NOTES ON THE GEOLOGICAL HISTOKY OF THE WAL- 

 NUTS AND HICKORIES.^ 



By Edward W. Bekby, 

 The Johns Hopkins University, Baltimore, Md. 



The walnut family ( Juglandaceae) , which in the popular mind 

 is fully rounded out by the enumeration of the walnut, butternut, 

 hickory, and pignut, while relatively small, is by no means as limited 

 as this might indicate. According to current interpretations there 

 are 6 genera and about 40 species widely scattered throughout the 

 warmer parts of the North Temperate Zone and penetrating some 

 distance south of the Equator along the Andes in South America, and 

 in the East Indies. 



The Juglandaceae are of considerable interest for a variety of 

 reasons, chief among which, aside from their great economic impor- 

 tance, are their long line of ancestors reaching back some millions 

 of years to the mid-Cretaceous, and the former wide range and 

 abundance of these ancestors, which also serves to explain the curious 

 geographical distribution of the still existing species. They are 

 also interesting because of the much discussed question as to whether 

 their morphological characters shall be interpreted as primitive or as 

 mere simplifications of a more highly organized stock. 



Not all of the genera have adopted the same methods of seed 

 dispersal and certain genera have kept the seed part of their fruits 

 comparatively small and light, thus enabling them to produce large 

 numbers of seeds with the same expenditure of energy required for 

 a single walnut. Furthermore, instead of depending altogether upon 

 chance for the dissemination of their latent progeny, the bracts which 

 are normally present have developed enormously and serve as wings. 

 This is especially true in the genera Eiigelhardtia and Oreomunnea 

 and will be referred to on a subsequent page. 



The fruits unmistakably indicate the genera — those of the hickory 

 have smooth shells and a husk which splits more or less, the walnuts 

 and butternuts have a very rugose surface and an entire husk, while 

 Engelhardtla, Oreomi^&nea^ and Pterocarya have small compound 

 winged fruits. The leaves are always compound, indicating a trop- 



1 Reprinted by permission from The Plant World, vol. 15, No. 10, October, 1912. 



319 



