190 



GROWTH AND WORK OF PLANTS 



I 



curious structure, and is supposed to be composed of a crown of 

 numerous small leaves (involucre) at the base of the pistillate 



flower, which become united 

 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 the 

 wedge-like pressure exerted 

 by the emerging root during 

 germination. 



316. The hazelnut, chest- 

 nut, and beechnut. In these 

 fruits a crown of leaves (invo- 

 lucre) 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. 



317. 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 consoli- 

 dated, 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- 



Fig. 150. 



Fruit of corn, husks spread to show ear. 



