canal, in which there are some loose projecting cells, a continuation of the placenta, constituting what is called conducting tissue, which ends in the stigma.
The ovule is attached to the placenta, and destined to become the seed.
In Leontice thalictroides (Blue Cohosh), species of Ophiopogon, Peliosanthes and Stateria, the ovary ruptures immediately after flowering, and the ovules are exposed; and in species of Cuphea the placenta ultimately bursts through the ovary and corolla, and becomes erect, bearing the exposed ovules.
It may hang from an apicilar placenta at the summit of the ovary, its apex being directed downwards, and is inverted or pendulous, as in Hippuris vulgaris; or from a parietal placenta near the summit, and then is suspended, as in Daphne Mezereum, Polygalaceae and Euphorbiaceae.
When the ovules are very numerous (indefinite), while at the same time the placenta is not much developed, their position exhibits great variation, some being directed upwards, others downwards, others transversely; and their form is altered by pressure into various polyhedral shapes.
As a rule there is no allantoic placenta forming the means of communication between the blood of the parent and the foetus, and when such a structure does occur its development is incomplete.
Placenta diffused or cotyledonary.
In Casuarina, Juglans and the order Corylaceae, the pollen-tube does not enter by means of the micropyle, but passing down the ovary wall and through the placenta, enters at the chalazal end of the ovule.
Placenta with few cotyledons.
The placenta is diffuse, not cotyledonary.
The part by which the ovule is attached to the placenta or cord is its base or hilum, the opposite extremity being its apex.
The ovule is sometimes embedded in the placenta, as in Hydnora.
When the ovule is so developed that the chalaza is at the hilum (next the placenta), and the micropyle is at the opposite extremity, there being a short funicle, the ovule is orthotropous.
I I 1) is the commonest form amongst angiosperms. In this ovule the apex with the micropyle is turned towards the point of attachment of the funicle to the placenta, the chalaza being situated at the opposite extremity; and the funicle, which runs along the side usually next the placenta, coalesces with the ovule and constitutes the raphe (r), which often forms a ridge.
When there is a single ovule, with its axis vertical, it may be attached to the placenta at the base of the ovary (basal placenta), and is then erect, as in Polygonaceae and Compositae; or it may be inserted a little above the base, on a parietal placenta, with its apex upwards, and then is ascending, as in Parietaria.
In Araucaria the cone-scale is regarded as consisting of a flat carpel, of which the placenta has not grown out into the scale-like structure.
Near affinity; and while more nearly related to the Marsupialia (q.v.), in which an imperfect allantoic placenta is sometimes developed, it is broadly distinguished therefrom by the invariable presence of a functional placenta by the aid of which the foetus is nourished throughout the greater portion of intra-uterine life.
While most of the species are of interest chiefly to the conchologist, there are a number of edible forms. The shells of Placuna placenta, L., split into thin flat plates and, cut into small squares, are almost universally used in place of window glass.
The superior ovary - half-inferior in Samolus - bears a simple style ending in a capitate entire stigma, and contains a free-central placenta bearing generally a large number of ovules, which are exceptional in the group Gamopetalae in having two integuments.
The petaloid perianth consists of two series, each with three members, which are joined below into a longer or shorter tube, followed by one whorl of three stamens; the inferior ovary is three-celled and contains numerous ovules on an axile placenta; the style is branched and the branches are often petaloid.
Malformations of the pelvis, accidental injuries and the diseases and displacements to which the uterus is liable, on the one hand; and, on the other, various morbid conditions of the ovum or placenta leading to the death of the foetus, are among the direct local causes.