In general fruits are _dry_ or _fleshy_. Dry fruits may be grouped under two heads. Those which open at maturity and scatter the seed are _dehiscent_. Those which do not open are _indehiscent_. II. Indehiscent Fruits. =869. The akene.=—The thin dry wall of the ovary encloses the single seed. It usually does not open and free the seed within. Such a fruit is an _akene_. An _akene_ is a dry, _indehiscent_ fruit. All of the crowded but separate pistils in the buttercup flower when ripe make a head of akenes, which form the fruit of the buttercup. Other examples of akenes are found in other members of the buttercup family, also in the composites, etc. The sunflower seed is a good example of an akene. [Illustration: Fig. 472. Seed, or akene, of buttercup.] [Illustration: Fig. 473. Fruit of red oak. An acorn.] =870. The samara.=—The winged fruits of the maple, elm, etc., are indehiscent fruits. They are sometimes called key fruits. =871. The caryopsis= is a dry fruit in which the seed is consolidated with the wall of the ovary, as in the wheat, corn, and other grasses. =872. The schizocarp= is a dry fruit consisting of several locules (from a _syncarpous gynœcium_). At maturity the carpels separate from each other, but do not themselves dehisce and free the seed, as in the carrot family, mallow family. =873. The acorn.=—The acorn fruit consists of the acorn and the “cup” at the base in which the acorn sits. The cup is a curious structure, and is supposed to be composed of an involucre of numerous small leaves at the base of the pistillate flower, which become consolidated into a hard cup-shaped body. When the acorn is ripe it easily separates from the cup, but the hard pericarp forming the “shell” of the acorn remains closed. Frost may cause it to crack, but very often the pericarp is split open at the smaller end by wedge-like pressure exerted by the emerging radicle during germination. [Illustration: Fig. 474. Germinating acorn of white oak.] =874. The hazelnut, chestnut, and beechnut.=—In these fruits a crown of leaves (involucre) at the base of the flower grows around the nut and completely envelops it, forming the husk or burr. When the fruit is ripe the nut is easily shelled out from the husk. In the beechnut and chestnut the burr dehisces as it dries and allows the nut to drop out. But the fruit is not dehiscent, since the pericarp is still intact and encloses the seed.