Ovary
The part of the pistil that in the flower incloses the ovules, and in the fruit incloses the seeds. Its true nature will be best understood if we examine the flower of a garden Pea, or of any similar plant. In the centre of the flower, hidden from view by the lower petals, and by a tube formed of the united stalks (filaments) of the stamens, lies the pistil, which, in the order Leguminosoe, to which the Pea belongs, consists of a single carpel. To expose it to view, the petals and stamens must be removed; it is then seen to have the form shown in Fig. 789. The part next the stalk (o) is broader and greener, and, on being slit open, is found to contain two rows of small bodies, which are the ovules: this part is the Ovary. It becomes narrowed rather abruptly into a paler body (s), which is called the style, and which bears at its tip, and for a little distance down the hollow side, a part (st) called the stigma, suited by its structure to receive the pollen grains, and to permit of their fertilising the ovules through it. By comparing a ripe pea-pod (Fig.790) with the carpel of the flower, it will be found that the Ovary has increased very much in size, and that the style and stigma have shrivelled to a mere shred (s) on the tip of the pod, but no new parts can be distinguished. The ovules have developed into seeds. In such a carpel as a pea-pod, it is not difficult to recognise a great likeness to a leaf, folded along the midrib, so that the lower surface is outside, and the upper surface lines the cavity, which is closed by the inturned edges being joined. It is now generally believed that carpels are in reality leaves modified in this way, and that the style is a prolongation of the leaf. Thus, a single chamber is formed, and in it lie all the ovules. The ovules are borne along the two edges of the carpellary leaf, forming a double row, as may be readily seen when a pea-pod is opened. The thickened edges are called the placentae. The microscopic structure of a pea-pod differs only in details from that of a leaf, and the arrangement of the veins is readily seen to be similar in both. Along the back of the pod (dorsal suture) is a large vein, corresponding to the midrib of the leaf, and along each half of the placenta runs a vein, corresponding to the marginal arched veins in leaves, but strengthened to give a branch to each ovule. The carpel is seen, in the Pea, to be situated nearer the tip of the receptacle than is any other part of the flower. The carpels of Actaea and of Paeonia show a very similar cross section to the pea-pod. The pea-pod may be taken as an excellent example of a carpel in which the Ovary is quite typically formed, and remains so throughout its growth; but this simple structure of the carpel is widely departed from in various ways, and it is necessary to look now at some of the less typical forms to ascertain to what extent they differ from it. 1. In most plants the pistil is made up of two, or more, carpels. These may be, as in Paeonia and Ranunculus, free from one another down to the base, or they may be united to any extent, from near the base almost, or altogether, to the top of the Ovary, and even the styles and stigmas may be completely united to each other. The Ovaries may be united only by their outer surfaces, each being closed, and remaining distinct from the rest internally, or they may open into a central space common to all. The ovules may be borne along the edges of the neighbouring Ovaries, either on projecting upright plates, e.g., Poppy, or in rows along the inside of the wall of the Ovary, as in Gooseberry. In all these cases, it is easy to recognise that the compound Ovary is really made up of as many simple Ovaries as there are placentae, and that the ovules are borne on the edges of the Ovaries. This will be better understood by a reference to Figs. 792, 793, and 794. In all fresh Ovaries, cross sections prove most instructive for study. In the Primrose and its allies, there occurs what is known as free-central placentation; i.e., the Ovules are borne on a head (placenta), which looks like a direct growth upwards of the receptacle; and the carpels from only a covering to the head and ovules, which latter do not adhere to any part of them It is not possible here, in the flower, to tell the number of carpels that unite to form the Ovary, though the number may be inferred. In Lychnis and other Caryophylleae, the structure is almost like that of the Primrose in the mature flower; but it is possible to trace a connection, at a very early stage, between the placentae and the carpels; and this connection, and the number of carpels, are indicated by lines radiating from the middle of the central mass; and the number of rows of ovules also indicates the number of carpels. The shape of the Ovary, when made up of two or more carpels, usually depends on the number that form it, and is best seen in a cross section of the Ovary made when fresh. 2. Care must be taken not to confound, with truly compound or united Ovaries, a condition sometimes met with-e.g., in Astragalus with a simple carpel, and in Flax, with the Ovary composed of several united carpels-in which each carpel is divided, or nearly so, into two cavities, lengthwise, by a partition, which springs from the surface along the midrib of the carpel, enlarging as the seeds ripen. 3. The typical structure of the Ovary is departed from in some plants, in the growth, after the seeds form, of a number of cross partitions; so that each carpel is made up of a row of divisions, with one seed in each. 4. The typical situation of the Ovary is, as we have seen, at the tip of the flower-stalk (receptacle), or, at least, nearer to it than are the other parts of the flower But in many plants the Ovary seems to be more or less completely sunk in the flower-stalk, and to be surrounded by the sepals, petals, and stamens grown together. More careful observation leads to the belief that the receptacle produces, below its apex, a flat ring (e.g., in Strawberry), or a cup, which may surround the Ovary either (e.g., Cherry) without being united with it, or united to it in part (e.g., Saxifrages), or throughout its whole length (e.g., Carrot, Campanula, &c.), or even projecting beyond it (e.g., Fuchsia), so that it is concealed. Great use has been made of these differences in Systematic Botany, though absolute reliance on them at times tends to separate nearly allied plants. 5. Still another mode of departure from the leaf-type shows itself in many Ovaries, as the seeds ripen in them, due to alterations in the texture of their walls. They may become uniformly hard, as in the Acorn, and in the Hazel-nut, or they may become pulpy inside, with a thin, leathery, outer skin, as in the Grape; or there may be a thin, tough, or hard outer skin, a more or less abundant pulpy middle layer, and a hard inner shell, known as the "stone" in stone-fruits This change is frequent both in simple and in compound or united Ovaries. Such differences in structure are best understood with reference to the modes of distribution of the seeds. The ripe fruit is, in most plants, the Ovary merely enlarged, or changed in one or more of the ways indicated; but in some plants the receptacle also enlarges, either into a fleshy head, as in the Strawberry, or the receptacle cup around the Ovary, mentioned above, becomes fleshy, or, in some form or other, becomes a part of the fruit. The methods of opening, to allow of the escape of the seed, differ greatly in those Ovaries from which the seeds escape when ripe; while one-seeded carpels, or such as have fleshy walls in the ripe condition, usually do not split. The consideration of these differences also belong more to the great subject of the distribution of the seeds than to the subject immediately under notice.