The simplest kind of fruit is a _ripened 1-loculed ovary_. The first stage in complexity is a ripened _2- or many-loculed ovary_. Very complex forms may arise by the _attachment of other parts to the ovary_. Sometimes the style persists and becomes a beak (mustard pods, dentaria, Fig. 224), or a tail as in clematis; or the calyx may be attached to the ovary; or the ovary may be embedded in the receptacle, and ovary and receptacle together constitute the fruit: or an involucre may become a part of the fruit, as possibly in the walnut and hickory (Fig. 225), and cup of the acorn (Fig. 226). The chestnut and the beech bear a prickly involucre, but the nuts, or true fruits, are not grown fast to it, and the involucre can scarcely be called a part of the fruit. A ripened ovary is a =pericarp=. A pericarp to which other parts adhere has been called an accessory or reënforced fruit. (Page 169.) [Illustration: FIG. 225.--HICKORY-NUT. The nut is the fruit, contained in a husk.] [Illustration: FIG. 226.--LIVE-OAK ACORN. The fruit is the “seed” part; the involucre is the “cup.”] [Illustration: FIG. 227.--KEY OF SUGAR MAPLE.] [Illustration: FIG. 228.--KEY OF COMMON AMERICAN ELM.] Some fruits are =dehiscent=, or split open at maturity and liberate the seeds; others are =indehiscent=, or do not open. A dehiscent pericarp is called a =pod=. The parts into which such a pod breaks or splits are known as =valves=. In indehiscent fruits the seed is liberated by the decay of the envelope, or by the rupturing of the envelope by the germinating seed. Indehiscent winged pericarps are known as =samaras= or =key fruits=. Maple (Fig. 227), elm (Fig. 228), and ash (Fig. 93) are examples. =Pericarps.=--The simplest pericarp is a dry, one-seeded, indehiscent body. It is known as an =akene=. A head of akenes is shown in Fig. 229, and the structure is explained in Fig. 230. Akenes may be seen in buttercup, hepatica, anemone, smartweed, buckwheat. [Illustration: FIG. 229.--AKENES OF BUTTERCUP.] [Illustration: FIG. 230.--AKENES OF BUTTERCUP, one in longitudinal section.] A 1-loculed pericarp which dehisces along the front edge (that is, the inner edge, next the center of the flower) is a =follicle=. The fruit of the larkspur (Fig. 231) is a follicle. There are usually five of these fruits (sometimes three or four) in each larkspur flower, each pistil ripening into a follicle. If these pistils were united, a single compound pistil would be formed. Columbine, peony, ninebark, milkweed, also have follicles. [Illustration: FIG. 231.--FOLLICLE OF LARKSPUR.] [Illustration: FIG. 233.--CAPSULE OF CASTOR-OIL BEAN AFTER DEHISCENCE.] [Illustration: FIG. 232.--A BEAN POD.]