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.
875. The hickory-nut, walnut, and butternut.—In these fruits the “shuck” of the hickory-nut and the “hull” of the walnut and butternut are different from the involucre of the acorn or hazelnut, etc. In the hickory-nut the “shuck” probably consists partly of calyx and partly of involucral bracts consolidated, probably the calyx part predominating. This part of the fruit splits open as it dries and frees the “nut,” the pericarp being very hard and indehiscent. In the walnut and butternut the “hull” is probably of like origin as the “shuck” of the hickory nut, but it does not split open as it ripens. It remains fleshy. The walnut and butternut are often called drupes or stone-fruits, but the fleshy part of the fruit is not of the same origin as the fleshy part of the true drupes, like the cherry, peach, plum, etc.
[III. Dehiscent Fruits.]
876. Of the dehiscent fruits several prominent types are recognized, and in general they are sometimes called pods. There is a single carpel (simple pistil), and the pericarp is dry (gynœcium apocarpous); or where there are several carpels united the pistil is compound (gynœcium syncarpous).
Fig. 475.
Diagrams illustrating three types
(in cross-section) of the dehiscence
of dry fruits. Loc, loculicidal;
Sep, Septicidal, Septifragal.
Fig. 476.
Fruit of sweet pea; a pod.