Nuts.—A fruit which has a dry, woody pericarp and does not open of itself is called a nut. The fruits of the hazel, oak, and beech are good examples. The shell (pericarp) of the hazel nut (Fig. 139) is composed of three layers, and encloses a single seed, or kernel. The nut of the oak (Fig. 113) is called an acorn. Its lower part lies in a cup developed from a wrinkled disc by which the lower part of the female flower was surrounded. When the fruit is ripe the nut easily separates from the cup. The acorn contains one seed only, which consists largely of two swollen cotyledons. If the ovary of [Pg 178] the female flower of the oak is cut across in June six ovules are found in it. As in the case of the hazel only one ovule is allowed to reach maturity; the rest are sacrificed in order that the remaining one may be more perfectly developed. In the beech, two three-sided nuts occur together, surrounded by a bristly, woody cup, which splits into four valves when the nuts are ripe. Fig. 139.—A, group of Hazel Nuts (× ½); B, longitudinal section of fruit. The dispersal of nuts.—Squirrels and other nut-eating animals are instrumental in the dispersal of the seeds in a somewhat indirect manner. They have a habit of storing up nuts and seeds in holes; but in their active life these animals often forget where their larder is. The seeds, thus left to themselves, sprout and grow into trees. The unripe nuts of the beech are protected from the attacks of squirrels by the hard bristles on the outer husk. Fig. 140.—A, Plum; B, longitudinal section; C, cross section; E, M, En, outer, middle, and inner layers of pericarp; S, seed. (× ½.) [Pg 179] Stone fruits.—A ripe plum (Fig. 140) or cherry (Fig. 77), like a nut, consists of a seed and a pericarp; but here the pericarp is specially modified to tempt animals and at the same time to protect the seed from them. The inner pericarp-layer is a hard, woody shell called the stone; the middle part swells up to form a juicy, sweet mass; while the outer layer constitutes the “skin,” and is often beautifully coloured. Fig. 141.— Blackberry. (× 1.) Blackberries (Fig. 141 F) and raspberries consist of several small fruits of the plum type, which are arranged round an axis derived from the receptacle of the flower. Berries.—The gooseberry (Fig. 142) is a type of this class of fruit. It contains several seeds. The pericarp is thick and fleshy before the fruit is ripe, but during ripening the greater part of it becomes a soft, sweet pulp in which the seeds are embedded. The rest of the pericarp is a membranous skin. Grapes, currants, oranges, and vegetable marrow fruits are also berries. The vegetable marrow has obviously a general resemblance to a half-ripe gooseberry but is on a much larger scale. Fig. 142.—A, Gooseberry; B, longitudinal section; C, cross section. (× ⅔.)