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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.)

Fig. 225.—Hickory-nut. The nut is the fruit, contained in a husk.

Fig. 226.—Live-oak Acorn. The fruit is the "seed" part; the involucre is the "cup."

Fig. 227.—Key of Sugar Maple.

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.