ipis.] OF EASTERN NORTH AMERICA. 169 



Leucobalanus: Abortive ovules basal, styles very short, spatulate, acorn not 

 tomentose within, cup scales often thickened at base, fruit ripening in 

 one year, leaves deciduous or evergreen, lobes when present rounded. 



The most stable characters in this classification seem to be the 

 position of the abortive ovules, the lining of the acorn shell and the 

 form of the style. Appression of scales, time for ripening fruit, 

 and time of keeping leaves are all more or less variable among the 

 white oaks. 



The earliest home of Leucohalanus on this continent, using the 

 term to include the white oaks as they separated themselves from 

 the black oaks in America, seems to have been northern Mexico 

 and the southwestern states. The older type (A. below) still pre- 

 dominates in this region, which has probably long been stable, with 

 a climate similar to the present. It is a region which seems to have 

 been for many species a center of distribution to other parts of the 

 continent. Since the Cretaceous, much differentiation has taken 

 place, the main lines of which may be represented by the following 

 division of North American white oaks : 



A. Leaves persistent, usually evergreen, entire, sinuate or dentate, or, if 

 deeper lobed, with pungent tips. 



1. Many species, southwestern U. S. and Mexico. 



2. Virginiana and varieties — an early offshoot. 



B. Leaves deciduous, lobed or divided, or serrate ; lobes rounded, obtuse or 

 acute but not pungent. 



The evergreen series, represented, say, by Q. undulata, is the 

 more direct continuation of the Cretaceous type, the deciduous the 

 more modern form. 



It is barely possible that not all of this differentiation took place 

 on this continent. Leucohalanus reached Europe at some time ; 

 and the possibility that this took place early (by means of ScharfT's 

 Mediterranean land bridge), and that the deciduous oaks originated 

 there, rather than on this continent, must be taken into account. 

 Species of this type occur also in Asia, but there seems to be little 

 doubt that they are sharply separated from the ancient Asiatic 

 species like semecarpifolia, and reached Asia in the Tertiary from 

 the eastward. The fact that the range of these species, in the Ter- 



PROC. AMER. PHIL. SOC. , LIV. 217 L, PRINTED JULY 21, I915. 



