THE LARGE FRUITED AMERICAN OAKS, 



By WILLIAM TRELEASE, Sc.D., LL.D. 



Plates I-III. 



{Read April 23, 1915.) 



When Alphonse de Candolle monographed the oaks of the 

 world something over a generation ago/ he distinguished with a 

 varietal name a form of our common white oak with small acorns 

 some 8 X 14 mm., which Engelmann had sent him — the usual fruit 

 of Quercus alba measuring about 14 X 18 mm. Those who have 

 examined numerous specimens of our common red oak, Q. rubra, 

 and its double, Q. Schneckii, have noted that they occur in forms 

 varying in diameter of the acorn from about 10 to about 20 mm. 

 The assemblage of forms clustering about the Calif ornian Q. chry- 

 solepis, the oldest of our existing types of oak, geologically, also 

 show a comparable or even greater difference in the size of the fruit 

 of what are otherwise held to be mere variants of a single species ; 

 and the polymorphic Q. dumosa presents a similar if less extended 

 range of fruit size.^ 



The most surprising of our species in this respect is the bur oak^ 

 which joins to its great range in size a difference in fruit which is 

 even more startling; for while the usual diameter of the acorns of 

 this species is somewhere about 25 mm., and of the cup five or ten 

 millimeters more, the acorns frequently measure 40 mm. in diameter 

 with a cup fully 50 mm. across on the one hand, while on the other 

 hand they may scarcely reach a diameter of 10 mm. Perhaps no 

 oak presents so great a range of cup characters as this species does.^ 

 While the round or ovoid scaly-fringed form covering the acorn 

 nearly or quite to its top is taken as the most typical and has given 

 to the tree its common name of mossy or overcup oak, it is not un- 



1 Quercus alba microcarpa A. de Candolle, Prodromus, 162 22. 1864. 



2 On these consult Sargent, Silva, vol. 8. 



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