[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.
Fig. 472.
Seed, or akene, of buttercup.
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.