311. =One-celled, with free Central Placenta.= The commoner case is that of Purslane (Fig. 272) and of the Pink and Chickweed families (Fig. 331, 332). This is explained by supposing that the partitions (such as those of Fig. 329) have early vanished or have been suppressed. Indeed, traces of them may often be detected in Pinks. On the other hand, it is equally supposable that in the Primula family the free central is derived from parietal placentation by the carpels bearing ovules only at base, and forming a consolidated common placenta in the axis. Mitella and Dionæa help out this conception. [Illustration: Fig. 333. Plan of a one-celled ovary of three carpel-leaves, with parietal placentæ, cut across below, where it is complete; the upper part showing the top of the three leaves it is composed of, approaching, but not united.] [Illustration: Fig. 334. Cross section of the ovary of Frost weed (Helianthemum), with three parietal placentæ, bearing ovules.] [Illustration: Fig. 335. Cross section of an ovary of Hypericum graveolens, the three large placentæ meeting in the centre, so as to form a three-celled ovary. 336. Same in fruit, the placentæ now separate and rounded.] 312. =One-celled, with Parietal Placentæ.= In this not uncommon case it is conceived that the two or three or more carpel-leaves of such a compound pistil coalesce by their adjacent edges, just as sepal-leaves do to form a gamosepalous calyx, or petals to form a gamopetalous corolla, and as is shown in the diagram, Fig. 333, and in an actual cross-section, Fig. 334. Here each carpel is an open leaf, or with some introflexion, bearing ovules along its margins; and each placenta consists of the contiguous margins of two pistil-leaves grown together. There is every gradation between this and the three-celled ovary with the placentæ in the axis, even in the same genus, sometimes even in different stages in the same pistil (Fig. 335, 336). § 2. GYMNOSPERMOUS GYNŒCIUM. 313. The ordinary pistil has a closed ovary, and accordingly the pollen can act upon the contained ovules only indirectly, through the stigma. This is expressed in a term of Greek derivation, viz.:-- _Angiospermous_, meaning that the seeds are borne in a sac or closed vessel. The counterpart term is _Gymnospermous_, meaning naked-seeded. This kind of pistil, or gynœcium, the simplest of all, yet the most peculiar, characterizes the Pine family and its relatives. [Illustration: Fig. 337. A pistil, that is, a scale of the cone, of a Larch, at the time of flowering; inside view, showing its pair of naked ovules.] [Illustration: Fig. 338. Branchlet of the American Arbor-Vitæ, considerably larger than in nature, terminated by its pistillate flowers, each consisting of a single scale (an open pistil), together forming a small cone.] [Illustration: Fig. 339. One of the scales or carpels of the last, removed and more enlarged, the inside exposed to view, showing a pair of ovules on its base.]