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leaf as if rolled into a tube; and an anther, a leaf of which the edges may have been turned in on the midrib.

Fig. 176.—Simple Pistils of Buttercup, one in longitudinal section.

The pistil may be of one part or compartment, or of many parts. The different units or parts of which it is composed are carpels. Each carpel is homologous with a leaf. Each carpel bears one or more seeds. A pistil of one carpel is simple; of two or more carpels, compound. Usually the structure of the pistil may be determined by cutting horizontally across the lower or seed-bearing part, as Figs. 177, 178 explain. A flower may contain a simple pistil (one carpel), as the pea (Fig. 177); several simple pistils (several separate carpels), as the buttercup (Fig. 176); or a compound pistil with carpels united, as the Saint John's wort (Fig. 178) and apple. How many carpels in an apple? A peach? An okra pod? A bean pod? The seed cavity in each carpel is called a locule (Latin locus, a place). In these locules the seeds are borne.

Fig. 177.—Pistil of Garden Pea, the stamens being pulled down in order to disclose it; also a section showing the single compartment (compare Fig. 188).

Fig. 178.—Compound Pistil of a St. John's Wort. It has 5 carpels.

Conformation of the Flower.—A flower that has calyx, corolla, stamens, and pistils is said to be complete (Fig. 173); all others are incomplete. In some flowers both the floral envelopes are wanting: such are naked. When one of the floral envelope series is wanting, the remaining series is said to be calyx, and the flower is therefore apetalous (without petals). The knot-