45 2 RELATION TO ENVIRONMENT. 



874. The hazelnut, chestnut, and beechnut. In these fruits a 

 crown of leaves (involucre) at the base of the flower grows around 



Fig. 474- 

 Germinating acorn of white oak. 



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 con- 

 sists 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 rec- 

 ognized, and in general they are sometimes called pods. There 

 is a single carpel (simple pistil), and the pericarp is dry (gynce- 



