168 COBB— RELATIONSHIPS OF WHITE OAKS [April 23. 



the inference is that they all reached Europe westward from Asia. 

 Though the older fossil evidences in this continent have all been re- 

 ferred to Q. chry sole pis (these date back' to the Cretaceous), it 

 seems not improbable that types such as Q. emoryi and Q. hypo- 

 leuca were soon present, and that differentiation early took the 

 lines towards our American black oaks and zvhite oaks. Since in 

 Cyclohalanopsis, and in the pasanias, the abortive ovules are carried 

 upward in growth till in the mature acorn they are typically apical, 

 this may be considered the primitive condition in Quercus. Chryso- 

 lepis, which has them only lateral, is on the way towards having 

 them in the basal, white-oak, position. The black oaks, on the con- 

 trary, have preserved the primitive character in this as in other par- 

 ticulars. 



(Since the black oaks resemble Cyclohalanopsis in some ways, 

 it may be that they differentiated from Cyclohalanopsis, in the 

 Pacific region, before reaching America. Or all three may have 

 diverged together from the primitive Quercus. Distribution may 

 have been such that Cyclohalanopsis went to Asia, Erythrobalanus 

 to America, Lepidohalanus to both.) 



Having thus some conception of a possible Cretaceous history 

 for American oaks, black and white, and of their relationship to 

 the ancient types of Old World oaks, we may now limit ourselves 

 to the white oak group in North America {Leiicohalanus) . For 

 the black oaks, being limited to the western hemisphere and becom- 

 ing only more sharply differentiated, can give us no further light on 

 white oak relationships. To begin with, we may mark off Leuco- 

 halanus as follows : 



Quercus. 



Cyclohalanopsis: Abortive ovules apical, styles short, subcapitate, often re- 

 curved, cup scales grown into a solid ring, fruit ripening in one year, 

 leaves evergreen, tertiary nerves very fine. 



Erythrobalanus: Abortive ovules apical, styles elongated, subcapitate, often 

 recurved, acorn tomentose within, cup scales thin, appressed, fruit rip- 

 ening in two years, leaves deciduous or evergreen, lobes when present 

 with bristle points. 



Styles slender or very short and flattened, not cephalated at apex. Lepi- 

 dohalanus. 



Cerris: Abortive ovules basal, styles long, tapering, cup scales often long, 

 bractlike, fruit ripening in two years, leaves more or less dentate. 



