PLACENTA, a name given to the surface, whatever it be, which bears the ovules and seeds. The name may be needless when the ovules grow directly on the ventral suture, or from its top or bottom; but when there are many ovules there is usually some expansion of an ovule-bearing or seed-bearing surface; as is seen in our Mandrake or Podophyllum, Fig. 326. [Illustration: Fig. 326. Simple pistil of Podophyllum, cut across, showing ovules borne on placenta.] [Illustration: Fig. 327. Pistil of a Saxifrage, of two simple carpels or pistil-leaves, united at the base only, cut across both above and below.] [Illustration: Fig. 328. Compound 3-carpellary pistil of common St. John's-wort, cut across: the three styles separate.] [Illustration: Fig. 329. The same of shrubby St. John's-wort; the three styles as well as ovaries here united into one.] [Illustration: Fig. 330. Compound 3-carpellary pistil of Tradescantia or Spiderwort; the three stigmas as well as styles and ovary completely coalescent into one.] 309. =A Compound Pistil= is a combination of two, three, or a greater number of pistil-leaves or carpels in a circle, united into one body, at least by their ovaries. The annexed figures should make it clear. A series of Saxifrages might be selected the gynœcium of which would show every gradation between two simple pistils, or separate carpels, and their complete coalescence into one compound and two-celled ovary. Even when the constituent styles and stigmas are completely coalescent into one, the nature of the combination is usually revealed by some external lines or grooves, or (as in Fig. 328-330) by the internal partitions, or the number of the placentæ. The simplest case of compound pistil is that 310. =With two or more Cells and Axile Placentæ=, namely, with as many cells as there are carpels, that have united to compose the organ. Such a pistil is just what would be formed if the simple pistils (two, three, or five in a circle, as the case may be), like those of a Pæony or Stonecrop (Fig. 224, 225), pressed together in the centre of the flower, were to cohere by their contiguous parts. In such a case the placentæ are naturally _axile_, or all brought together in the axis or centre; and the ovary has as many DISSEPIMENTS, or internal _Partitions_, as there are carpels in its composition. For these are the contiguous and coalescent walls or sides of the component carpels. When such pistils ripen into pods, they often separate along these lines into their elementary carpels. [Illustration: Fig. 331, 332. Pistil of a Sandwort, with vertical and transverse section of the ovary: free central placenta.]